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

@@ -1,3 +1,14 @@
+import {
+    type PreparedTextWithSegments,
+    layoutWithLines,
+    prepareWithSegments,
+} from '@chenglou/pretext';
+
+/**
+ * Default render size in px when callers omit the `size` arg on `layout()`.
+ */
+const DEFAULT_RENDER_SIZE_PX = 16;
+
 /**
  * Internal shape of pretext's PreparedTextWithSegments — only `segments` is in
  * pretext's public TS type; the numeric arrays exist at runtime but are not in
@@ -26,6 +37,15 @@ interface PretextInternals {
     lineEndPaintAdvances: number[];
 }
 
+/**
+ * Asserts pretext's runtime shape. The public TS type exposes only `segments`;
+ * the numeric arrays exist at runtime but are absent from the published signature.
+ * Centralizing the cast keeps the engine body free of `as any`.
+ */
+function asPretextInternals(prepared: PreparedTextWithSegments): PretextInternals {
+    return prepared as unknown as PretextInternals;
+}
+
 /**
  * Per-grapheme data computed during dual-font layout. Internal to the engine;
  * consumed by computeLineRenderModel to derive the per-frame render model.
@@ -85,3 +105,213 @@ export interface ComparisonResult {
      */
     totalHeight: number;
 }
+
+/**
+ * Dual-font text layout engine backed by `@chenglou/pretext`.
+ *
+ * Computes identical line breaks for two fonts simultaneously by constructing a
+ * "unified" prepared-text object whose per-glyph widths are the worst-case maximum
+ * of font A and font B. This guarantees that both fonts wrap at exactly the same
+ * positions, making side-by-side or slider comparison visually coherent.
+ *
+ * **Two-level caching strategy**
+ * 1. Font-change cache (`#preparedA`, `#preparedB`, `#unifiedPrepared`): rebuilt only
+ *    when `text`, `fontA`, or `fontB` changes. `prepareWithSegments` is expensive
+ *    (canvas measurement), so this avoids re-measuring during slider interaction.
+ * 2. Layout cache (`#lastResult`): rebuilt when `width` or `lineHeight` changes but
+ *    the fonts have not changed. Line-breaking is cheap relative to measurement, but
+ *    still worth skipping on every render tick.
+ *
+ * Per-frame slider state derivation lives in `computeLineRenderModel`, not on the
+ * class. This class is pure layout + caching; it holds no reactive state.
+ */
+export class DualFontLayout {
+    #segmenter: Intl.Segmenter;
+
+    // Cached prepared data
+    #preparedA: PretextInternals | null = null;
+    #preparedB: PretextInternals | null = null;
+    #unifiedPrepared: PretextInternals | null = null;
+
+    #lastText = '';
+    #lastFontA = '';
+    #lastFontB = '';
+    #lastSpacing = 0;
+    #lastSize = 0;
+
+    // Cached layout results
+    #lastWidth = -1;
+    #lastLineHeight = -1;
+    #lastResult: ComparisonResult | null = null;
+
+    constructor(locale?: string) {
+        this.#segmenter = new Intl.Segmenter(locale, { granularity: 'grapheme' });
+    }
+
+    /**
+     * Lay out `text` using both fonts within `width` pixels.
+     *
+     * Line breaks are determined by the worst-case (maximum) glyph widths across
+     * both fonts, so both fonts always wrap at identical positions.
+     *
+     * @param text       Raw text to lay out.
+     * @param fontA      CSS font string for the first font: `"weight sizepx \"family\""`.
+     * @param fontB      CSS font string for the second font: `"weight sizepx \"family\""`.
+     * @param width      Available line width in pixels.
+     * @param lineHeight Line height in pixels (passed directly to pretext).
+     * @param spacing    Letter spacing in em (from typography settings).
+     * @param size       Current font size in pixels (used to convert spacing em to px).
+     * @returns          Per-line grapheme data for both fonts. Empty `lines` when `text` is empty.
+     */
+    layout(
+        text: string,
+        fontA: string,
+        fontB: string,
+        width: number,
+        lineHeight: number,
+        spacing: number = 0,
+        size: number = DEFAULT_RENDER_SIZE_PX,
+    ): ComparisonResult {
+        if (!text) {
+            return { lines: [], totalHeight: 0 };
+        }
+
+        const spacingPx = spacing * size;
+
+        const isFontChange = text !== this.#lastText
+            || fontA !== this.#lastFontA
+            || fontB !== this.#lastFontB
+            || spacing !== this.#lastSpacing
+            || size !== this.#lastSize;
+
+        const isLayoutChange = width !== this.#lastWidth || lineHeight !== this.#lastLineHeight;
+
+        if (!isFontChange && !isLayoutChange && this.#lastResult) {
+            return this.#lastResult;
+        }
+
+        // 1. Prepare (or use cache)
+        if (isFontChange) {
+            this.#preparedA = asPretextInternals(prepareWithSegments(text, fontA));
+            this.#preparedB = asPretextInternals(prepareWithSegments(text, fontB));
+            this.#unifiedPrepared = this.#createUnifiedPrepared(this.#preparedA, this.#preparedB, spacingPx);
+
+            this.#lastText = text;
+            this.#lastFontA = fontA;
+            this.#lastFontB = fontB;
+            this.#lastSpacing = spacing;
+            this.#lastSize = size;
+        }
+
+        if (!this.#unifiedPrepared || !this.#preparedA || !this.#preparedB) {
+            return { lines: [], totalHeight: 0 };
+        }
+
+        // pretext's `layoutWithLines` is typed against its public surface; pass the
+        // runtime-internal shape through with one boundary cast.
+        const { lines, height } = layoutWithLines(
+            this.#unifiedPrepared as unknown as PreparedTextWithSegments,
+            width,
+            lineHeight,
+        );
+
+        // 3. Map results back to both fonts
+        const preparedA = this.#preparedA;
+        const preparedB = this.#preparedB;
+        const resultLines: ComparisonLine[] = lines.map(line => {
+            const chars: ComparisonChar[] = [];
+            let currentXA = 0;
+            let currentXB = 0;
+
+            const start = line.start;
+            const end = line.end;
+
+            for (let sIdx = start.segmentIndex; sIdx <= end.segmentIndex; sIdx++) {
+                const segmentText = preparedA.segments[sIdx];
+                if (segmentText === undefined) {
+                    continue;
+                }
+
+                const graphemes = Array.from(this.#segmenter.segment(segmentText), s => s.segment);
+
+                const advA = preparedA.breakableFitAdvances[sIdx];
+                const advB = preparedB.breakableFitAdvances[sIdx];
+
+                const gStart = sIdx === start.segmentIndex ? start.graphemeIndex : 0;
+                const gEnd = sIdx === end.segmentIndex ? end.graphemeIndex : graphemes.length;
+
+                for (let gIdx = gStart; gIdx < gEnd; gIdx++) {
+                    const char = graphemes[gIdx];
+                    let wA = advA != null ? advA[gIdx]! : preparedA.widths[sIdx]!;
+                    let wB = advB != null ? advB[gIdx]! : preparedB.widths[sIdx]!;
+
+                    // Apply letter spacing (tracking) to the width of each character
+                    wA += spacingPx;
+                    wB += spacingPx;
+
+                    chars.push({
+                        char,
+                        xA: currentXA,
+                        widthA: wA,
+                        xB: currentXB,
+                        widthB: wB,
+                    });
+                    currentXA += wA;
+                    currentXB += wB;
+                }
+            }
+
+            return {
+                text: line.text,
+                width: line.width,
+                chars,
+            };
+        });
+
+        this.#lastWidth = width;
+        this.#lastLineHeight = lineHeight;
+        this.#lastResult = {
+            lines: resultLines,
+            totalHeight: height,
+        };
+
+        return this.#lastResult;
+    }
+
+    /**
+     * Merge two prepared texts into a worst-case unified version so both fonts
+     * wrap at identical positions. Per-segment widths are the elementwise max
+     * across both fonts, with `spacingPx` added to model letter-spacing.
+     */
+    #createUnifiedPrepared(
+        a: PretextInternals,
+        b: PretextInternals,
+        spacingPx: number = 0,
+    ): PretextInternals {
+        const unified: PretextInternals = { ...a };
+
+        unified.widths = a.widths.map((w, i) => Math.max(w, b.widths[i]) + spacingPx);
+        unified.lineEndFitAdvances = a.lineEndFitAdvances.map((w, i) =>
+            Math.max(w, b.lineEndFitAdvances[i]) + spacingPx
+        );
+        unified.lineEndPaintAdvances = a.lineEndPaintAdvances.map((w, i) =>
+            Math.max(w, b.lineEndPaintAdvances[i]) + spacingPx
+        );
+
+        unified.breakableFitAdvances = a.breakableFitAdvances.map((advA, i) => {
+            const advB = b.breakableFitAdvances[i];
+            if (!advA && !advB) {
+                return null;
+            }
+            if (!advA) {
+                return advB!.map(w => w + spacingPx);
+            }
+            if (!advB) {
+                return advA.map(w => w + spacingPx);
+            }
+            return advA.map((w, j) => Math.max(w, advB[j]) + spacingPx);
+        });
+
+        return unified;
+    }
+}