frontend-svelte/src/shared/lib/helpers/CharacterComparisonEngine/CharacterComparisonEngine.svelte.ts

import {
    type PreparedTextWithSegments,
    layoutWithLines,
    prepareWithSegments,
} from '@chenglou/pretext';

/**
 * A single laid-out line produced by dual-font comparison layout.
 *
 * Line breaking is determined by the unified worst-case widths, so both fonts
 * always break at identical positions. Per-character `xA`/`xB` offsets reflect
 * each font's actual advance widths independently.
 */
export interface ComparisonLine {
    /**
     * Full text of this line as returned by pretext.
     */
    text: string;
    /**
     * Rendered width of this line in pixels — maximum across font A and font B.
     */
    width: number;
    /**
     * Individual character metadata for both fonts in this line
     */
    chars: Array<{
        /**
         * The grapheme cluster string (may be >1 code unit for emoji, etc.).
         */
        char: string;
        /**
         * X offset from the start of the line in font A, in pixels.
         */
        xA: number;
        /**
         * Advance width of this grapheme in font A, in pixels.
         */
        widthA: number;
        /**
         * X offset from the start of the line in font B, in pixels.
         */
        xB: number;
        /**
         * Advance width of this grapheme in font B, in pixels.
         */
        widthB: number;
    }>;
}

/**
 * Aggregated output of a dual-font layout pass.
 */
export interface ComparisonResult {
    /**
     * Per-line grapheme data for both fonts. Empty when input text is empty.
     */
    lines: ComparisonLine[];
    /**
     * Total height in pixels. Equals `lines.length * lineHeight` (pretext guarantee).
     */
    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.
 *
 * **`as any` casts:** `PreparedTextWithSegments` exposes only the `segments` field in
 * its public TypeScript type. The numeric arrays (`widths`, `breakableFitAdvances`,
 * `lineEndFitAdvances`, `lineEndPaintAdvances`) are internal implementation details of
 * pretext that are not part of the published type signature. The casts are required to
 * access these fields; they are verified against the pretext source at
 * `node_modules/@chenglou/pretext/src/layout.ts`.
 */
export class CharacterComparisonEngine {
    #segmenter: Intl.Segmenter;

    // Cached prepared data
    #preparedA: PreparedTextWithSegments | null = null;
    #preparedB: PreparedTextWithSegments | null = null;
    #unifiedPrepared: PreparedTextWithSegments | null = null;

    #lastText = '';
    #lastFontA = '';
    #lastFontB = '';
    #lastSpacing = 0;
    #lastSize = 0;

    // Cached layout results
    #lastWidth = -1;
    #lastLineHeight = -1;
    #lastResult = $state<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 = 16,
    ): 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 = prepareWithSegments(text, fontA);
            this.#preparedB = 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 };
        }

        // 2. Layout using the unified widths.
        // `PreparedTextWithSegments` only exposes `segments` in its public type; cast to `any`
        // so pretext's layoutWithLines can read the internal numeric arrays at runtime.
        const { lines, height } = layoutWithLines(this.#unifiedPrepared as any, width, lineHeight);

        // 3. Map results back to both fonts
        const resultLines: ComparisonLine[] = lines.map(line => {
            const chars: ComparisonLine['chars'] = [];
            let currentXA = 0;
            let currentXB = 0;

            const start = line.start;
            const end = line.end;

            // Cast to `any`: accessing internal numeric arrays not in the public type signature.
            const intA = this.#preparedA as any;
            const intB = this.#preparedB as any;

            for (let sIdx = start.segmentIndex; sIdx <= end.segmentIndex; sIdx++) {
                const segmentText = this.#preparedA!.segments[sIdx];
                if (segmentText === undefined) {
                    continue;
                }

                const graphemes = Array.from(this.#segmenter.segment(segmentText), s => s.segment);

                const advA = intA.breakableFitAdvances[sIdx];
                const advB = intB.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]! : intA.widths[sIdx]!;
                    let wB = advB != null ? advB[gIdx]! : intB.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;
    }

    /**
     * Calculates character states for an entire line in a single sequential pass.
     *
     * Walks characters left-to-right, accumulating the running x position using
     * each character's actual rendered width: `widthB` for already-morphed characters
     * (isPast=true) and `widthA` for upcoming ones. This ensures thresholds stay
     * aligned with the visual DOM layout even when the two fonts have different widths.
     *
     * @param line           A single laid-out line from the last layout result.
     * @param sliderPos      Current slider position as a percentage (0–100) of `containerWidth`.
     * @param containerWidth Total container width in pixels.
     * @returns              Per-character `proximity` and `isPast` in the same order as `line.chars`.
     */
    getLineCharStates(
        line: ComparisonLine,
        sliderPos: number,
        containerWidth: number,
    ): Array<{ proximity: number; isPast: boolean }> {
        if (!line) {
            return [];
        }
        const chars = line.chars;
        const n = chars.length;
        const sliderX = (sliderPos / 100) * containerWidth;
        const range = 5;
        // Prefix sums of widthA (left chars will be past → use widthA).
        // Suffix sums of widthB (right chars will not be past → use widthB).
        // This lets us compute, for each char i, what the total line width and
        // char center would be at the exact moment the slider crosses that char:
        //   left side (0..i-1) already past → font A widths
        //   right side (i+1..n-1) not yet past → font B widths
        const prefA = new Float64Array(n + 1);
        const sufB = new Float64Array(n + 1);
        for (let i = 0; i < n; i++) { prefA[i + 1] = prefA[i] + chars[i].widthA; }
        for (let i = n - 1; i >= 0; i--) { sufB[i] = sufB[i + 1] + chars[i].widthB; }
        // Per-char threshold: slider x at which this char should toggle isPast.
        const thresholds = new Float64Array(n);
        for (let i = 0; i < n; i++) {
            const totalWidth = prefA[i] + chars[i].widthA + sufB[i + 1];
            const xOffset = (containerWidth - totalWidth) / 2;
            thresholds[i] = xOffset + prefA[i] + chars[i].widthA / 2;
        }
        // Determine isPast for each char at the current slider position.
        const isPastArr = new Uint8Array(n);
        for (let i = 0; i < n; i++) { isPastArr[i] = sliderX > thresholds[i] ? 1 : 0; }
        // Compute visual positions based on actual rendered widths (font A if past, B if not).
        const totalRendered = chars.reduce((s, c, i) => s + (isPastArr[i] ? c.widthA : c.widthB), 0);
        const xOffset = (containerWidth - totalRendered) / 2;
        let currentX = xOffset;
        return chars.map((char, i) => {
            const isPast = isPastArr[i] === 1;
            const charWidth = isPast ? char.widthA : char.widthB;
            const visualCenter = currentX + charWidth / 2;
            const charGlobalPercent = (visualCenter / containerWidth) * 100;
            const distance = Math.abs(sliderPos - charGlobalPercent);
            const proximity = Math.max(0, 1 - distance / range);
            currentX += charWidth;
            return { proximity, isPast };
        });
    }

    /**
     * Internal helper to merge two prepared texts into a "worst-case" unified version
     */
    #createUnifiedPrepared(
        a: PreparedTextWithSegments,
        b: PreparedTextWithSegments,
        spacingPx: number = 0,
    ): PreparedTextWithSegments {
        // Cast to `any`: accessing internal numeric arrays not in the public type signature.
        const intA = a as any;
        const intB = b as any;

        const unified = { ...intA };

        unified.widths = intA.widths.map((w: number, i: number) => Math.max(w, intB.widths[i]) + spacingPx);
        unified.lineEndFitAdvances = intA.lineEndFitAdvances.map((w: number, i: number) =>
            Math.max(w, intB.lineEndFitAdvances[i]) + spacingPx
        );
        unified.lineEndPaintAdvances = intA.lineEndPaintAdvances.map((w: number, i: number) =>
            Math.max(w, intB.lineEndPaintAdvances[i]) + spacingPx
        );

        unified.breakableFitAdvances = intA.breakableFitAdvances.map((advA: number[] | null, i: number) => {
            const advB = intB.breakableFitAdvances[i];
            if (!advA && !advB) {
                return null;
            }
            if (!advA) {
                return advB.map((w: number) => w + spacingPx);
            }
            if (!advB) {
                return advA.map((w: number) => w + spacingPx);
            }

            return advA.map((w: number, j: number) => Math.max(w, advB[j]) + spacingPx);
        });

        return unified;
    }
}