1 files changed, 25 insertions, 0 deletions
diff --git a/src/tag_layer.zig b/src/tag_layer.zig
index ac7c4ac2..7cfe5bbf 100644
--- a/src/tag_layer.zig
+++ b/src/tag_layer.zig
@@ -78,3 +78,28 @@ test "tag text capacity uses its own pitch and excludes the physical grip" {
     try std.testing.expect(layer.hitAt(120, 85, std.math.nan(f32), 20, 6, 12) == null);
     try std.testing.expect(layer.hitAt(120, 85, 10, 20, 0, 12) == null);
 }
+
+/// Whether two pointer hits name the same cell of the same tag line.
+pub fn sameCell(a: ?Hit, b: ?Hit) bool {
+    const first = a orelse return b == null;
+    const second = b orelse return false;
+    return first.kind == second.kind and first.id == second.id and first.serial == second.serial and first.line == second.line and first.col == second.col;
+}
+
+/// The tag column a host's pointer is at. Clamped, a point off the tag is
+/// brought back onto line `line` of it, for a drag that leaves it.
+pub fn columnAt(p: *const pardes.Pardes, kind: Kind, id: usize, supplied: ?Hit, clamp: bool, line: u16) ?u16 {
+    var point = supplied orelse return null;
+    if (point.kind != kind or point.id != id) return null;
+    if (clamp) for (p.surface.tagLayers()) |*layer| {
+        if (layer.cols == 0 or layer.kind != kind or layer.id != id or layer.serial != point.serial or layer.line != line) continue;
+        const bw: f32 = @floatFromInt(point.metrics.body_w);
+        const bh: f32 = @floatFromInt(point.metrics.body_h);
+        const left = @as(f32, @floatFromInt(layer.viewport.x)) * bw;
+        const right = @as(f32, @floatFromInt(layer.viewport.x + layer.viewport.w)) * bw;
+        point.pixel_x = std.math.clamp(point.pixel_x, left, @max(left, right - 0.001));
+        point.pixel_y = (@as(f32, @floatFromInt(layer.viewport.y)) + 0.5) * bh;
+        break;
+    };
+    return if (p.reprojectTagHit(point)) |mapped| mapped.col else null;
+}