terminal: bitmapallocator handles perfectly divisble chunk size

src/terminal/bitmap_allocator.zig

@@ -203,11 +203,12 @@ fn findFreeChunks(bitmaps: []u64, n: usize) ?usize {
             // then add one and continue, we're still accumulating blanks.
             if (seq != div) {
                 seq += 1;
-                continue;
+                if (seq != div or mod > 0) continue;
             }
 
             // We've reached the seq count see if this has mod starting empty
             // blanks.
+            if (mod > 0) {
                 const final = @as(u64, std.math.maxInt(u64)) >> @intCast(64 - mod);
                 if (bitmap.* & final == 0) {
                     // No blanks, reset.
@@ -215,10 +216,17 @@ fn findFreeChunks(bitmaps: []u64, n: usize) ?usize {
                     continue;
                 }
 
-            // Found! Set all in our sequence to full and mask our final.
-            const start_idx = idx - seq;
-            for (start_idx..idx) |i| bitmaps[i] = 0;
                 bitmap.* ^= final;
+            }
+
+            // Found! Set all in our sequence to full and mask our final.
+            // The "zero_mod" modifier below handles the case where we have
+            // a perfectly divisible number of chunks so we don't have to
+            // mark the trailing bitmap.
+            const zero_mod = @intFromBool(mod == 0);
+            const start_idx = idx - (seq - zero_mod);
+            const end_idx = idx + zero_mod;
+            for (start_idx..end_idx) |i| bitmaps[i] = 0;
 
             return (start_idx * 64);
         }
@@ -344,6 +352,25 @@ test "findFreeChunks larger than 64 chunks not at beginning" {
     try testing.expectEqual(@as(usize, 64), idx);
 }
 
+test "findFreeChunks larger than 64 chunks exact" {
+    const testing = std.testing;
+
+    var bitmaps = [_]u64{
+        0b11111111_11111111_11111111_11111111_11111111_11111111_11111111_11111111,
+        0b11111111_11111111_11111111_11111111_11111111_11111111_11111111_11111111,
+    };
+    const idx = findFreeChunks(&bitmaps, 128).?;
+    try testing.expectEqual(
+        0b00000000_00000000_00000000_00000000_00000000_00000000_00000000_00000000,
+        bitmaps[0],
+    );
+    try testing.expectEqual(
+        0b00000000_00000000_00000000_00000000_00000000_00000000_00000000_00000000,
+        bitmaps[1],
+    );
+    try testing.expectEqual(@as(usize, 0), idx);
+}
+
 test "BitmapAllocator layout" {
     const Alloc = BitmapAllocator(4);
     const cap = 64 * 4;