"""TP5 .td5 block header checksum - solved.

Block layout (first 0x13 bytes):
  +0x00  u8   0x43 'C'      block type tag
  +0x01  u16  CRC-16/ARC over block[3:blockLen]   <-- the "mystery field" low half
  +0x03  u32  rand()  (MSVCRT, 0..0x7FFF; srand(time(NULL)) once per export)
  +0x07  u32  blockLen
  +0x0b  u8   1
  +0x0c  u8   1
  +0x0d  u16  1
  +0x0f  u8   0x84
  +0x10  u16  0x0116  (offset of bitmap data = header size)
The 4 bytes read as one u32-LE at +1 are therefore CRC | (rand<<16).
"""
import struct
from parse import records

def crc16_arc(data, init=0, poly=0xA001):
    """TP5 sub_409150 with polyIndex=1 (table @0x447020 = [8480,A001,8621,E950])."""
    c = init
    for b in data:
        c ^= b
        for _ in range(8):
            c = (c >> 1) ^ poly if c & 1 else c >> 1
    return c & 0xFFFF

def td5_block_checksum(block):
    """block: the full block bytes (length == u32 at block[7:11]). Returns u16 for block[1:3]."""
    blen = struct.unpack_from('<I', block, 7)[0]
    return crc16_arc(block[3:blen])

def build_header(blen, rand_val, bitmap_off=0x0116):
    """Return the first 0x13 header bytes with a placeholder CRC (fill after body is built)."""
    return bytes([0x43, 0, 0]) + struct.pack('<I', rand_val) + struct.pack('<I', blen) \
         + bytes([1, 1]) + struct.pack('<H', 1) + bytes([0x84]) + struct.pack('<H', bitmap_off)

if __name__ == '__main__':
    rs = records()
    ok = 0
    print("%-18s %6s  %-9s %-9s %-6s %-6s %s" %
          ("record", "blen", "field(exp)", "field(calc)", "crcExp", "crcCalc", "rand"))
    for r in rs:
        b = r['blk']
        exp_crc = struct.unpack_from('<H', b, 1)[0]
        rnd = struct.unpack_from('<I', b, 3)[0]
        got = td5_block_checksum(b)
        field_exp = r['field']
        field_calc = (got | (rnd << 16)) & 0xFFFFFFFF
        good = (got == exp_crc)
        ok += good
        print("%-18s %6d  %08x  %08x  %04x   %04x   %04x %s" %
              (r['name'], r['blklen'], field_exp, field_calc, exp_crc, got, rnd,
               "OK" if good else "**MISMATCH**"))
    print("\n%d/%d records match" % (ok, len(rs)))
    print("all rand() values <= 0x7FFF (RAND_MAX):",
          all(struct.unpack_from('<I', r['blk'], 3)[0] <= 0x7FFF for r in rs))