Python script implementing decoder for LZ4 block format. License: 0BSD

lz4_decode.py

import lz4.block

f = open('lz4_Block_format.md', 'rb').read()
e = lz4.block.compress(f, compression=12, store_size=False)
d = [0] * len(f)

i = k = 0
while i < len(e):

    def get_len(l):
        # Get a variable length
        global i
        if l == 15:
            while True:
                n = e[i]
                i += 1
                l += n
                if n != 255:
                    break
        return l

    # Get token
    t = e[i]
    i += 1

    # Copy literals
    l = get_len(t >> 4)
    d[k:k + l] = e[i:i + l]
    i += l
    k += l

    # "The block ends right after the literals (no offset field)"
    if i == len(e):
        break

    # Get offset (16 bit little-endian)
    offt = (e[i + 1] << 8) | e[i]
    i += 2
    assert offt != 0 # zero offset is invalid

    # Copy match
    l = get_len(t & 0xf) + 4
    # We have to copy byte by byte because it may overlap and we're copying
    # from low to high
    m = k - offt
    for _ in range(l):
        d[k] = d[m]
        k += 1
        m += 1

print(bytes(d).decode('utf-8'))