from typing import Sequence

def ROTL(a,b):
    return (((a) << (b)) | ((a) >> (32 - (b))))

def QR(a, b, c, d):
    a = (a + b) & 0xffffffff
    d = (d ^ a) & 0xffffffff
    d = ROTL(d, 16) & 0xffffffff
    c = (c + d) & 0xffffffff
    b = (b ^ c) & 0xffffffff
    b = ROTL(b, 12)
    a = (a + b) & 0xffffffff
    d = (d ^ a) & 0xffffffff
    d = ROTL(d,  8) & 0xffffffff
    c = (c + d) & 0xffffffff
    b = (b ^ c) & 0xffffffff
    b = ROTL(b,  7) & 0xffffffff

    return a, b, c, d

ROUNDS = 20
def chacha_block(data_in: Sequence[int]) -> Sequence[int]:
    # make sure to copy this list so it doesn't get modified
    data = data_in[:]


    for i in range(0, ROUNDS, 2):
        data[0], data[4], data[8],  data[12] = QR(data[0], data[4], data[8],  data[12])
        data[1], data[5], data[9],  data[13] = QR(data[1], data[5], data[9],  data[13])
        data[2], data[6], data[10], data[14] = QR(data[2], data[6], data[10], data[14])
        data[3], data[7], data[11], data[15] = QR(data[3], data[7], data[11], data[15])

        data[0], data[5], data[10], data[15] = QR(data[0], data[5], data[10], data[15])
        data[1], data[6], data[11], data[12] = QR(data[1], data[6], data[11], data[12])
        data[2], data[7], data[8],  data[13] = QR(data[2], data[7], data[8],  data[13])
        data[3], data[4], data[9],  data[14] = QR(data[3], data[4], data[9],  data[14])

    for i in range(16):
        data[i] = (data[i] + data_in[i]) & 0xffffffff

    return data