vgm_to_bytebeat

# vgm_to_bytebeat (see https://battleofthebits.org/arena/Entry/vgm+engine/39556/)

import sys

def rev_end(num, run=0, pos=0):
    if not num:
        return run
    return rev_end(num[1:], run | (num[0] << (pos * 8)), pos + 1)

def convert(b):
    d = b + 35
    if d >= 92:
        d += 1
    return d

if len(sys.argv) < 3:
    print('Usage: {} <infile> <outfile>'.format(sys.argv[0]))
    sys.exit(1)

infile = open(sys.argv[1], 'rb')
vgm_data = infile.read()
infile.close()

if vgm_data[0x00:0x04] != b'Vgm ':
    raise Exception('Not VGM file?')

vgm_eof = rev_end(vgm_data[0x04:0x08])
vgm_ver = rev_end(vgm_data[0x08:0x0a])
if vgm_ver >= 0x0150:
    vgm_data_offset = rev_end(vgm_data[0x34:0x38]) + 0x34
else:
    vgm_data_offset = 0x40

outfile = open(sys.argv[2], 'wb')

tone = [0, 0, 0, 0]
vol = [0xf, 0xf, 0xf, 0xf]

latch_ch = 0
latch_vol = False

index = vgm_data_offset
while index < vgm_eof:
    inc = 1
    cmd = vgm_data[index]

    if cmd == 0x00:
        print('0x00 - nop?')
    elif cmd == 0x50:
        data = vgm_data[index+1]
        print('0x50 - write 0x{:02x}'.format(data))

        if data & 0x80:
            latch_ch = (data & 0x60) >> 5
            latch_vol = bool(data & 0x10)
            print('  latch ch{} {} = 0x{:02x}'.format(latch_ch, 'vol' if latch_vol else 'noise' if latch_ch == 3 else 'tone', data & 0x0f))

            if latch_vol:
                vol[latch_ch] = data & 0x0f
            else:
                if latch_ch == 3:
                    tone[latch_ch] = data & 0x07
                else:
                    tone[latch_ch] = (tone[latch_ch] & 0xfff0) | (data & 0x000f)
        else:
            print('  data 0x{:02x}'.format(data & 0x3f))

            if latch_vol:
                vol[latch_ch] = data & 0x0f
            else:
                if latch_ch == 3:
                    tone[latch_ch] = data & 0x07
                else:
                    tone[latch_ch] = (tone[latch_ch] & 0x000f) | ((data & 0x003f) << 4)

        inc = 2
    elif 0x61 <= cmd <= 0x63:
        print('0x{:02x} - wait frame'.format(cmd))
        inc = 3 if cmd == 0x61 else 1
        times = 1
        if cmd == 0x61:
            samps = rev_end(vgm_data[index+1:index+3])
            frames = samps / 735
            times = round(frames)
            if frames != times:
                print('!! WARNING !! - non-NTSC frame wait ({} frames), timing will be off!'.format(frames))
        elif cmd == 0x63:
            print('!! WARNING !! - non-NTSC frame wait, timing will be off!')

        print('   ', tone, vol)

        tone_high = [(t & 0x3c0) >> 6 for t in tone[0:3]]
        tone_low = [(t & 0x03f) for t in tone[0:3]]
        tone_data = [x for y in zip(tone_high, tone_low) for x in y] + [tone[3]]
        outdata = tone_data + vol

        print('    {}'.format(outdata))
        outbytes = bytes(convert(b) for b in outdata)
        for _ in range(times):
            outfile.write(outbytes)
        print('    {} (x{})'.format(str(outbytes, 'ascii'), times))
    elif cmd == 0x66:
        print('0x66 - end of sound data')
        break
    else:
        print('!! WARNING !! - unknown command 0x{:02x} @ 0x{:08x}'.format(cmd, index))

    index += inc

outfile.close()