#!/usr/bin/env python3
"""实测音频 BPM 并找出首个强拍起点（librosa）。

用法: .venv-tts/bin/python tools/bpm_check.py <audio> <target_bpm>
输出 JSON: {"measured_bpm": ..., "octave_ok": bool, "first_beat_sec": ..., "n_beats": ...}
octave_ok: 实测值与目标偏差 <=2%（含半速/倍速的节拍八度等价）。
"""
import json
import sys

import librosa
import numpy as np

path, target = sys.argv[1], float(sys.argv[2])
y, sr = librosa.load(path, sr=22050, mono=True)
onset = librosa.onset.onset_strength(y=y, sr=sr)
tempo, beats = librosa.beat.beat_track(y=y, sr=sr, onset_envelope=onset, start_bpm=target)
tempo = float(np.atleast_1d(tempo)[0])

def dev(a, b):
    return abs(a - b) / b

ok = any(dev(tempo, target * k) <= 0.02 for k in (0.5, 1.0, 2.0))

beat_times = librosa.frames_to_time(beats, sr=sr)
first_beat = 0.0
if len(beat_times):
    # 第一个 onset 强度足够高的拍（跳过 intro 弱起）
    thresh = 0.4 * float(np.max(onset))
    for bf, bt in zip(beats, beat_times):
        if bf < len(onset) and onset[bf] >= thresh:
            first_beat = float(bt)
            break
    else:
        first_beat = float(beat_times[0])

print(json.dumps({
    "file": path,
    "measured_bpm": round(tempo, 2),
    "target": target,
    "deviation_pct": round(dev(tempo, target) * 100, 2),
    "octave_ok": ok,
    "first_beat_sec": round(first_beat, 3),
    "n_beats": int(len(beats)),
}))