// Recent known-pilot observations. Fixed memory; no audio or content retained.
const CAPACITY = 1000;

export class PilotWindow {
    constructor(windowMs) {
        if (!Number.isFinite(windowMs) || windowMs <= 0 || windowMs > 100) {
            throw new RangeError('pilot observation duration');
        }
        this.windowMs = windowMs;
        this.history = new Uint8Array(CAPACITY);
        this.next = 0;
        this.count = 0;
        this.lastEndMs = null;
        this.interruptions = 0;
    }

    interrupt() {
        this.next = 0;
        this.count = 0;
        this.lastEndMs = null;
        this.interruptions = Math.min(Number.MAX_SAFE_INTEGER, this.interruptions + 1);
    }

    observe(present, endMs) {
        if (typeof present !== 'boolean' || !Number.isFinite(endMs) || endMs < this.windowMs) {
            this.interrupt();
            return;
        }
        // The detector emits equal-sized, contiguous sample windows. Missing
        // callbacks are unknown coverage, not evidence of missing pilot PCM.
        if (this.lastEndMs !== null &&
            Math.abs(endMs - this.lastEndMs - this.windowMs) > 0.001) {
            this.interrupt();
        }
        this.history[this.next] = present ? 1 : 0;
        this.next = (this.next + 1) % CAPACITY;
        this.count = Math.min(CAPACITY, this.count + 1);
        this.lastEndMs = endMs;
    }

    summary(nowMs) {
        let missing = 0, run = 0, longest = 0;
        const start = (this.next + CAPACITY - this.count) % CAPACITY;
        for (let n = 0; n < this.count; n++) {
            if (this.history[(start + n) % CAPACITY]) run = 0;
            else { missing++; run++; longest = Math.max(longest, run); }
        }
        const age = this.lastEndMs !== null && Number.isFinite(nowMs) && nowMs >= this.lastEndMs
            ? nowMs - this.lastEndMs : null;
        return {
            windowLimitMs: CAPACITY * this.windowMs,
            observedWindows: this.count,
            observedMs: this.count * this.windowMs,
            endMs: this.lastEndMs,
            observationAgeMs: age,
            coverageComplete: this.count === CAPACITY && age !== null && age <= 2 * this.windowMs,
            absentFraction: this.count ? missing / this.count : null,
            absenceMaxMs: this.count ? longest * this.windowMs : null,
            trailingAbsenceMs: this.count ? run * this.windowMs : null,
            interruptions: this.interruptions,
        };
    }
}