scrolling in the screen (no scrollback yet)

src/terminal/Screen.zig

@@ -1,11 +1,14 @@
 const Screen = @This();
 
 const std = @import("std");
+const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
 const color = @import("color.zig");
 
-/// A line is a set of cells.
-pub const Line = []Cell;
+const log = std.log.scoped(.screen);
+
+/// A row is a set of cells.
+pub const Row = []Cell;
 
 /// Cell is a single cell within the screen.
 pub const Cell = struct {
@@ -23,45 +26,39 @@ pub const Cell = struct {
     }
 };
 
-pub const LineIterator = struct {
+pub const RowIterator = struct {
     screen: *const Screen,
     index: usize,
 
-    pub fn next(self: *LineIterator) ?Line {
-        if (self.index >= self.screen.lines) return null;
-
-        // Get the index of the first byte of the the line at index.
-        const idx = self.screen.lineIndex(self.index) * self.screen.cols;
-
-        // The storage is sliced to return exactly the number of columns.
-        const line = self.screen.storage[idx .. idx + self.screen.cols];
-
+    pub fn next(self: *RowIterator) ?Row {
+        if (self.index >= self.screen.rows) return null;
+        const res = self.screen.getRow(self.index);
         self.index += 1;
-        return line;
+        return res;
     }
 };
 
-/// The full list of lines, including any scrollback.
+/// The full list of rows, including any scrollback.
 storage: []Cell,
 
-/// The first visible line.
+/// The first visible row.
 zero: usize,
 
-/// The number of lines and columns in the visible space.
-lines: usize,
+/// The number of rows and columns in the visible space.
+rows: usize,
 cols: usize,
 
 /// Initialize a new screen.
-pub fn init(alloc: Allocator, lines: usize, cols: usize) !Screen {
-    // Allocate enough storage to cover every line and column in the visible
+pub fn init(alloc: Allocator, rows: usize, cols: usize) !Screen {
+    // Allocate enough storage to cover every row and column in the visible
     // area. This wastes some up front memory but saves allocations later.
-    const buf = try alloc.alloc(Cell, lines * cols);
+    const buf = try alloc.alloc(Cell, rows * cols);
     std.mem.set(Cell, buf, .{ .char = 0 });
 
     return Screen{
         .storage = buf,
         .zero = 0,
-        .lines = lines,
+        .rows = rows,
         .cols = cols,
     };
 }
@@ -71,32 +68,68 @@ pub fn deinit(self: *Screen, alloc: Allocator) void {
     self.* = undefined;
 }
 
-/// Returns an iterator that can be used to iterate over all of the lines
+/// Returns an iterator that can be used to iterate over all of the rows
 /// from index zero.
-pub fn lineIterator(self: *const Screen) LineIterator {
+pub fn rowIterator(self: *const Screen) RowIterator {
     return .{ .screen = self, .index = 0 };
 }
 
-/// Returns the index for the given line (0-indexed) into the underlying
+/// Get a single row by index (0-indexed).
+pub fn getRow(self: Screen, idx: usize) Row {
+    // Get the index of the first byte of the the row at index.
+    const real_idx = self.rowIndex(idx);
+
+    // The storage is sliced to return exactly the number of columns.
+    return self.storage[real_idx .. real_idx + self.cols];
+}
+
+/// Get a single cell in the visible area. row and col are 0-indexed.
+pub fn getCell(self: Screen, row: usize, col: usize) *Cell {
+    assert(row < self.rows);
+    assert(col < self.cols);
+    const row_idx = self.rowIndex(row);
+    return self.storage[row_idx + col];
+}
+
+/// Returns the index for the given row (0-indexed) into the underlying
 /// storage array.
-pub fn lineIndex(self: Screen, line: usize) usize {
-    const idx = self.zero + line;
-    if (idx < self.storage.len) return idx;
-    return idx - self.storage.len;
+pub fn rowIndex(self: Screen, idx: usize) usize {
+    assert(idx < self.rows);
+    const val = (self.zero + idx) * self.cols;
+    if (val < self.storage.len) return val;
+    return val - self.storage.len;
+}
+
+/// Scroll the screen up (negative) or down (positive). Scrolling direction
+/// is the direction opposite text would move. For example, scrolling down would
+/// move existing text upward. This sounds confusing but is the natural way
+/// that humans scroll a screen.
+pub fn scroll(self: *Screen, count: isize) void {
+    if (count < 0) {
+        self.zero -|= @intCast(usize, -count);
+    } else {
+        self.zero += @intCast(usize, count);
+    }
+    if (self.zero > self.storage.len) {
+        self.zero -= self.storage.len;
+    }
 }
 
 /// Turns the screen into a string.
 fn testString(self: Screen, alloc: Allocator) ![]const u8 {
-    const buf = try alloc.alloc(u8, self.storage.len + self.lines);
+    const buf = try alloc.alloc(u8, self.storage.len + self.rows);
     var i: usize = 0;
-    var lines = self.lineIterator();
-    for (lines.next()) |line, y| {
+    var y: usize = 0;
+    var rows = self.rowIterator();
+    while (rows.next()) |row| {
+        defer y += 1;
+
         if (y > 0) {
             buf[i] = '\n';
             i += 1;
         }
 
-        for (line) |cell| {
+        for (row) |cell| {
             // Turn NUL into space.
             const char = if (cell.char == 0) 0x20 else cell.char;
             i += try std.unicode.utf8Encode(@intCast(u21, char), buf[i..]);
@@ -106,16 +139,70 @@ fn testString(self: Screen, alloc: Allocator) ![]const u8 {
     return buf[0..i];
 }
 
+/// Writes a basic string into the screen for testing. Newlines (\n) separate
+/// each row.
+fn testWriteString(self: *Screen, text: []const u8) void {
+    var y: usize = 0;
+    var x: usize = 0;
+    var row = self.getRow(y);
+    for (text) |c| {
+        if (c == '\n') {
+            y += 1;
+            x = 0;
+            row = self.getRow(y);
+            continue;
+        }
+
+        assert(x < self.cols);
+        row[x].char = @intCast(u32, c);
+        x += 1;
+    }
+}
+
 test "Screen" {
     const testing = std.testing;
     const alloc = testing.allocator;
 
-    var s = try init(alloc, 5, 10);
+    var s = try init(alloc, 3, 5);
     defer s.deinit(alloc);
 
-    var i: usize = 0;
-    var iter = s.lineIterator();
-    while (iter.next() != null) i += 1;
+    // Sanity check that our test helpers work
+    const str = "1ABCD\n2EFGH\n3IJKL";
+    s.testWriteString(str);
+    var contents = try s.testString(alloc);
+    defer alloc.free(contents);
+    try testing.expectEqualStrings(str, contents);
 
-    try testing.expectEqual(@as(usize, 5), i);
+    // Test the row iterator
+    var count: usize = 0;
+    var iter = s.rowIterator();
+    while (iter.next()) |row| {
+        // Rows should be pointer equivalent to getRow
+        const row_other = s.getRow(count);
+        try testing.expectEqual(row.ptr, row_other.ptr);
+        count += 1;
+    }
+
+    // Should go through all rows
+    try testing.expectEqual(@as(usize, 3), count);
+}
+
+test "Screen: scrolling" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var s = try init(alloc, 3, 5);
+    defer s.deinit(alloc);
+    s.testWriteString("1ABCD\n2EFGH\n3IJKL");
+    s.scroll(1);
+
+    // Test our row index
+    try testing.expectEqual(@as(usize, 5), s.rowIndex(0));
+    try testing.expectEqual(@as(usize, 10), s.rowIndex(1));
+    try testing.expectEqual(@as(usize, 0), s.rowIndex(2));
+
+    // Test our contents rotated
+    var contents = try s.testString(alloc);
+    defer alloc.free(contents);
+    try testing.expectEqualStrings("2EFGH\n3IJKL\n1ABCD", contents);
 }