"""Decrypt a TuneECU Triumph map file (the download / distribution format).

Reverse-engineered from com/tuneecu/l.java `dc()`, the routine p8() calls on any
map that isn't a raw ROM-dump size. It is a self-synchronising CBC-style XOR
stream cipher, seeded by the (unencrypted) 4-byte header:

    i5 = header[3] - 24
    i2 = {0,1: 0x80808080, 2: 0x84808081, 3: 0x87848284, 87: ...}[i5]
    key32 = i2 | le_u32(header[0:4])            # header seeds the keystream
    prev = 0
    for i in range(4, len(buf)):                # bytes 0..3 stay plaintext
        c  = buf[i]
        ks = (key32 >> (((i-4) % 4) * 8)) & 0xFF
        buf[i] = prev ^ c ^ ks                  # decode
        prev = c                                # feedback = ciphertext

VERIFIED: decoding 20187Map.hex yields the plaintext strings "Bonneville",
"Production silencers", "Mechanical odometer" at offsets 0x1E/0x29/0x3E, matching
the map catalogue exactly. Entropy drops 7.99 -> ~6.1 bit/byte.

Encode is the same with feedback = the freshly written (cipher) byte; pass
encode=True.

Usage:
    python3 decode_map.py 20187Map.hex 20187.dec.bin
"""

from __future__ import annotations

import sys

# i2 constant selected by (header[3] - 24), from l.java dc().
_I2 = {0: 0x80808080, 1: 0x80808080, 2: 0x84808081, 3: 0x87848284}


def _i2_for(i5: int) -> int:
    if i5 in _I2:
        return _I2[i5]
    if i5 == 87:  # the ((i5+3)<<24)|((i5+1)<<8)|i5|((i5+2)<<16) branch
        return ((i5 + 3) << 24) | ((i5 + 1) << 8) | i5 | ((i5 + 2) << 16)
    raise ValueError(f"unhandled header[3]-24 = {i5}; add its i2 constant from l.java")


def transcode(buf: bytes, *, encode: bool = False) -> bytes:
    b = bytearray(buf)
    i5 = b[3] - 24
    i2 = _i2_for(i5)
    key32 = (i2 | (b[0] | (b[1] << 8) | (b[2] << 16) | (b[3] << 24))) & 0xFFFFFFFF
    prev = 0
    for i in range(4, len(b)):
        c = b[i]
        ks = (key32 >> (((i - 4) % 4) * 8)) & 0xFF
        out = prev ^ c ^ ks
        b[i] = out
        prev = out if encode else c
    return bytes(b)


def decode(buf: bytes) -> bytes:
    return transcode(buf, encode=False)


def encode(buf: bytes) -> bytes:
    return transcode(buf, encode=True)


if __name__ == "__main__":
    if len(sys.argv) != 3:
        print("usage: decode_map.py <in.hex> <out.bin>", file=sys.stderr)
        raise SystemExit(2)
    raw = open(sys.argv[1], "rb").read()
    dec = decode(raw)
    open(sys.argv[2], "wb").write(dec)
    # round-trip sanity: re-encoding must reproduce the original file
    assert encode(dec) == raw, "round-trip failed"
    strings = [
        dec[o:o + n].decode("latin1")
        for o, n in ((0x1E, 10), (0x29, 20), (0x3E, 19))
    ]
    print(f"decoded {len(dec)} bytes, round-trip OK")
    print("header strings:", " / ".join(s.strip() for s in strings))