Speed group_texture_runs for two-texture batches.

engine/app.odin

@@ -504,19 +504,78 @@ group_texture_runs :: proc(list: []Queued_Sprite) {
 			end += 1
 		}
 
-		// Stable insertion sort is sufficient while MAX_SPRITES is 128.
-		for i in start + 1 ..< end {
-			item := list[i]
-			j := i
-			for j > start {
-				if uintptr(list[j - 1].texture) <= uintptr(item.texture) {
-					break
-				}
-				list[j] = list[j - 1]
-				j -= 1
-			}
-			list[j] = item
-		}
+		stable_group_by_texture(list[start:end])
 		start = end
 	}
 }
+
+// Stably orders window by texture pointer. Fast path for 1–2 textures
+// (common); insertion sort for 3+.
+stable_group_by_texture :: proc(window: []Queued_Sprite) {
+	n := len(window)
+	if n <= 1 do return
+
+	already := true
+	for i in 1 ..< n {
+		if uintptr(window[i].texture) < uintptr(window[i - 1].texture) {
+			already = false
+			break
+		}
+	}
+	if already do return
+
+	first := uintptr(window[0].texture)
+	second: uintptr
+	has_second := false
+	third := false
+	for i in 1 ..< n {
+		t := uintptr(window[i].texture)
+		if t == first do continue
+		if !has_second {
+			second = t
+			has_second = true
+			continue
+		}
+		if t != second {
+			third = true
+			break
+		}
+	}
+
+	if !has_second do return
+
+	if third {
+		for i in 1 ..< n {
+			item := window[i]
+			j := i
+			for j > 0 {
+				if uintptr(window[j - 1].texture) <= uintptr(item.texture) {
+					break
+				}
+				window[j] = window[j - 1]
+				j -= 1
+			}
+			window[j] = item
+		}
+		return
+	}
+
+	lo, hi := first, second
+	if lo > hi do lo, hi = hi, lo
+
+	tmp: [MAX_SPRITES]Queued_Sprite
+	w := 0
+	for q in window {
+		if uintptr(q.texture) == lo {
+			tmp[w] = q
+			w += 1
+		}
+	}
+	for q in window {
+		if uintptr(q.texture) == hi {
+			tmp[w] = q
+			w += 1
+		}
+	}
+	copy(window, tmp[:w])
+}

engine/batching_test.odin

@@ -190,6 +190,51 @@ group_texture_runs_already_optimal :: proc(t: ^testing.T) {
 	testing.expect_value(t, marker_of(list[3]), f32(40))
 }
 
+@(test)
+group_texture_runs_alternating_many_stable :: proc(t: ^testing.T) {
+	// Characterization: large alternating group must collapse to two runs
+	// while preserving same-texture submission order (markers).
+	N :: 64
+	list := make([]Queued_Sprite, N)
+	defer delete(list)
+	for i in 0 ..< N {
+		tex: uintptr = 2 if (i % 2) == 0 else 1
+		list[i] = queued(tex, 1, f32(i + 1))
+	}
+	testing.expect_value(t, texture_run_count(list), N)
+	group_texture_runs(list)
+	testing.expect_value(t, texture_run_count(list), 2)
+	testing.expect(t, list[0].texture == fake_tex(1))
+	testing.expect(t, list[N / 2 - 1].texture == fake_tex(1))
+	testing.expect(t, list[N / 2].texture == fake_tex(2))
+	testing.expect(t, list[N - 1].texture == fake_tex(2))
+	for i in 0 ..< N / 2 {
+		testing.expect_value(t, marker_of(list[i]), f32(2 * i + 2)) // odd markers: 2,4,...,N
+		testing.expect_value(t, marker_of(list[N / 2 + i]), f32(2 * i + 1)) // even markers: 1,3,...,N-1
+	}
+}
+
+@(test)
+group_texture_runs_three_textures_stable :: proc(t: ^testing.T) {
+	list := []Queued_Sprite {
+		queued(3, 1, 1),
+		queued(1, 1, 2),
+		queued(2, 1, 3),
+		queued(3, 1, 4),
+		queued(1, 1, 5),
+		queued(2, 1, 6),
+	}
+	testing.expect_value(t, texture_run_count(list), 6)
+	group_texture_runs(list)
+	testing.expect_value(t, texture_run_count(list), 3)
+	testing.expect_value(t, marker_of(list[0]), f32(2))
+	testing.expect_value(t, marker_of(list[1]), f32(5))
+	testing.expect_value(t, marker_of(list[2]), f32(3))
+	testing.expect_value(t, marker_of(list[3]), f32(6))
+	testing.expect_value(t, marker_of(list[4]), f32(1))
+	testing.expect_value(t, marker_of(list[5]), f32(4))
+}
+
 @(test)
 group_texture_runs_noncontiguous_same_group_id :: proc(t: ^testing.T) {
 	// Same nonzero id split by group 0: each window regroups alone.