test(font): computeLineRenderModel handles empty line

src/entities/Font/lib/dualFontLayout/computeLineRenderModel/computeLineRenderModel.test.ts

@@ -1,5 +1,45 @@
-import { describe } from 'vitest';
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import type { ComparisonLine } from '../DualFontLayout/DualFontLayout';
+import { computeLineRenderModel } from './computeLineRenderModel';
+
+/**
+ * Build a ComparisonLine fixture with given per-char widths. xA/xB are
+ * cumulative prefix sums of widthA/widthB respectively.
+ */
+function makeLine(
+    chars: { char: string; widthA: number; widthB: number }[],
+): ComparisonLine {
+    let xA = 0;
+    let xB = 0;
+    const out: ComparisonLine = {
+        text: chars.map(c => c.char).join(''),
+        width: chars.reduce((s, c) => s + Math.max(c.widthA, c.widthB), 0),
+        chars: chars.map(c => {
+            const entry = {
+                char: c.char,
+                xA,
+                xB,
+                widthA: c.widthA,
+                widthB: c.widthB,
+            };
+            xA += c.widthA;
+            xB += c.widthB;
+            return entry;
+        }),
+    };
+    return out;
+}
 
 describe('computeLineRenderModel', () => {
-    // tests added in subsequent tasks
+    it('returns empty model for an empty line', () => {
+        const line = makeLine([]);
+        const model = computeLineRenderModel(line, 50, 500, 5);
+        expect(model.leftText).toBe('');
+        expect(model.windowChars).toEqual([]);
+        expect(model.rightText).toBe('');
+    });
 });

src/entities/Font/lib/dualFontLayout/computeLineRenderModel/computeLineRenderModel.ts

@@ -41,5 +41,8 @@ export function computeLineRenderModel(
     containerWidth: number,
     windowSize: number,
 ): LineRenderModel {
+    if (line.chars.length === 0) {
+        return { leftText: '', windowChars: [], rightText: '' };
+    }
     throw new Error('not implemented');
 }