"""Pure-Python Keccak-256 (the Ethereum variant, pad 0x01), only for computing 4-byte function selectors.
Checked against the known selector of transfer(address,uint256) = a9059cbb in the self-test below."""
RC = [0x0000000000000001, 0x0000000000008082, 0x800000000000808A, 0x8000000080008000, 0x000000000000808B,
      0x0000000080000001, 0x8000000080008081, 0x8000000000008009, 0x000000000000008A, 0x0000000000000088,
      0x0000000080008009, 0x000000008000000A, 0x000000008000808B, 0x800000000000008B, 0x8000000000008089,
      0x8000000000008003, 0x8000000000008002, 0x8000000000000080, 0x000000000000800A, 0x800000008000000A,
      0x8000000080008081, 0x8000000000008080, 0x0000000080000001, 0x8000000080008008]
ROT = [[0, 36, 3, 41, 18], [1, 44, 10, 45, 2], [62, 6, 43, 15, 61], [28, 55, 25, 21, 56], [27, 20, 39, 8, 14]]
M = (1 << 64) - 1
rol = lambda x, n: ((x << n) | (x >> (64 - n))) & M if n else x

def _f(A):
    for rc in RC:
        C = [A[x][0] ^ A[x][1] ^ A[x][2] ^ A[x][3] ^ A[x][4] for x in range(5)]
        D = [C[(x - 1) % 5] ^ rol(C[(x + 1) % 5], 1) for x in range(5)]
        A = [[A[x][y] ^ D[x] for y in range(5)] for x in range(5)]
        B = [[0] * 5 for _ in range(5)]
        for x in range(5):
            for y in range(5):
                B[y][(2 * x + 3 * y) % 5] = rol(A[x][y], ROT[x][y])
        A = [[B[x][y] ^ ((~B[(x + 1) % 5][y]) & B[(x + 2) % 5][y]) for y in range(5)] for x in range(5)]
        A[0][0] ^= rc
    return A

def keccak256(data: bytes) -> bytes:
    rate = 136
    p = bytearray(data) + b"\x01" + b"\x00" * ((-len(data) - 1) % rate)
    p[-1] |= 0x80
    A = [[0] * 5 for _ in range(5)]
    for off in range(0, len(p), rate):
        blk = p[off:off + rate]
        for i in range(rate // 8):
            A[i % 5][i // 5] ^= int.from_bytes(blk[8 * i:8 * i + 8], "little")
        A = _f(A)
    return b"".join(A[i % 5][i // 5].to_bytes(8, "little") for i in range(4))

def selector(sig: str) -> str:
    return keccak256(sig.encode()).hex()[:8]

assert selector("transfer(address,uint256)") == "a9059cbb"
assert keccak256(b"").hex() == "c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470"