perf: introduce RefCountedSet structure, use it for Style.Set

src/terminal/PageList.zig

@@ -1158,17 +1158,17 @@ fn reflowPage(
 
                         // If the source cell has a style, we need to copy it.
                         if (src_cursor.page_cell.style_id != stylepkg.default_id) {
-                            const src_style = src_cursor.page.styles.lookupId(
+                            const src_style = src_cursor.page.styles.get(
                                 src_cursor.page.memory,
                                 src_cursor.page_cell.style_id,
-                            ).?.*;
+                            ).*;
-
+                            if (try dst_cursor.page.styles.addWithId(
-                            const dst_md = try dst_cursor.page.styles.upsert(
                                 dst_cursor.page.memory,
                                 src_style,
-                            );
+                                src_cursor.page_cell.style_id,
-                            dst_md.ref += 1;
+                            )) |id| {
-                            dst_cursor.page_cell.style_id = dst_md.id;
+                                dst_cursor.page_cell.style_id = id;
+                            }
                             dst_cursor.page_row.styled = true;
                         }
                     }
@@ -1905,18 +1905,6 @@ pub fn adjustCapacity(
     return new_page;
 }
 
-/// Compact a page, reallocating to minimize the amount of memory
-/// required for the page. This is useful when we've overflowed ID
-/// spaces, are archiving a page, etc.
-///
-/// Note today: this doesn't minimize the memory usage, but it does
-/// fix style ID overflow. A future update can shrink the memory
-/// allocation.
-pub fn compact(self: *PageList, page: *List.Node) !*List.Node {
-    // Adjusting capacity with no adjustments forces a reallocation.
-    return try self.adjustCapacity(page, .{});
-}
-
 /// Create a new page node. This does not add it to the list and this
 /// does not do any memory size accounting with max_size/page_size.
 fn createPage(
@@ -3039,10 +3027,10 @@ pub const Pin = struct {
     /// Returns the style for the given cell in this pin.
     pub fn style(self: Pin, cell: *pagepkg.Cell) stylepkg.Style {
         if (cell.style_id == stylepkg.default_id) return .{};
-        return self.page.data.styles.lookupId(
+        return self.page.data.styles.get(
             self.page.data.memory,
             cell.style_id,
-        ).?.*;
+        ).*;
     }
 
     /// Check if this pin is dirty.
@@ -3302,10 +3290,10 @@ const Cell = struct {
     /// Not meant for non-test usage since this is inefficient.
     pub fn style(self: Cell) stylepkg.Style {
         if (self.cell.style_id == stylepkg.default_id) return .{};
-        return self.page.data.styles.lookupId(
+        return self.page.data.styles.get(
             self.page.data.memory,
             self.cell.style_id,
-        ).?.*;
+        ).*;
     }
 
     /// Gets the screen point for the given cell.
@@ -7363,18 +7351,20 @@ test "PageList resize reflow less cols copy style" {
 
         // Create a style
         const style: stylepkg.Style = .{ .flags = .{ .bold = true } };
-        const style_md = try page.styles.upsert(page.memory, style);
+        const style_id = try page.styles.add(page.memory, style);
 
         for (0..s.cols - 1) |x| {
             const rac = page.getRowAndCell(x, 0);
             rac.cell.* = .{
                 .content_tag = .codepoint,
                 .content = .{ .codepoint = @intCast(x) },
-                .style_id = style_md.id,
+                .style_id = style_id,
             };
-
+            page.styles.use(page.memory, style_id);
-            style_md.ref += 1;
         }
+
+        // We're over-counted by 1 because `add` implies `use`.
+        page.styles.release(page.memory, style_id);
     }
 
     // Resize
@@ -7391,10 +7381,10 @@ test "PageList resize reflow less cols copy style" {
             const style_id = rac.cell.style_id;
             try testing.expect(style_id != 0);
 
-            const style = offset.page.data.styles.lookupId(
+            const style = offset.page.data.styles.get(
                 offset.page.data.memory,
                 style_id,
-            ).?;
+            );
             try testing.expect(style.flags.bold);
 
             const row = rac.row;

src/terminal/Screen.zig

@@ -100,7 +100,6 @@ pub const Cursor = struct {
     /// we change pages we need to ensure that we update that page with
     /// our style when used.
     style_id: style.Id = style.default_id,
-    style_ref: ?*size.CellCountInt = null,
 
     /// The pointers into the page list where the cursor is currently
     /// located. This makes it faster to move the cursor.
@@ -202,31 +201,6 @@ pub fn assertIntegrity(self: *const Screen) void {
         ) orelse unreachable;
         assert(self.cursor.x == pt.active.x);
         assert(self.cursor.y == pt.active.y);
-
-        if (self.cursor.style_id == style.default_id) {
-            // If our style is default, we should have no refs.
-            assert(self.cursor.style.default());
-            assert(self.cursor.style_ref == null);
-        } else {
-            // If our style is not default, we should have a ref.
-            assert(!self.cursor.style.default());
-            assert(self.cursor.style_ref != null);
-
-            // Further, the ref should be valid within the current page.
-            const page = &self.cursor.page_pin.page.data;
-            const page_style = page.styles.lookupId(
-                page.memory,
-                self.cursor.style_id,
-            ).?.*;
-            assert(self.cursor.style.eql(page_style));
-
-            // The metadata pointer should be equal to the ref.
-            const md = page.styles.upsert(
-                page.memory,
-                page_style,
-            ) catch unreachable;
-            assert(&md.ref == self.cursor.style_ref.?);
-        }
     }
 }
 
@@ -524,14 +498,6 @@ pub fn cursorReload(self: *Screen) void {
     // If we have a style, we need to ensure it is in the page because this
     // method may also be called after a page change.
     if (self.cursor.style_id != style.default_id) {
-        // We set our ref to null because manualStyleUpdate will refresh it.
-        // If we had a valid ref and it was zero before, then manualStyleUpdate
-        // will reload the same ref.
-        //
-        // We want to avoid the scenario this was non-null but the pointer
-        // is now invalid because it pointed to a page that no longer exists.
-        self.cursor.style_ref = null;
-
         self.manualStyleUpdate() catch |err| {
             // This failure should not happen because manualStyleUpdate
             // handles page splitting, overflow, and more. This should only
@@ -540,7 +506,6 @@ pub fn cursorReload(self: *Screen) void {
             log.err("failed to update style on cursor reload err={}", .{err});
             self.cursor.style = .{};
             self.cursor.style_id = 0;
-            self.cursor.style_ref = null;
         };
     }
 }
@@ -595,7 +560,6 @@ pub fn cursorDownScroll(self: *Screen) !void {
                 log.err("failed to update style on cursor scroll err={}", .{err});
                 self.cursor.style = .{};
                 self.cursor.style_id = 0;
-                self.cursor.style_ref = null;
             };
         }
     } else {
@@ -683,7 +647,6 @@ pub fn cursorCopy(self: *Screen, other: Cursor) !void {
     // invalid.
     self.cursor.style = .{};
     self.cursor.style_id = 0;
-    self.cursor.style_ref = null;
 
     // We need to keep our old x/y because that is our cursorAbsolute
     // will fix up our pointers.
@@ -698,7 +661,6 @@ pub fn cursorCopy(self: *Screen, other: Cursor) !void {
     // We keep the old style ref so manualStyleUpdate can clean our old style up.
     self.cursor.style = other.style;
     self.cursor.style_id = old.style_id;
-    self.cursor.style_ref = old.style_ref;
     try self.manualStyleUpdate();
 }
 
@@ -744,7 +706,6 @@ fn cursorChangePin(self: *Screen, new: Pin) void {
         log.err("failed to update style on cursor change err={}", .{err});
         self.cursor.style = .{};
         self.cursor.style_id = 0;
-        self.cursor.style_ref = null;
     };
 }
 
@@ -889,33 +850,7 @@ pub fn clearCells(
     if (row.styled) {
         for (cells) |*cell| {
             if (cell.style_id == style.default_id) continue;
-
+            page.styles.release(page.memory, cell.style_id);
-            // Fast-path, the style ID matches, in this case we just update
-            // our own ref and continue. We never delete because our style
-            // is still active.
-            if (page == &self.cursor.page_pin.page.data and
-                cell.style_id == self.cursor.style_id)
-            {
-                self.cursor.style_ref.?.* -= 1;
-                continue;
-            }
-
-            // Slow path: we need to lookup this style so we can decrement
-            // the ref count. Since we've already loaded everything, we also
-            // just go ahead and GC it if it reaches zero, too.
-            if (page.styles.lookupId(
-                page.memory,
-                cell.style_id,
-            )) |prev_style| {
-                // Below upsert can't fail because it should already be present
-                const md = page.styles.upsert(
-                    page.memory,
-                    prev_style.*,
-                ) catch unreachable;
-                assert(md.ref > 0);
-                md.ref -= 1;
-                if (md.ref == 0) page.styles.remove(page.memory, cell.style_id);
-            }
         }
 
         // If we have no left/right scroll region we can be sure that
@@ -1044,17 +979,37 @@ pub fn resizeWithoutReflow(
 }
 
 /// Resize the screen.
-// TODO: replace resize and resizeWithoutReflow with this.
 fn resizeInternal(
     self: *Screen,
     cols: size.CellCountInt,
     rows: size.CellCountInt,
     reflow: bool,
 ) !void {
+    defer self.assertIntegrity();
+
     // No matter what we mark our image state as dirty
     self.kitty_images.dirty = true;
 
-    // Perform the resize operation. This will update cursor by reference.
+    // Release the cursor style while resizing just
+    // in case the cursor ends up on a different page.
+    const cursor_style = self.cursor.style;
+    self.cursor.style = .{};
+    self.manualStyleUpdate() catch unreachable;
+    defer {
+        // Restore the cursor style.
+        self.cursor.style = cursor_style;
+        self.manualStyleUpdate() catch |err| {
+            // This failure should not happen because manualStyleUpdate
+            // handles page splitting, overflow, and more. This should only
+            // happen if we're out of RAM. In this case, we'll just degrade
+            // gracefully back to the default style.
+            log.err("failed to update style on cursor reload err={}", .{err});
+            self.cursor.style = .{};
+            self.cursor.style_id = 0;
+        };
+    }
+
+    // Perform the resize operation.
     try self.pages.resize(.{
         .rows = rows,
         .cols = cols,
@@ -1072,7 +1027,6 @@ fn resizeInternal(
     // If our cursor was updated, we do a full reload so all our cursor
     // state is correct.
     self.cursorReload();
-    self.assertIntegrity();
 }
 
 /// Set a style attribute for the current cursor.
@@ -1223,21 +1177,14 @@ pub fn manualStyleUpdate(self: *Screen) !void {
 
     // std.log.warn("active styles={}", .{page.styles.count(page.memory)});
 
-    // Remove our previous style if is unused.
+    // Release our previous style if it was not default.
-    if (self.cursor.style_ref) |ref| {
+    if (self.cursor.style_id != style.default_id) {
-        if (ref.* == 0) {
+        page.styles.release(page.memory, self.cursor.style_id);
-            page.styles.remove(page.memory, self.cursor.style_id);
-        }
-
-        // Reset our ID and ref to null since the ref is now invalid.
-        self.cursor.style_id = 0;
-        self.cursor.style_ref = null;
     }
 
     // If our new style is the default, just reset to that
     if (self.cursor.style.default()) {
         self.cursor.style_id = 0;
-        self.cursor.style_ref = null;
         return;
     }
 
@@ -1251,42 +1198,29 @@ pub fn manualStyleUpdate(self: *Screen) !void {
     // if that makes a meaningful difference. Our priority is to keep print
     // fast because setting a ton of styles that do nothing is uncommon
     // and weird.
-    const md = page.styles.upsert(
+    const id = page.styles.add(
         page.memory,
         self.cursor.style,
-    ) catch |err| md: {
+    ) catch id: {
-        switch (err) {
+        // Our style map is full. Let's allocate a new
-            // Our style map is full. Let's allocate a new page by doubling
+        // page by doubling the size and then try again.
-            // the size and then try again.
+        const node = try self.pages.adjustCapacity(
-            error.OutOfMemory => {
+            self.cursor.page_pin.page,
-                const node = try self.pages.adjustCapacity(
+            .{ .styles = page.capacity.styles * 2 },
-                    self.cursor.page_pin.page,
+        );
-                    .{ .styles = page.capacity.styles * 2 },
-                );
 
-                page = &node.data;
+        page = &node.data;
-            },
-
-            // We've run out of style IDs. This is fixed by doing a page
-            // compaction.
-            error.Overflow => {
-                const node = try self.pages.compact(
-                    self.cursor.page_pin.page,
-                );
-                page = &node.data;
-            },
-        }
 
         // Since this modifies our cursor page, we need to reload
         cursor_reload = true;
 
-        break :md try page.styles.upsert(
+        break :id try page.styles.add(
             page.memory,
             self.cursor.style,
         );
     };
-    self.cursor.style_id = md.id;
+    self.cursor.style_id = id;
-    self.cursor.style_ref = &md.ref;
+
     if (cursor_reload) self.cursorReload();
     self.assertIntegrity();
 }
@@ -2271,8 +2205,9 @@ pub fn testWriteString(self: *Screen, text: []const u8) !void {
                 };
 
                 // If we have a ref-counted style, increase.
-                if (self.cursor.style_ref) |ref| {
+                if (self.cursor.style_id != style.default_id) {
-                    ref.* += 1;
+                    const page = self.cursor.page_pin.page.data;
+                    page.styles.use(page.memory, self.cursor.style_id);
                     self.cursor.page_row.styled = true;
                 }
             },
@@ -2310,6 +2245,14 @@ pub fn testWriteString(self: *Screen, text: []const u8) !void {
                     .wide = .spacer_tail,
                     .protected = self.cursor.protected,
                 };
+
+                // If we have a ref-counted style, increase twice.
+                if (self.cursor.style_id != style.default_id) {
+                    const page = self.cursor.page_pin.page.data;
+                    page.styles.use(page.memory, self.cursor.style_id);
+                    page.styles.use(page.memory, self.cursor.style_id);
+                    self.cursor.page_row.styled = true;
+                }
             },
 
             else => unreachable,
@@ -2792,10 +2735,10 @@ test "Screen: cursorDown across pages preserves style" {
     try s.setAttribute(.{ .bold = {} });
     {
         const page = &s.cursor.page_pin.page.data;
-        const styleval = page.styles.lookupId(
+        const styleval = page.styles.get(
             page.memory,
             s.cursor.style_id,
-        ).?;
+        );
         try testing.expect(styleval.flags.bold);
     }
 
@@ -2803,10 +2746,10 @@ test "Screen: cursorDown across pages preserves style" {
     s.cursorDown(1);
     {
         const page = &s.cursor.page_pin.page.data;
-        const styleval = page.styles.lookupId(
+        const styleval = page.styles.get(
             page.memory,
             s.cursor.style_id,
-        ).?;
+        );
         try testing.expect(styleval.flags.bold);
     }
 }
@@ -2835,10 +2778,10 @@ test "Screen: cursorUp across pages preserves style" {
     try s.setAttribute(.{ .bold = {} });
     {
         const page = &s.cursor.page_pin.page.data;
-        const styleval = page.styles.lookupId(
+        const styleval = page.styles.get(
             page.memory,
             s.cursor.style_id,
-        ).?;
+        );
         try testing.expect(styleval.flags.bold);
     }
 
@@ -2848,10 +2791,10 @@ test "Screen: cursorUp across pages preserves style" {
         const page = &s.cursor.page_pin.page.data;
         try testing.expect(start_page == page);
 
-        const styleval = page.styles.lookupId(
+        const styleval = page.styles.get(
             page.memory,
             s.cursor.style_id,
-        ).?;
+        );
         try testing.expect(styleval.flags.bold);
     }
 }
@@ -2880,10 +2823,10 @@ test "Screen: cursorAbsolute across pages preserves style" {
     try s.setAttribute(.{ .bold = {} });
     {
         const page = &s.cursor.page_pin.page.data;
-        const styleval = page.styles.lookupId(
+        const styleval = page.styles.get(
             page.memory,
             s.cursor.style_id,
-        ).?;
+        );
         try testing.expect(styleval.flags.bold);
     }
 
@@ -2893,10 +2836,10 @@ test "Screen: cursorAbsolute across pages preserves style" {
         const page = &s.cursor.page_pin.page.data;
         try testing.expect(start_page == page);
 
-        const styleval = page.styles.lookupId(
+        const styleval = page.styles.get(
             page.memory,
             s.cursor.style_id,
-        ).?;
+        );
         try testing.expect(styleval.flags.bold);
     }
 }
@@ -3013,10 +2956,10 @@ test "Screen: scrolling across pages preserves style" {
     const page = &s.pages.pages.last.?.data;
     try testing.expect(start_page != page);
 
-    const styleval = page.styles.lookupId(
+    const styleval = page.styles.get(
         page.memory,
         s.cursor.style_id,
-    ).?;
+    );
     try testing.expect(styleval.flags.bold);
 }
 

src/terminal/Terminal.zig

@@ -600,8 +600,19 @@ fn printCell(
         );
     }
 
-    // Keep track of the previous style so we can decrement the ref count
+    // We don't need to update the style refs unless the
-    const prev_style_id = cell.style_id;
+    // cell's new style will be different after writing.
+    const style_changed = cell.style_id != self.screen.cursor.style_id;
+
+    if (style_changed) {
+        var page = &self.screen.cursor.page_pin.page.data;
+
+        // Release the old style.
+        if (cell.style_id != style.default_id) {
+            assert(self.screen.cursor.page_row.styled);
+            page.styles.release(page.memory, cell.style_id);
+        }
+    }
 
     // Write
     cell.* = .{
@@ -612,50 +623,12 @@ fn printCell(
         .protected = self.screen.cursor.protected,
     };
 
-    if (comptime std.debug.runtime_safety) {
+    if (style_changed) {
-        // We've had bugs around this, so let's add an assertion: every
+        var page = &self.screen.cursor.page_pin.page.data;
-        // style we use should be present in the style table.
-        if (self.screen.cursor.style_id != style.default_id) {
-            const page = &self.screen.cursor.page_pin.page.data;
-            if (page.styles.lookupId(
-                page.memory,
-                self.screen.cursor.style_id,
-            ) == null) {
-                log.err("can't find style page={X} id={}", .{
-                    @intFromPtr(&self.screen.cursor.page_pin.page.data),
-                    self.screen.cursor.style_id,
-                });
-                @panic("style not found");
-            }
-        }
-    }
 
-    // Handle the style ref count handling
+        // Use the new style.
-    style_ref: {
+        if (cell.style_id != style.default_id) {
-        if (prev_style_id != style.default_id) {
+            page.styles.use(page.memory, cell.style_id);
-            const row = self.screen.cursor.page_row;
-            assert(row.styled);
-
-            // If our previous cell had the same style ID as us currently,
-            // then we don't bother with any ref counts because we're the same.
-            if (prev_style_id == self.screen.cursor.style_id) break :style_ref;
-
-            // Slow path: we need to lookup this style so we can decrement
-            // the ref count. Since we've already loaded everything, we also
-            // just go ahead and GC it if it reaches zero, too.
-            var page = &self.screen.cursor.page_pin.page.data;
-            if (page.styles.lookupId(page.memory, prev_style_id)) |prev_style| {
-                // Below upsert can't fail because it should already be present
-                const md = page.styles.upsert(page.memory, prev_style.*) catch unreachable;
-                assert(md.ref > 0);
-                md.ref -= 1;
-                if (md.ref == 0) page.styles.remove(page.memory, prev_style_id);
-            }
-        }
-
-        // If we have a ref-counted style, increase.
-        if (self.screen.cursor.style_ref) |ref| {
-            ref.* += 1;
             self.screen.cursor.page_row.styled = true;
         }
     }
@@ -2190,8 +2163,12 @@ pub fn decaln(self: *Terminal) !void {
         });
 
         // If we have a ref-counted style, increase
-        if (self.screen.cursor.style_ref) |ref| {
+        if (self.screen.cursor.style_id != style.default_id) {
-            ref.* += @intCast(cells.len);
+            page.styles.useMultiple(
+                page.memory,
+                self.screen.cursor.style_id,
+                @intCast(cells.len),
+            );
             row.styled = true;
         }
 
@@ -7066,7 +7043,8 @@ test "Terminal: bold style" {
         const cell = list_cell.cell;
         try testing.expectEqual(@as(u21, 'A'), cell.content.codepoint);
         try testing.expect(cell.style_id != 0);
-        try testing.expect(t.screen.cursor.style_ref.?.* > 0);
+        const page = t.screen.cursor.page_pin.page.data;
+        try testing.expect(page.styles.refCount(page.memory, t.screen.cursor.style_id) > 1);
     }
 }
 

src/terminal/page.zig

@@ -197,6 +197,7 @@ pub const Page = struct {
             .styles = style.Set.init(
                 buf.add(l.styles_start),
                 l.styles_layout,
+                style.StyleSetContext{},
             ),
             .grapheme_alloc = GraphemeAlloc.init(
                 buf.add(l.grapheme_alloc_start),
@@ -324,17 +325,11 @@ pub const Page = struct {
 
                 if (cell.style_id != style.default_id) {
                     // If a cell has a style, it must be present in the styles
-                    // set.
+                    // set. Accessing it with `get` asserts that.
-                    _ = self.styles.lookupId(
+                    _ = self.styles.get(
                         self.memory,
                         cell.style_id,
-                    ) orelse {
+                    );
-                        log.warn(
-                            "page integrity violation y={} x={} style missing id={}",
-                            .{ y, x, cell.style_id },
-                        );
-                        return IntegrityError.MissingStyle;
-                    };
 
                     if (!row.styled) {
                         log.warn(
@@ -424,12 +419,11 @@ pub const Page = struct {
         {
             var it = styles_seen.iterator();
             while (it.next()) |entry| {
-                const style_val = self.styles.lookupId(self.memory, entry.key_ptr.*).?.*;
+                const ref_count = self.styles.refCount(self.memory, entry.key_ptr.*);
-                const md = self.styles.upsert(self.memory, style_val) catch unreachable;
+                if (ref_count < entry.value_ptr.*) {
-                if (md.ref < entry.value_ptr.*) {
                     log.warn(
                         "page integrity violation style ref count mismatch id={} expected={} actual={}",
-                        .{ entry.key_ptr.*, entry.value_ptr.*, md.ref },
+                        .{ entry.key_ptr.*, entry.value_ptr.*, ref_count },
                     );
                     return IntegrityError.MismatchedStyleRef;
                 }
@@ -474,7 +468,7 @@ pub const Page = struct {
         return result;
     }
 
-    pub const CloneFromError = Allocator.Error || style.Set.UpsertError;
+    pub const CloneFromError = Allocator.Error || error{OutOfMemory};
 
     /// Clone the contents of another page into this page. The capacities
     /// can be different, but the size of the other page must fit into
@@ -586,11 +580,22 @@ pub const Page = struct {
                     for (cps) |cp| try self.appendGrapheme(dst_row, dst_cell, cp);
                 }
                 if (src_cell.style_id != style.default_id) {
-                    const other_style = other.styles.lookupId(other.memory, src_cell.style_id).?.*;
-                    const md = try self.styles.upsert(self.memory, other_style);
-                    md.ref += 1;
-                    dst_cell.style_id = md.id;
                     dst_row.styled = true;
+
+                    if (other == self) {
+                        // If it's the same page we don't have to worry about
+                        // copying the style, we can use the style ID directly.
+                        dst_cell.style_id = src_cell.style_id;
+                        self.styles.use(self.memory, dst_cell.style_id);
+                        continue;
+                    }
+
+                    // Slow path: Get the style from the other
+                    // page and add it to this page's style set.
+                    const other_style = other.styles.get(other.memory, src_cell.style_id).*;
+                    if (try self.styles.addWithId(self.memory, other_style, src_cell.style_id)) |id| {
+                        dst_cell.style_id = id;
+                    }
                 }
             }
         }
@@ -767,13 +772,7 @@ pub const Page = struct {
             for (cells) |*cell| {
                 if (cell.style_id == style.default_id) continue;
 
-                if (self.styles.lookupId(self.memory, cell.style_id)) |prev_style| {
+                self.styles.release(self.memory, cell.style_id);
-                    // Below upsert can't fail because it should already be present
-                    const md = self.styles.upsert(self.memory, prev_style.*) catch unreachable;
-                    assert(md.ref > 0);
-                    md.ref -= 1;
-                    if (md.ref == 0) self.styles.remove(self.memory, cell.style_id);
-                }
             }
 
             if (cells.len == self.size.cols) row.styled = false;
@@ -2112,7 +2111,7 @@ test "Page verifyIntegrity styles good" {
     defer page.deinit();
 
     // Upsert a style we'll use
-    const md = try page.styles.upsert(page.memory, .{ .flags = .{
+    const id = try page.styles.add(page.memory, .{ .flags = .{
         .bold = true,
     } });
 
@@ -2123,11 +2122,15 @@ test "Page verifyIntegrity styles good" {
         rac.cell.* = .{
             .content_tag = .codepoint,
             .content = .{ .codepoint = @intCast(x + 1) },
-            .style_id = md.id,
+            .style_id = id,
         };
-        md.ref += 1;
+        page.styles.use(page.memory, id);
     }
 
+    // The original style add would have incremented the
+    // ref count too, so release it to balance that out.
+    page.styles.release(page.memory, id);
+
     try page.verifyIntegrity(testing.allocator);
 }
 
@@ -2140,7 +2143,7 @@ test "Page verifyIntegrity styles ref count mismatch" {
     defer page.deinit();
 
     // Upsert a style we'll use
-    const md = try page.styles.upsert(page.memory, .{ .flags = .{
+    const id = try page.styles.add(page.memory, .{ .flags = .{
         .bold = true,
     } });
 
@@ -2151,13 +2154,17 @@ test "Page verifyIntegrity styles ref count mismatch" {
         rac.cell.* = .{
             .content_tag = .codepoint,
             .content = .{ .codepoint = @intCast(x + 1) },
-            .style_id = md.id,
+            .style_id = id,
         };
-        md.ref += 1;
+        page.styles.use(page.memory, id);
     }
 
+    // The original style add would have incremented the
+    // ref count too, so release it to balance that out.
+    page.styles.release(page.memory, id);
+
     // Miss a ref
-    md.ref -= 1;
+    page.styles.release(page.memory, id);
 
     try testing.expectError(
         Page.IntegrityError.MismatchedStyleRef,

src/terminal/ref_counted_set.zig

@@ -0,0 +1,573 @@
+const size = @import("size.zig");
+const Offset = size.Offset;
+const OffsetBuf = size.OffsetBuf;
+
+const fastmem = @import("../fastmem.zig");
+
+const std = @import("std");
+const assert = std.debug.assert;
+
+/// A reference counted set.
+///
+/// This set is created with some capacity in mind. You can determine
+/// the exact memory requirement of a given capacity by calling `layout`
+/// and checking the total size.
+///
+/// When the set exceeds capacity, `error.OutOfMemory` is returned from
+/// any memory-using methods. The caller is responsible for determining
+/// a path forward.
+///
+/// This set is reference counted. Each item in the set has an associated
+/// reference count. The caller is responsible for calling release for an
+/// item when it is no longer being used. Items with 0 references will be
+/// kept until another item is written to their bucket. This allows items
+/// to be ressurected if they are re-added before they get overwritten.
+///
+/// The backing data structure of this set is an open addressed hash table
+/// with linear probing and Robin Hood hashing, and a flat array of items.
+///
+/// The table maps values to item IDs, which are indices in the item array
+/// which contain the item's value and its reference count. Item IDs can be
+/// used to efficiently access an item and update its reference count after
+/// it has been added to the table, to avoid having to use the hash map to
+/// look the value back up.
+///
+/// ID 0 is reserved and will never be assigned.
+///
+/// Parameters:
+///
+/// `Context`
+///   A type containing methods to define behaviors.
+///   - `fn hash(*Context, T) u64`    - Return a hash for an item.
+///   - `fn eql(*Context, T, T) bool` - Check two items for equality.
+///
+///   - `fn deleted(*Context, T) void` - [OPTIONAL] Deletion callback.
+///     If present, called whenever an item is finally deleted.
+///     Useful if the item has memory that needs to be freed.
+///
+pub fn RefCountedSet(
+    comptime T: type,
+    comptime Id: type,
+    comptime RefCountInt: type,
+    comptime Context: type,
+) type {
+    return struct {
+        const Self = RefCountedSet(T, Id, RefCountInt, Context);
+
+        pub const base_align = @max(
+            @alignOf(Context),
+            @alignOf(Layout),
+            @alignOf(Item),
+            @alignOf(Id),
+        );
+
+        /// Set item
+        pub const Item = struct {
+            /// The value this item represents.
+            value: T = undefined,
+            /// Metadata for this item.
+            meta: Metadata = .{},
+
+            pub const Metadata = struct {
+                /// The bucket in the hash table where this item
+                /// is referenced.
+                bucket: Id = std.math.maxInt(Id),
+
+                /// The length of the probe sequence between this
+                /// item's starting bucket and the bucket it's in,
+                /// used for Robin Hood hashing.
+                psl: Id = 0,
+
+                /// The reference count for this item.
+                ref: RefCountInt = 0,
+            };
+        };
+
+        /// A hash table of item indices
+        table: Offset(Id),
+
+        /// By keeping track of the max probe sequence length
+        /// we can bail out early when looking up values that
+        /// aren't present.
+        max_psl: Id = 0,
+
+        /// We keep track of how many items have a PSL of any
+        /// given length, so that we can shrink max_psl when
+        /// we delete items.
+        ///
+        /// A probe sequence of length 32 or more is astronomically
+        /// unlikely. Roughly a (1/table_cap)^32 -- with any normal
+        /// table capacity that is so unlikely that it's not worth
+        /// handling.
+        psl_stats: [32]Id = [_]Id{0} ** 32,
+
+        /// The backing store of items
+        items: Offset(Item),
+
+        /// The next index to store an item at.
+        /// Id 0 is reserved for unused items.
+        next_id: Id = 1,
+
+        layout: Layout,
+
+        /// An instance of the context structure.
+        context: Context,
+
+        /// Returns the memory layout for the given base offset and
+        /// desired capacity. The layout can be used by the caller to
+        /// determine how much memory to allocate, and the layout must
+        /// be used to initialize the set so that the set knows all
+        /// the offsets for the various buffers.
+        ///
+        /// The capacity passed for cap will be used for the hash table,
+        /// which has a load factor of `0.8125` (13/16), so the number of
+        /// items which can actually be stored in the set will be smaller.
+        ///
+        /// The laid out capacity will be at least `cap`, but may be higher,
+        /// since it is rounded up to the next power of 2 for efficiency.
+        ///
+        /// The returned layout `cap` property will be 1 more than the number
+        /// of items that the set can actually store, since ID 0 is reserved.
+        pub fn layout(cap: Id) Layout {
+            // Experimentally, this load factor works quite well.
+            const load_factor = 0.8125;
+
+            const table_cap: Id = std.math.ceilPowerOfTwoAssert(Id, cap);
+            const table_mask: Id = (@as(Id, 1) << std.math.log2_int(Id, table_cap)) - 1;
+            const items_cap: Id = @intFromFloat(load_factor * @as(f64, @floatFromInt(table_cap)));
+
+            const table_start = 0;
+            const table_end = table_start + table_cap * @sizeOf(Id);
+
+            const items_start = std.mem.alignForward(usize, table_end, @alignOf(Item));
+            const items_end = items_start + items_cap * @sizeOf(Item);
+
+            const total_size = items_end;
+
+            return .{
+                .cap = items_cap,
+                .table_cap = table_cap,
+                .table_mask = table_mask,
+                .table_start = table_start,
+                .items_start = items_start,
+                .total_size = total_size,
+            };
+        }
+
+        pub const Layout = struct {
+            cap: Id,
+            table_cap: Id,
+            table_mask: Id,
+            table_start: usize,
+            items_start: usize,
+            total_size: usize,
+        };
+
+        pub fn init(base: OffsetBuf, l: Layout, context: Context) Self {
+            const table = base.member(Id, l.table_start);
+            const items = base.member(Item, l.items_start);
+
+            @memset(table.ptr(base)[0..l.table_cap], 0);
+            @memset(items.ptr(base)[0..l.cap], .{});
+
+            return .{
+                .table = table,
+                .items = items,
+                .layout = l,
+                .context = context,
+            };
+        }
+
+        /// Add an item to the set if not present
+        /// and increment its reference count.
+        ///
+        /// Returns the item's ID.
+        ///
+        /// If the set has no more room, then an
+        /// OutOfMemory error is returned instead.
+        pub fn add(self: *Self, base: anytype, value: T) error{OutOfMemory}!Id {
+            const items = self.items.ptr(base);
+
+            // Trim dead items from the end of the list.
+            while (self.next_id > 1 and items[self.next_id - 1].meta.ref == 0) {
+                self.next_id -= 1;
+
+                self.deleteItem(base, self.next_id);
+            }
+
+            // If we still don't have an available ID, we're out of memory.
+            if (self.next_id >= self.layout.cap) {
+                return error.OutOfMemory;
+            }
+
+            const id = self.upsert(base, value, self.next_id);
+            items[id].meta.ref += 1;
+
+            if (id == self.next_id) {
+                self.next_id += 1;
+            }
+
+            return id;
+        }
+
+        /// Add an item to the set if not present
+        /// and increment its reference count.
+        /// If possible, use the provided ID.
+        ///
+        /// Returns the item's ID, or null
+        /// if the provided ID was used.
+        ///
+        /// If the set has no more room, then an
+        /// OutOfMemory error is returned instead.
+        pub fn addWithId(self: *Self, base: anytype, value: T, id: Id) error{OutOfMemory}!?Id {
+            const items = self.items.ptr(base);
+
+            if (id < self.next_id) {
+                if (items[id].meta.ref == 0) {
+                    self.deleteItem(base, id);
+
+                    const added_id = self.upsert(base, value, id);
+
+                    items[added_id].meta.ref += 1;
+
+                    return if (added_id == id) null else added_id;
+                } else if (self.context.eql(value, items[id].value)) {
+                    items[id].meta.ref += 1;
+
+                    return null;
+                }
+            }
+
+            return try self.add(base, value);
+        }
+
+        /// Increment an item's reference count by 1.
+        ///
+        /// Asserts that the item's reference count is greater than 0.
+        pub fn use(self: *const Self, base: anytype, id: Id) void {
+            assert(id > 0);
+            assert(id < self.layout.cap);
+
+            const items = self.items.ptr(base);
+            const item = &items[id];
+
+            // If `use` is being called on an item with 0 references, then
+            // either someone forgot to call it before, released too early
+            // or lied about releasing. In any case something is wrong and
+            // shouldn't be allowed.
+            assert(item.meta.ref > 0);
+
+            item.meta.ref += 1;
+        }
+
+        /// Increment an item's reference count by a specified number.
+        ///
+        /// Asserts that the item's reference count is greater than 0.
+        pub fn useMultiple(self: *const Self, base: anytype, id: Id, n: RefCountInt) void {
+            assert(id > 0);
+            assert(id < self.layout.cap);
+
+            const items = self.items.ptr(base);
+            const item = &items[id];
+
+            // If `use` is being called on an item with 0 references, then
+            // either someone forgot to call it before, released too early
+            // or lied about releasing. In any case something is wrong and
+            // shouldn't be allowed.
+            assert(item.meta.ref > 0);
+
+            item.meta.ref += n;
+        }
+
+        /// Get an item by its ID without incrementing its reference count.
+        ///
+        /// Asserts that the item's reference count is greater than 0.
+        pub fn get(self: *const Self, base: anytype, id: Id) *T {
+            assert(id > 0);
+            assert(id < self.layout.cap);
+
+            const items = self.items.ptr(base);
+            const item = &items[id];
+
+            assert(item.meta.ref > 0);
+
+            return @ptrCast(&item.value);
+        }
+
+        /// Releases a reference to an item by its ID.
+        ///
+        /// Asserts that the item's reference count is greater than 0.
+        pub fn release(self: *Self, base: anytype, id: Id) void {
+            assert(id > 0);
+            assert(id < self.layout.cap);
+
+            const items = self.items.ptr(base);
+            const item = &items[id];
+
+            assert(item.meta.ref > 0);
+            item.meta.ref -= 1;
+        }
+
+        /// Release a specified number of references to an item by its ID.
+        ///
+        /// Asserts that the item's reference count is at least `n`.
+        pub fn releaseMultiple(self: *Self, base: anytype, id: Id, n: Id) void {
+            assert(id > 0);
+            assert(id < self.layout.cap);
+
+            const items = self.items.ptr(base);
+            const item = &items[id];
+
+            assert(item.meta.ref >= n);
+            item.meta.ref -= n;
+        }
+
+        /// Get the ref count for an item by its ID.
+        pub fn refCount(self: *const Self, base: anytype, id: Id) RefCountInt {
+            assert(id > 0);
+            assert(id < self.layout.cap);
+
+            const items = self.items.ptr(base);
+            const item = &items[id];
+            return item.meta.ref;
+        }
+
+        /// Get the current number of non-dead items in the set.
+        ///
+        /// NOT DESIGNED TO BE USED OUTSIDE OF TESTING, this is a very slow
+        /// operation, since it traverses the entire structure to count.
+        ///
+        /// Additionally, because this is a testing method, it does extra
+        /// work to verify the integrity of the structure when called.
+        pub fn count(self: *const Self, base: anytype) usize {
+            const table = self.table.ptr(base);
+            const items = self.items.ptr(base);
+
+            // The number of items accessible through the table.
+            var tb_ct: usize = 0;
+
+            for (table[0..self.layout.table_cap]) |id| {
+                if (id != 0) {
+                    const item = items[id];
+                    if (item.meta.ref > 0) {
+                        tb_ct += 1;
+                    }
+                }
+            }
+
+            // The number of items accessible through the backing store.
+            // The two counts should always match- it shouldn't be possible
+            // to have untracked items in the backing store.
+            var it_ct: usize = 0;
+
+            for (items[0..self.layout.cap]) |it| {
+                if (it.meta.ref > 0) {
+                    it_ct += 1;
+                }
+            }
+
+            assert(tb_ct == it_ct);
+
+            return tb_ct;
+        }
+
+        //================================================//
+        // The functions below are all internal functions //
+        // for performing operations on the hash table.   //
+        //================================================//
+
+        /// Delete an item, removing any references from
+        /// the table, and freeing its ID to be re-used.
+        fn deleteItem(self: *Self, base: anytype, id: Id) void {
+            const table = self.table.ptr(base);
+            const items = self.items.ptr(base);
+
+            const item = items[id];
+
+            if (item.meta.bucket > self.layout.table_cap) {
+                return;
+            }
+
+            if (table[item.meta.bucket] != id) {
+                return;
+            }
+
+            if (comptime @hasDecl(Context, "deleted")) {
+                // Inform the context struct that we're
+                // deleting the dead item's value for good.
+                self.context.deleted(item.value);
+            }
+
+            self.psl_stats[item.meta.psl] -= 1;
+            table[item.meta.bucket] = 0;
+            items[id] = .{};
+
+            var p: Id = item.meta.bucket;
+            var n: Id = (p + 1) & self.layout.table_mask;
+
+            while (table[n] != 0 and items[table[n]].meta.psl > 0) {
+                items[table[n]].meta.bucket = p;
+                self.psl_stats[items[table[n]].meta.psl] -= 1;
+                items[table[n]].meta.psl -= 1;
+                self.psl_stats[items[table[n]].meta.psl] += 1;
+                table[p] = table[n];
+                p = n;
+                n = (n + 1) & self.layout.table_mask;
+            }
+
+            while (self.max_psl > 0 and self.psl_stats[self.max_psl] == 0) {
+                self.max_psl -= 1;
+            }
+
+            table[p] = 0;
+        }
+
+        /// Find an item in the table and return its ID.
+        /// If the item does not exist in the table, null is returned.
+        fn lookup(self: *Self, base: anytype, value: T) ?Id {
+            const table = self.table.ptr(base);
+            const items = self.items.ptr(base);
+
+            const hash: u64 = self.context.hash(value);
+
+            for (0..self.max_psl + 1) |i| {
+                const p = (hash + i) & self.layout.table_mask;
+
+                const id = table[p];
+
+                // Empty bucket, our item cannot have probed to
+                // any point after this, meaning it's not present.
+                if (id == 0) {
+                    return null;
+                }
+
+                const item = items[id];
+
+                // An item with a shorter probe sequence length would never
+                // end up in the middle of another sequence, since it would
+                // be swapped out if inserted before the new sequence, and
+                // would not be swapped in if inserted afterwards.
+                //
+                // As such, our item cannot be present.
+                if (item.meta.psl < i) {
+                    return null;
+                }
+
+                // We don't bother checking dead items.
+                if (item.meta.ref == 0) {
+                    continue;
+                }
+
+                // If the item is a part of the same probe sequence,
+                // we check if it matches the value we're looking for.
+                if (item.meta.psl == i and
+                    self.context.eql(value, item.value))
+                {
+                    return id;
+                }
+            }
+
+            return null;
+        }
+
+        /// Find the provided value in the hash table, or add a new item
+        /// for it if not present. If a new item is added, `new_id` will
+        /// be used as the ID.
+        fn upsert(self: *Self, base: anytype, value: T, new_id: Id) Id {
+            // If the item already exists, return it.
+            if (self.lookup(base, value)) |id| {
+                return id;
+            }
+
+            const table = self.table.ptr(base);
+            const items = self.items.ptr(base);
+
+            // The new item that we'll put in to the table.
+            var new_item: Item = .{
+                .value = value,
+                .meta = .{
+                    .psl = 0,
+                    .ref = 0,
+                },
+            };
+
+            const hash: u64 = self.context.hash(value);
+
+            var held_id: Id = new_id;
+            var held_item: *Item = &new_item;
+
+            var chosen_p: ?Id = null;
+            var chosen_id: Id = new_id;
+
+            for (0..self.layout.table_cap - 1) |i| {
+                const p: Id = @intCast((hash + i) & self.layout.table_mask);
+                const id = table[p];
+
+                // Empty bucket, put our held item in to it and break.
+                if (id == 0) {
+                    table[p] = held_id;
+                    held_item.meta.bucket = p;
+                    self.psl_stats[held_item.meta.psl] += 1;
+                    self.max_psl = @max(self.max_psl, held_item.meta.psl);
+
+                    break;
+                }
+
+                const item = &items[id];
+
+                // If there's a dead item then we resurrect it
+                // for our value so that we can re-use its ID.
+                if (item.meta.ref == 0) {
+                    if (comptime @hasDecl(Context, "deleted")) {
+                        // Inform the context struct that we're
+                        // deleting the dead item's value for good.
+                        self.context.deleted(item.value);
+                    }
+
+                    chosen_id = id;
+
+                    held_item.meta.bucket = p;
+                    self.psl_stats[held_item.meta.psl] += 1;
+                    self.max_psl = @max(self.max_psl, held_item.meta.psl);
+
+                    // If we're not still holding our new item then we
+                    // need to make sure that we put the re-used ID in
+                    // the right place, where we previously put new_id.
+                    if (chosen_p) |c| {
+                        table[c] = id;
+                        table[p] = held_id;
+                    } else {
+                        // If we're still holding our new item then we
+                        // don't actually have to do anything, because
+                        // the table already has the correct ID here.
+                    }
+
+                    break;
+                }
+
+                // This item has a lower PSL, swap it out with our held item.
+                if (item.meta.psl < held_item.meta.psl) {
+                    if (held_id == new_id) {
+                        chosen_p = p;
+                        new_item.meta.bucket = p;
+                    }
+
+                    table[p] = held_id;
+                    items[held_id].meta.bucket = p;
+                    self.psl_stats[held_item.meta.psl] += 1;
+                    self.max_psl = @max(self.max_psl, held_item.meta.psl);
+
+                    held_id = id;
+                    held_item = item;
+                    self.psl_stats[item.meta.psl] -= 1;
+                }
+
+                // Advance to the next probe position for our held item.
+                held_item.meta.psl += 1;
+            }
+
+            items[chosen_id] = new_item;
+            return chosen_id;
+        }
+    };
+}

src/terminal/style.zig

@@ -6,8 +6,11 @@ const page = @import("page.zig");
 const size = @import("size.zig");
 const Offset = size.Offset;
 const OffsetBuf = size.OffsetBuf;
-const hash_map = @import("hash_map.zig");
+
-const AutoOffsetHashMap = hash_map.AutoOffsetHashMap;
+const Wyhash = std.hash.Wyhash;
+const autoHash = std.hash.autoHash;
+
+const RefCountedSet = @import("ref_counted_set.zig").RefCountedSet;
 
 /// The unique identifier for a style. This is at most the number of cells
 /// that can fit into a terminal page.
@@ -43,11 +46,34 @@ pub const Style = struct {
         none: void,
         palette: u8,
         rgb: color.RGB,
+
+        /// Formatting to make debug logs easier to read
+        /// by only including non-default attributes.
+        pub fn format(
+            self: Color,
+            comptime fmt: []const u8,
+            options: std.fmt.FormatOptions,
+            writer: anytype,
+        ) !void {
+            _ = fmt;
+            _ = options;
+            switch (self) {
+                .none => {
+                    _ = try writer.write("Color.none");
+                },
+                .palette => |p| {
+                    _ = try writer.print("Color.palette{{ {} }}", .{ p });
+                },
+                .rgb => |rgb| {
+                    _ = try writer.print("Color.rgb{{ {}, {}, {} }}", .{ rgb.r, rgb.g, rgb.b });
+                }
+            }
+        }
     };
 
     /// True if the style is the default style.
     pub fn default(self: Style) bool {
-        return std.meta.eql(self, .{});
+        return self.eql(.{});
     }
 
     /// True if the style is equal to another style.
@@ -133,6 +159,83 @@ pub const Style = struct {
         };
     }
 
+    /// Formatting to make debug logs easier to read
+    /// by only including non-default attributes.
+    pub fn format(
+        self: Style,
+        comptime fmt: []const u8,
+        options: std.fmt.FormatOptions,
+        writer: anytype,
+    ) !void {
+        _ = fmt;
+        _ = options;
+
+        const dflt: Style = .{};
+
+        _ = try writer.write("Style{ ");
+
+        var started = false;
+
+        inline for (std.meta.fields(Style)) |f| {
+            if (std.mem.eql(u8, f.name, "flags")) {
+                if (started) {
+                    _ = try writer.write(", ");
+                }
+
+                _ = try writer.write("flags={ ");
+
+                started = false;
+
+                inline for (std.meta.fields(@TypeOf(self.flags))) |ff| {
+                    const v = @as(ff.type, @field(self.flags, ff.name));
+                    const d = @as(ff.type, @field(dflt.flags, ff.name));
+                    if (ff.type == bool) {
+                        if (v) {
+                            if (started) {
+                                _ = try writer.write(", ");
+                            }
+                            _ = try writer.print("{s}", .{ff.name});
+                            started = true;
+                        }
+                    } else if (!std.meta.eql(v, d)) {
+                        if (started) {
+                            _ = try writer.write(", ");
+                        }
+                        _ = try writer.print(
+                            "{s}={any}",
+                            .{ ff.name, v },
+                        );
+                        started = true;
+                    }
+                }
+                _ = try writer.write(" }");
+
+                started = true;
+                comptime continue;
+            }
+            const value = @as(f.type, @field(self, f.name));
+            const d_val = @as(f.type, @field(dflt, f.name));
+            if (!std.meta.eql(value, d_val)) {
+                if (started) {
+                    _ = try writer.write(", ");
+                }
+                _ = try writer.print(
+                    "{s}={any}",
+                    .{ f.name, value },
+                );
+                started = true;
+            }
+        }
+
+        _ = try writer.write(" }");
+    }
+
+    pub fn hash(self: *const Style) u64 {
+        var hasher = Wyhash.init(0);
+        autoHash(&hasher, self.*);
+        return hasher.final();
+    }
+
     test {
         // The size of the struct so we can be aware of changes.
         const testing = std.testing;
@@ -140,170 +243,24 @@ pub const Style = struct {
     }
 };
 
-/// A set of styles.
+pub const StyleSetContext = struct {
-///
+    pub fn hash(self: *StyleSetContext, style: Style) u64 {
-/// This set is created with some capacity in mind. You can determine
+        _ = self;
-/// the exact memory requirement for a capacity by calling `layout`
+        return style.hash();
-/// and checking the total size.
-///
-/// When the set exceeds capacity, `error.OutOfMemory` is returned
-/// from memory-using methods. The caller is responsible for determining
-/// a path forward.
-///
-/// The general idea behind this structure is that it is optimized for
-/// the scenario common in terminals where there aren't many unique
-/// styles, and many cells are usually drawn with a single style before
-/// changing styles.
-///
-/// Callers should call `upsert` when a new style is set. This will
-/// return a stable pointer to metadata. You should use this metadata
-/// to keep a ref count of the style usage. When it falls to zero you
-/// can remove it.
-pub const Set = struct {
-    pub const base_align = @max(MetadataMap.base_align, IdMap.base_align);
-
-    /// The mapping of a style to associated metadata. This is
-    /// the map that contains the actual style definitions
-    /// (in the form of the key).
-    styles: MetadataMap,
-
-    /// The mapping from ID to style.
-    id_map: IdMap,
-
-    /// The next ID to use for a style that isn't in the set.
-    /// When this overflows we'll begin returning an IdOverflow
-    /// error and the caller must manually compact the style
-    /// set.
-    ///
-    /// Id zero is reserved and always is the default style. The
-    /// default style isn't present in the map, its dependent on
-    /// the terminal configuration.
-    next_id: Id = 1,
-
-    /// Maps a style definition to metadata about that style.
-    const MetadataMap = AutoOffsetHashMap(Style, Metadata);
-
-    /// Maps the unique style ID to the concrete style definition.
-    const IdMap = AutoOffsetHashMap(Id, Offset(Style));
-
-    /// Returns the memory layout for the given base offset and
-    /// desired capacity. The layout can be used by the caller to
-    /// determine how much memory to allocate, and the layout must
-    /// be used to initialize the set so that the set knows all
-    /// the offsets for the various buffers.
-    pub fn layout(cap: usize) Layout {
-        const md_layout = MetadataMap.layout(@intCast(cap));
-        const md_start = 0;
-        const md_end = md_start + md_layout.total_size;
-
-        const id_layout = IdMap.layout(@intCast(cap));
-        const id_start = std.mem.alignForward(usize, md_end, IdMap.base_align);
-        const id_end = id_start + id_layout.total_size;
-
-        const total_size = id_end;
-
-        return .{
-            .md_start = md_start,
-            .md_layout = md_layout,
-            .id_start = id_start,
-            .id_layout = id_layout,
-            .total_size = total_size,
-        };
     }
 
-    pub const Layout = struct {
+    pub fn eql(self: *StyleSetContext, a: Style, b: Style) bool {
-        md_start: usize,
+        _ = self;
-        md_layout: MetadataMap.Layout,
+        return a.eql(b);
-        id_start: usize,
-        id_layout: IdMap.Layout,
-        total_size: usize,
-    };
-
-    pub fn init(base: OffsetBuf, l: Layout) Set {
-        const styles_buf = base.add(l.md_start);
-        const id_buf = base.add(l.id_start);
-        return .{
-            .styles = MetadataMap.init(styles_buf, l.md_layout),
-            .id_map = IdMap.init(id_buf, l.id_layout),
-        };
-    }
-
-    /// Possible errors for upsert.
-    pub const UpsertError = error{
-        /// No more space in the backing buffer. Remove styles or
-        /// grow and reinitialize.
-        OutOfMemory,
-
-        /// No more available IDs. Perform a garbage collection
-        /// operation to compact ID space.
-        Overflow,
-    };
-
-    /// Upsert a style into the set and return a pointer to the metadata
-    /// for that style. The pointer is valid for the lifetime of the set
-    /// so long as the style is not removed.
-    ///
-    /// The ref count for new styles is initialized to zero and
-    /// for existing styles remains unmodified.
-    pub fn upsert(self: *Set, base: anytype, style: Style) UpsertError!*Metadata {
-        // If we already have the style in the map, this is fast.
-        var map = self.styles.map(base);
-        const gop = try map.getOrPut(style);
-        if (gop.found_existing) return gop.value_ptr;
-
-        // New style, we need to setup all the metadata. First thing,
-        // let's get the ID we'll assign, because if we're out of space
-        // we need to fail early.
-        errdefer map.removeByPtr(gop.key_ptr);
-        const id = self.next_id;
-        self.next_id = try std.math.add(Id, self.next_id, 1);
-        errdefer self.next_id -= 1;
-        gop.value_ptr.* = .{ .id = id };
-
-        // Setup our ID mapping
-        var id_map = self.id_map.map(base);
-        const id_gop = try id_map.getOrPut(id);
-        errdefer id_map.removeByPtr(id_gop.key_ptr);
-        assert(!id_gop.found_existing);
-        id_gop.value_ptr.* = size.getOffset(Style, base, gop.key_ptr);
-        return gop.value_ptr;
-    }
-
-    /// Lookup a style by its unique identifier.
-    pub fn lookupId(self: *const Set, base: anytype, id: Id) ?*Style {
-        const id_map = self.id_map.map(base);
-        const offset = id_map.get(id) orelse return null;
-        return @ptrCast(offset.ptr(base));
-    }
-
-    /// Remove a style by its id.
-    pub fn remove(self: *Set, base: anytype, id: Id) void {
-        // Lookup by ID, if it doesn't exist then we return. We use
-        // getEntry so that we can make removal faster later by using
-        // the entry's key pointer.
-        var id_map = self.id_map.map(base);
-        const id_entry = id_map.getEntry(id) orelse return;
-
-        var style_map = self.styles.map(base);
-        const style_ptr: *Style = @ptrCast(id_entry.value_ptr.ptr(base));
-
-        id_map.removeByPtr(id_entry.key_ptr);
-        style_map.removeByPtr(style_ptr);
-    }
-
-    /// Return the number of styles currently in the set.
-    pub fn count(self: *const Set, base: anytype) usize {
-        return self.id_map.map(base).count();
     }
 };
 
-/// Metadata about a style. This is used to track the reference count
+pub const Set = RefCountedSet(
-/// and the unique identifier for a style. The unique identifier is used
+    Style,
-/// to track the style in the full style map.
+    Id,
-pub const Metadata = struct {
+    size.CellCountInt,
-    ref: size.CellCountInt = 0,
+    StyleSetContext,
-    id: Id = 0,
+);
-};
 
 test "Set basic usage" {
     const testing = std.testing;
@@ -313,29 +270,49 @@ test "Set basic usage" {
     defer alloc.free(buf);
 
     const style: Style = .{ .flags = .{ .bold = true } };
+    const style2: Style = .{ .flags = .{ .italic = true } };
 
-    var set = Set.init(OffsetBuf.init(buf), layout);
+    var set = Set.init(OffsetBuf.init(buf), layout, StyleSetContext{});
 
-    // Upsert
+    // Add style
-    const meta = try set.upsert(buf, style);
+    const id = try set.add(buf, style);
-    try testing.expect(meta.id > 0);
+    try testing.expect(id > 0);
 
-    // Second upsert should return the same metadata.
+    // Second add should return the same metadata.
     {
-        const meta2 = try set.upsert(buf, style);
+        const id2 = try set.add(buf, style);
-        try testing.expectEqual(meta.id, meta2.id);
+        try testing.expectEqual(id, id2);
     }
 
     // Look it up
     {
-        const v = set.lookupId(buf, meta.id).?;
+        const v = set.get(buf, id);
         try testing.expect(v.flags.bold);
 
-        const v2 = set.lookupId(buf, meta.id).?;
+        const v2 = set.get(buf, id);
         try testing.expectEqual(v, v2);
     }
 
-    // Removal
+    // Add a second style
-    set.remove(buf, meta.id);
+    const id2 = try set.add(buf, style2);
-    try testing.expect(set.lookupId(buf, meta.id) == null);
+
+    // Look it up
+    {
+        const v = set.get(buf, id2);
+        try testing.expect(v.flags.italic);
+    }
+
+    // Ref count
+    try testing.expect(set.refCount(buf, id) == 2);
+    try testing.expect(set.refCount(buf, id2) == 1);
+
+    // Release
+    set.release(buf, id);
+    try testing.expect(set.refCount(buf, id) == 1);
+    set.release(buf, id2);
+    try testing.expect(set.refCount(buf, id2) == 0);
+
+    // We added the first one twice, so
+    set.release(buf, id);
+    try testing.expect(set.refCount(buf, id) == 0);
 }