1 files changed, 28 insertions, 0 deletions
diff --git a/src/Layer.zig b/src/Layer.zig
index a9827304..7a5fcf7e 100644
--- a/src/Layer.zig
+++ b/src/Layer.zig
@@ -199,3 +199,31 @@ test "a taller tag's rows are grid rows, each hit on its own line" {
     try std.testing.expect(layer.tagHit(120, 140, 10, 20, 6, 12) == null);
     try std.testing.expect(layer.bodyHit(120, 100, 10, 20, 6, 12) == null);
 }
+
+// Pointer hits on tag bands: comparing two, and bringing one that left
+// its band back onto it.
+
+/// Whether two pointer hits name the same cell of the same tag line.
+pub fn sameCell(a: ?TagHit, b: ?TagHit) 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: ?TagHit, 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.rows == 0 or layer.kind != kind or layer.id != id or layer.serial != point.serial or line >= layer.rows) 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 + line)) + 0.5) * bh;
+        break;
+    };
+    return if (p.reprojectTagHit(point)) |mapped| mapped.col else null;
+}