From 600b905e016bc26f54b946261bdda7855af7f81d Mon Sep 17 00:00:00 2001
From: Ilia Mashkov <hikkikomori7@gmail.com>
Date: Wed, 3 Jun 2026 16:00:29 +0300
Subject: [PATCH] feat(Font): add windowSizeForLine crossfade-window policy

---
 src/entities/Font/domain/index.ts             |  1 +
 .../windowSizeForLine.test.ts                 | 38 +++++++++++++++++++
 .../windowSizeForLine/windowSizeForLine.ts    | 34 +++++++++++++++++
 src/entities/Font/index.ts                    |  1 +
 4 files changed, 74 insertions(+)
 create mode 100644 src/entities/Font/domain/windowSizeForLine/windowSizeForLine.test.ts
 create mode 100644 src/entities/Font/domain/windowSizeForLine/windowSizeForLine.ts

diff --git a/src/entities/Font/domain/index.ts b/src/entities/Font/domain/index.ts
index bffc09f..0f8a2ea 100644
--- a/src/entities/Font/domain/index.ts
+++ b/src/entities/Font/domain/index.ts
@@ -8,3 +8,4 @@ export {
     findSplitIndex,
     type LineRenderModel,
 } from './computeLineRenderModel/computeLineRenderModel';
+export { windowSizeForLine } from './windowSizeForLine/windowSizeForLine';
diff --git a/src/entities/Font/domain/windowSizeForLine/windowSizeForLine.test.ts b/src/entities/Font/domain/windowSizeForLine/windowSizeForLine.test.ts
new file mode 100644
index 0000000..392871b
--- /dev/null
+++ b/src/entities/Font/domain/windowSizeForLine/windowSizeForLine.test.ts
@@ -0,0 +1,38 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { windowSizeForLine } from './windowSizeForLine';
+
+describe('windowSizeForLine', () => {
+    it('returns 0 for an empty or non-positive line', () => {
+        expect(windowSizeForLine(0)).toBe(0);
+        expect(windowSizeForLine(-3)).toBe(0);
+    });
+
+    it('floors non-empty short lines at the minimum window of 1', () => {
+        expect(windowSizeForLine(1)).toBe(1);
+        expect(windowSizeForLine(2)).toBe(1);
+        expect(windowSizeForLine(3)).toBe(1);
+    });
+
+    it('scales with round(n / 3) in the mid range', () => {
+        expect(windowSizeForLine(6)).toBe(2);
+        expect(windowSizeForLine(12)).toBe(4);
+    });
+
+    it('caps at the maximum window of 5', () => {
+        expect(windowSizeForLine(15)).toBe(5);
+        expect(windowSizeForLine(16)).toBe(5);
+        expect(windowSizeForLine(100)).toBe(5);
+    });
+
+    it('rounds to nearest at fractional boundaries', () => {
+        // round(4/3)=1, round(5/3)=2, round(13/3)=4, round(14/3)=5
+        expect(windowSizeForLine(4)).toBe(1);
+        expect(windowSizeForLine(5)).toBe(2);
+        expect(windowSizeForLine(13)).toBe(4);
+        expect(windowSizeForLine(14)).toBe(5);
+    });
+});
diff --git a/src/entities/Font/domain/windowSizeForLine/windowSizeForLine.ts b/src/entities/Font/domain/windowSizeForLine/windowSizeForLine.ts
new file mode 100644
index 0000000..daa9f6d
--- /dev/null
+++ b/src/entities/Font/domain/windowSizeForLine/windowSizeForLine.ts
@@ -0,0 +1,34 @@
+/**
+ * Crossfade-window sizing policy for the dual-font slider.
+ *
+ * The slider renders a band of per-char `Character` cells that opacity-crossfade
+ * between the two fonts; everything outside the band is committed native bulk
+ * text. A fixed band looked wrong on short lines — a 6-grapheme line left almost
+ * no bulk, so nearly the whole line shimmered as per-char DOM. The band size
+ * therefore scales with the line's grapheme count and caps so long lines don't
+ * pay for an oversized per-char DOM band.
+ */
+
+/**
+ * Fraction of a line's graphemes that sit in the crossfade band.
+ */
+const WINDOW_RATIO = 1 / 3;
+/**
+ * Smallest band for a non-empty line — guarantees at least one crossfading char.
+ */
+const WINDOW_MIN = 1;
+/**
+ * Largest band regardless of line length — bounds per-char DOM cost.
+ */
+const WINDOW_MAX = 5;
+
+/**
+ * Crossfade window size, in graphemes, for a line of `n` graphemes.
+ * `clamp(round(n / 3), 1, 5)`; an empty/non-positive line gets no window.
+ */
+export function windowSizeForLine(n: number): number {
+    if (n <= 0) {
+        return 0;
+    }
+    return Math.min(WINDOW_MAX, Math.max(WINDOW_MIN, Math.round(n * WINDOW_RATIO)));
+}
diff --git a/src/entities/Font/index.ts b/src/entities/Font/index.ts
index b93f0fc..a59a859 100644
--- a/src/entities/Font/index.ts
+++ b/src/entities/Font/index.ts
@@ -2,6 +2,7 @@ export {
     computeLineRenderModel,
     DualFontLayout,
     findSplitIndex,
+    windowSizeForLine,
 } from './domain';
 export type {
     ComparisonLine,