frontend-svelte/src/widgets/ComparisonView/model/stores/comparisonStore.svelte.ts

/**
 * Font comparison store — TanStack Query refactor
 *
 * Manages the state for comparing two fonts character by character.
 * Persists font selection to localStorage and handles font loading
 * with the CSS Font Loading API to prevent Flash of Unstyled Text (FOUT).
 *
 * Features:
 * - Persistent font selection (survives page refresh)
 * - Font loading state tracking via FontsByIdsStore + TanStack Query
 * - Sample text management
 * - Typography controls (size, weight, line height, spacing)
 * - Slider position for character-by-character morphing
 */

import {
    type FontLoadRequestConfig,
    type UnifiedFont,
    fontCatalogStore,
    fontLifecycleManager,
    getFontUrl,
} from '$entities/Font';
import { typographySettingsStore } from '$features/AdjustTypography/model';
import { FontsByIdsStore } from '$features/FetchFontsByIds';
import { createPersistentStore } from '$shared/lib';
import { untrack } from 'svelte';
import { getPretextFontString } from '../../lib';

/**
 * Storage schema for comparison state
 */
interface ComparisonState {
    /**
     * Unique identifier for the primary font being compared (Side A)
     */
    fontAId: string | null;
    /**
     * Unique identifier for the secondary font being compared (Side B)
     */
    fontBId: string | null;
}

export type Side = 'A' | 'B';

const STORAGE_KEY = 'glyphdiff:comparison';

/**
 * Max time the UI waits after a font-load failure before unblocking
 * (#fontsReady = true). Acts as a safety net so a transient load error
 * can't strand the comparison view in a permanent loading state.
 */
const FONT_READY_FALLBACK_MS = 1000;

// Persistent storage for selected comparison fonts
const storage = createPersistentStore<ComparisonState>(STORAGE_KEY, {
    fontAId: null,
    fontBId: null,
});

/**
 * Store for managing font comparison state.
 *
 * Uses FontsByIdsStore (TanStack Query) to fetch fonts by ID, replacing
 * the previous hand-rolled async fetch approach. Three reactive effects
 * handle: (1) syncing batch results into fontA/fontB, (2) triggering the
 * CSS Font Loading API, and (3) falling back to default fonts when
 * storage is empty.
 */
export class ComparisonStore {
    /**
     * The primary font model for Side A (left/top)
     */
    #fontA = $state<UnifiedFont | undefined>();
    /**
     * The secondary font model for Side B (right/bottom)
     */
    #fontB = $state<UnifiedFont | undefined>();
    /**
     * The preview text string displayed in the comparison area
     */
    #sampleText = $state('The quick brown fox jumps over the lazy dog');
    /**
     * Flag indicating if both fonts are successfully loaded and ready for rendering
     */
    #fontsReady = $state(false);
    /**
     * Currently active side (A or B) for single-font adjustments
     */
    #side = $state<Side>('A');
    /**
     * Interactive slider position (0-100) used for morphing/layout transitions
     */
    #sliderPosition = $state(50);
    /**
     * TanStack Query-backed store for efficient batch font retrieval
     */
    #fontsByIdsStore: FontsByIdsStore;

    constructor() {
        // Synchronously seed the batch store with any IDs already in storage
        const { fontAId, fontBId } = storage.value;
        this.#fontsByIdsStore = new FontsByIdsStore(fontAId && fontBId ? [fontAId, fontBId] : []);

        $effect.root(() => {
            // Effect 1: Sync batch results → fontA / fontB
            $effect(() => {
                const fonts = this.#fontsByIdsStore.fonts;
                if (fonts.length === 0) {
                    return;
                }

                const { fontAId: aId, fontBId: bId } = storage.value;
                if (aId) {
                    const fa = fonts.find(f => f.id === aId);
                    if (fa) {
                        this.#fontA = fa;
                    }
                }
                if (bId) {
                    const fb = fonts.find(f => f.id === bId);
                    if (fb) {
                        this.#fontB = fb;
                    }
                }
            });

            // Effect 2: Trigger font loading whenever selection or weight changes
            $effect(() => {
                const fa = this.#fontA;
                const fb = this.#fontB;
                const weight = typographySettingsStore.weight;

                if (!fa || !fb) {
                    return;
                }

                const configs: FontLoadRequestConfig[] = [];
                [fa, fb].forEach(f => {
                    const url = getFontUrl(f, weight);
                    if (url) {
                        configs.push({
                            id: f.id,
                            name: f.name,
                            weight,
                            url,
                            isVariable: f.features?.isVariable,
                        });
                    }
                });

                if (configs.length > 0) {
                    fontLifecycleManager.touch(configs);
                    this.#checkFontsLoaded();
                }
            });

            // Effect 3: Set default fonts when storage is empty
            $effect(() => {
                if (this.#fontA && this.#fontB) {
                    return;
                }

                const fonts = fontCatalogStore.fonts;
                if (fonts.length >= 2) {
                    untrack(() => {
                        const id1 = fonts[0].id;
                        const id2 = fonts[fonts.length - 1].id;
                        storage.value = { fontAId: id1, fontBId: id2 };
                        this.#fontsByIdsStore.setIds([id1, id2]);
                    });
                }
            });

            // Effect 4: Pin fontA/fontB so eviction never removes on-screen fonts
            $effect(() => {
                const fa = this.#fontA;
                const fb = this.#fontB;
                const w = typographySettingsStore.weight;
                if (fa) {
                    fontLifecycleManager.pin(fa.id, w, fa.features?.isVariable);
                }
                if (fb) {
                    fontLifecycleManager.pin(fb.id, w, fb.features?.isVariable);
                }
                return () => {
                    if (fa) {
                        fontLifecycleManager.unpin(fa.id, w, fa.features?.isVariable);
                    }
                    if (fb) {
                        fontLifecycleManager.unpin(fb.id, w, fb.features?.isVariable);
                    }
                };
            });
        });
    }

    /**
     * Checks if fonts are actually loaded in the browser at current weight.
     *
     * Uses CSS Font Loading API to prevent FOUT. Waits for fonts to load
     * and forces a layout/paint cycle before marking as ready.
     */
    async #checkFontsLoaded() {
        if (!('fonts' in document)) {
            this.#fontsReady = true;
            return;
        }

        const weight = typographySettingsStore.weight;
        const size = typographySettingsStore.renderedSize;
        const fontAName = this.#fontA?.name;
        const fontBName = this.#fontB?.name;

        if (!fontAName || !fontBName) {
            return;
        }

        const fontAString = getPretextFontString(weight, size, fontAName);
        const fontBString = getPretextFontString(weight, size, fontBName);

        // Check if already loaded to avoid UI flash
        const isALoaded = document.fonts.check(fontAString);
        const isBLoaded = document.fonts.check(fontBString);

        if (isALoaded && isBLoaded) {
            this.#fontsReady = true;
            return;
        }

        this.#fontsReady = false;

        try {
            await Promise.all([
                document.fonts.load(fontAString),
                document.fonts.load(fontBString),
            ]);
            await document.fonts.ready;
            await new Promise(resolve => {
                requestAnimationFrame(() => {
                    requestAnimationFrame(resolve);
                });
            });
            this.#fontsReady = true;
        } catch (error) {
            console.warn('[ComparisonStore] Font loading failed:', error);
            setTimeout(() => (this.#fontsReady = true), FONT_READY_FALLBACK_MS);
        }
    }

    /**
     * Updates persistent storage with the current font selection.
     */
    private updateStorage() {
        storage.value = {
            fontAId: this.#fontA?.id ?? null,
            fontBId: this.#fontB?.id ?? null,
        };
    }

    /**
     * Primary font for comparison (reactive)
     */
    get fontA() {
        return this.#fontA;
    }

    set fontA(font: UnifiedFont | undefined) {
        this.#fontA = font;
        this.updateStorage();
    }

    /**
     * Secondary font for comparison (reactive)
     */
    get fontB() {
        return this.#fontB;
    }

    set fontB(font: UnifiedFont | undefined) {
        this.#fontB = font;
        this.updateStorage();
    }

    /**
     * Shared preview text string (reactive)
     */
    get text() {
        return this.#sampleText;
    }

    set text(value: string) {
        this.#sampleText = value;
    }

    /**
     * Side currently selected for focused manipulation (reactive)
     */
    get side() {
        return this.#side;
    }

    set side(value: Side) {
        this.#side = value;
    }

    /**
     * Morphing slider position (0-100) used by Character components (reactive)
     */
    get sliderPosition() {
        return this.#sliderPosition;
    }

    set sliderPosition(value: number) {
        this.#sliderPosition = value;
    }

    /**
     * True if both fonts are ready for side-by-side display (reactive)
     */
    get isReady() {
        return !!this.#fontA && !!this.#fontB && this.#fontsReady;
    }

    /**
     * True if any font is currently being fetched or loaded (reactive)
     */
    get isLoading() {
        return this.#fontsByIdsStore.isLoading || !this.#fontsReady;
    }

    /**
     * Resets all state, clears storage, and disables the batch query.
     */
    resetAll() {
        this.#fontA = undefined;
        this.#fontB = undefined;
        this.#fontsByIdsStore.setIds([]);
        storage.clear();
        typographySettingsStore.reset();
    }
}

/**
 * Singleton comparison store instance
 */
export const comparisonStore = new ComparisonStore();