// BeatGrid — beat data management, calibration, and snap logic for LightSync
// Fetches beat data from /api/audio/beats and provides calibrated snap-to-beat.

export class BeatGrid {
    constructor() {
        this.beatTimes = [];        // raw beat times from librosa (never mutated)
        this.onsetTimes = [];       // raw onset times (empty — API does not provide onset data yet)
        this.tempoBpm = null;       // detected tempo in BPM
        this.calibrationOffset = 0.03;  // seconds, default 30ms latency compensation
        this.snapEnabled = true;
        this.loading = false;
        this.error = null;
    }

    /** Load beat data from backend API */
    async load(audioPath) {
        this.loading = true;
        this.error = null;
        this.beatTimes = [];
        this.onsetTimes = [];
        this.tempoBpm = null;
        try {
            const res = await fetch(`/api/audio/beats?path=${encodeURIComponent(audioPath)}`);
            if (!res.ok) throw new Error(`HTTP ${res.status}`);
            const data = await res.json();
            // API returns { beats: [...], tempo: float }
            // Support both field name variants for forward compatibility
            this.beatTimes = data.beats || data.beat_times || [];
            this.onsetTimes = data.onset_times || [];
            this.tempoBpm = data.tempo_bpm || data.tempo || null;
        } catch (e) {
            this.error = 'BEAT DETECTION FAILED';
            console.warn('[beats]', e);
        } finally {
            this.loading = false;
        }
    }

    /** Get calibrated beat times for display (offset subtracted) */
    getCalibratedBeats() {
        return this.beatTimes.map(t => t - this.calibrationOffset);
    }

    /** Get calibrated onset times for display (offset subtracted) */
    getCalibratedOnsets() {
        return this.onsetTimes.map(t => t - this.calibrationOffset);
    }

    /** Snap a time value to nearest calibrated beat within threshold */
    snap(t) {
        if (!this.snapEnabled || this.beatTimes.length === 0) return t;
        const SNAP_THRESHOLD = 0.1;  // 100ms
        const calibrated = this.getCalibratedBeats();
        let nearest = null;
        let minDist = Infinity;
        for (const beat of calibrated) {
            const d = Math.abs(t - beat);
            if (d < minDist) { minDist = d; nearest = beat; }
        }
        return (nearest !== null && minDist < SNAP_THRESHOLD) ? nearest : t;
    }

    /** Check if beats are loaded and available */
    get hasBeats() {
        return this.beatTimes.length > 0;
    }
}