import { useContext, useEffect, useRef, useState } from 'react'
import type { FrameEvent } from '../../core/frame.js'
import FrameStatsContext from '../components/FrameStatsContext.js'

/**
 * Frame rate statistics returned by `useFps`.
 */
export type FpsStats = {
	/** Current frames per second (rolling average over the sample window). */
	fps: number
	/** Last frame duration in milliseconds. */
	lastFrameMs: number
	/** Total frames counted since mount. */
	totalFrames: number
	/** Per-phase timing from the most recent frame (if available). */
	phases: FrameEvent['phases'] | undefined
}

const EMPTY_STATS: FpsStats = {
	fps: 0,
	lastFrameMs: 0,
	totalFrames: 0,
	phases: undefined,
}

/**
 * Hook that measures the actual terminal rendering frame rate.
 *
 * Subscribes to Ink's frame events and computes a rolling FPS average.
 * Returns `{ fps, lastFrameMs, totalFrames, phases }`.
 *
 * @param sampleWindowMs - Rolling window for FPS calculation (default 1000ms)
 */
export function useFps(sampleWindowMs = 1000): FpsStats {
	const ctx = useContext(FrameStatsContext)
	const [stats, setStats] = useState<FpsStats>(EMPTY_STATS)

	// Ring buffer of frame timestamps for rolling FPS
	const timestampsRef = useRef<number[]>([])
	const totalRef = useRef(0)

	useEffect(() => {
		if (!ctx) return

		return ctx.subscribe((event: FrameEvent) => {
			const now = performance.now()
			const timestamps = timestampsRef.current
			timestamps.push(now)

			// Trim timestamps outside the sample window
			const cutoff = now - sampleWindowMs
			while (timestamps.length > 0 && timestamps[0]! < cutoff) {
				timestamps.shift()
			}

			totalRef.current++

			// FPS = frames in window / window duration (in seconds)
			const fps =
				timestamps.length >= 2
					? ((timestamps.length - 1) / (now - timestamps[0]!)) * 1000
					: 0

			setStats({
				fps: Math.round(fps * 10) / 10,
				lastFrameMs: Math.round(event.durationMs * 100) / 100,
				totalFrames: totalRef.current,
				phases: event.phases,
			})
		})
	}, [ctx, sampleWindowMs])

	return stats
}