#!/usr/bin/env python3
"""Wie weit daneben landet ein Browser-Seek in dieser MP3?

Ein Browser springt in einer VBR-Datei über die Xing-TOC: 100 Stützstellen,
linear interpoliert -> Byte-Position. Er meldet danach die ANGEFRAGTE Zeit,
dekodiert aber ab dem gefundenen Byte. Dieses Skript rechnet beides nach:
Byte-Position wie der Browser, echte Zeit per Frame-Zählung.
"""
import sys, struct

path = sys.argv[1]
targets = [float(x) for x in sys.argv[2].split(',')]
data = open(path, 'rb').read()

BITRATES = [0, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 0]   # MPEG1 Layer3
SRATES = [44100, 48000, 32000]

def frame_len(hdr):
    if hdr[0] != 0xFF or (hdr[1] & 0xE0) != 0xE0:
        return None, None, None
    ver = (hdr[1] >> 3) & 3          # 3 = MPEG1
    layer = (hdr[1] >> 1) & 3        # 1 = Layer III
    if ver != 3 or layer != 1:
        return None, None, None
    br = BITRATES[(hdr[2] >> 4) & 0xF] * 1000
    sr = SRATES[(hdr[2] >> 2) & 3]
    pad = (hdr[2] >> 1) & 1
    if br == 0 or sr == 0:
        return None, None, None
    return int(144 * br / sr) + pad, sr, 1152

# --- alle Frames einmal durchzählen: Byte -> exakte Zeit -------------------
pos, table = 0, []          # (byte_offset, start_time)
t = 0.0
while pos < len(data) - 4:
    ln, sr, spf = frame_len(data[pos:pos + 4])
    if not ln:
        pos += 1
        continue
    table.append((pos, t))
    t += spf / sr
    pos += ln
print(f'{len(table)} frames, echte Dauer {t:.1f} s, Dateigroesse {len(data)} bytes')

# --- Xing-TOC ---------------------------------------------------------------
i = data.find(b'Xing')
if i < 0:
    i = data.find(b'Info')
flags = struct.unpack('>I', data[i + 4:i + 8])[0]
p = i + 8
frames = struct.unpack('>I', data[p:p + 4])[0] if flags & 1 else None
if flags & 1: p += 4
bytes_total = struct.unpack('>I', data[p:p + 4])[0] if flags & 2 else len(data)
if flags & 2: p += 4
toc = list(data[p:p + 100]) if flags & 4 else None
print(f'Xing: frames={frames}, bytes={bytes_total}, TOC={"ja" if toc else "nein"}')
if not toc:
    sys.exit('kein TOC -> Browser extrapoliert rein linear')

def real_time_at(byte):
    lo, best = 0, table[0][1]
    for off, tt in table:
        if off <= byte: best = tt
        else: break
    return best

print(f'\n{"Ziel":>8} {"Byte(TOC)":>12} {"echte Zeit":>12} {"Fehler":>10}')
for target in targets:
    pct = target / t * 100.0
    a = int(pct)
    fa = toc[a]
    fb = toc[a + 1] if a < 99 else 256.0
    frac = pct - a
    byte = int((fa + (fb - fa) * frac) / 256.0 * bytes_total)
    real = real_time_at(byte)
    lin_byte = int(target / t * len(data))
    lin_real = real_time_at(lin_byte)
    print(f'{target:8.1f} {byte:12d} {real:12.1f} {real - target:+10.1f} s'
          f'   | linear: {lin_real:8.1f} {lin_real - target:+8.1f} s')