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Most significant nibble meaning

0001 - 1111

single-byte symbol, literal-only file

[literal sequence length] = [byte value] - 17

Examples:

0x    1    2
0b 0001 0010  ( 18)  =   1 literal

0x    F    F
0b 1111 1111  (255)  = 238 literals

0000

one byte or more

one byte:
[literal sequence length] = [byte value] + 3

two bytes or more:
[literal sequence length] = ([zero byte count] - 1) * 255 + [last byte value] + 18

Examples:

0x    0    1
0b 0000 0001  (  1)  =   4 literals

0x    0    f
0b 0000 1111  ( 15)  =  18 literals

0x    0    0           0    1
0b 0000 0000  (  0) 0000 0001  (  1)  =  19 literals

0x    0    0           f    f
0b 0000 0000  (  0) 1111 1111  (255)  = 273 literals

0x    0    0           0    0           0    1
0b 0000 0000  (  0) 0000 0000  (  0) 0000 0001  (  1)  = 274 literals

0x    0    0           0    0           f    f
0b 0000 0000  (  0) 0000 0000  (  0) 1111 1111  (255)  = 528 literals

0x    0    0           0    0           0    0           0    1
0b 0000 0000  (  0) 0000 0000  (  0) 0000 0000  (  0) 0000 0001  (  1)  = 529 literals

0x    0    0           0    0           0    0           f    f
0b 0000 0000  (  0) 0000 0000  (  0) 0000 0000  (  0) 1111 1111  (255)  = 783 literals

0001

three bytes or more

0001 yxxx yyyy yyzz yyyy yyyy
      ^^^ długość kopii (wartość + 2)
      210
                 ^^ długość literału-następnika (wartość)
                 10
     ^    ^^^^ ^^   ^^^^ ^^^^ odległość (wartość + 16384)
     e    5432 10   dcba 9876


0001 y000 xxxx xxxx yyyy yyzz yyyy yyyy
          ^^^^ ^^^^ długość kopii (wartość + 9)
          7654 3210
                           ^^ długość literału-następnika (wartość)
                           10
     ^              ^^^^ ^^   ^^^^ ^^^^ odległość (wartość + 16384)
     e              5432 10   dcba 9876


0001 y000 0000 0000 xxxx xxxx yyyy yyzz yyyy yyyy
                    ^^^^ ^^^^ długość kopii (wartość + 264)
                    7654 3210
                                     ^^ długość literału-następnika (wartość)
                                     10
     ^                        ^^^^ ^^   ^^^^ ^^^^ odległość (wartość + 16384)
     e                        5432 10   dcba 9876

zero distance or length is invalid

0010 - 0011

three bytes or more

001x xxxx yyyy yyzz yyyy yyyy
   ^ ^^^^
   4 3210 długość kopii (wartość + 2)
                 ^^ długość literału-następnika (wartość)
                 10
          ^^^^ ^^   ^^^^ ^^^^ odległość (wartość + 1)
          5432 10   dcba 9876


0010 0000 xxxx xxxx yyyy yyzz yyyy yyyy
          ^^^^ ^^^^
          7654 3210 długość kopii (wartość + 33)
                           ^^ długość literału-następnika (wartość)
                           10
                    ^^^^ ^^   ^^^^ ^^^^ odległość (wartość + 1)
                    5432 10   dcba 9876

0100 - 1111

two bytes

xxxy yyzz yyyy yyyy
^^^ długość kopii (1 + wartość)
210
   ^ ^^   ^^^^ ^^^^ odległość (wartość + 1) } źródło jest zapętlane: długość kopii % odległość
   2 10   a987 6543
       ^^ długość literału-następnika (wartość)
       10
HEADER_LENGTH = 2 * 4
ETX_MARKER = b'\x11\x00\x00'
MINIMAL_ENCODED_LENGTH = HEADER_LENGTH + len(ETX_MARKER)
def read_int(data: bytes):
return int.from_bytes(data[:4], 'little')
def read_short(data: bytes):
return int.from_bytes(data[:2], 'little')
def read_byte(data: bytes):
return data[:1][0]
def read_most_significant_nibble(data: bytes):
return read_byte(data) >> 4
def most_significant_nibble_matches(data: bytes, ones_mask: int, zeros_mask: int) -> bool:
assert 0b0000 <= ones_mask <= 0b1111
assert 0b0000 <= zeros_mask <= 0b1111
most_significant_nibble = read_byte(data) >> 4
ones_mask_matches = (most_significant_nibble & ones_mask) == ones_mask
zeros_mask_matches = ((0b1111 - most_significant_nibble) & zeros_mask) == zeros_mask
return ones_mask_matches and zeros_mask_matches
def decode(data: bytes) -> bytes:
assert len(data) >= MINIMAL_ENCODED_LENGTH, f'Encoded data too short ({len(data)}, expected {MINIMAL_ENCODED_LENGTH} bytes)'
header = data[:HEADER_LENGTH]
data = data[HEADER_LENGTH:]
decoded_length = read_int(header)
encoded_length = read_int(header[4:])
assert len(data) == encoded_length, f'Length of encoded data {len(data)} does not match length from the header {encoded_length}'
decoded_data = bytearray(decoded_length)
enc_ptr = 0
dec_ptr = 0
literal_encountered = False
while True:
assert enc_ptr < len(data), f'Out of bounds access to encoded data at index {enc_ptr} (data length: {len(data)})'
if data[enc_ptr:enc_ptr + 3] == ETX_MARKER:
# print(f'Encountered ETX marker: {ETX_MARKER} at position {enc_ptr}')
enc_ptr += 3
break
assert dec_ptr < len(decoded_data), f'Out of bounds access to decoded data at index {dec_ptr} (data length: {len(decoded_data)})'
if read_most_significant_nibble(data[enc_ptr:enc_ptr + 1]) == 0b0000:
# print(f'Encountered {0b0000:04b} literal symbol at position {enc_ptr}')
zero_bytes_count = 0
while read_byte(data[enc_ptr:enc_ptr + 1]) == 0:
zero_bytes_count += 1
enc_ptr += 1
literal_length = read_byte(data[enc_ptr:enc_ptr + 1])
enc_ptr += 1
if zero_bytes_count == 0:
literal_length += 3
else:
literal_length += 18 + (zero_bytes_count - 1) * 255
# print(f' length: {literal_length}')
decoded_data[dec_ptr:dec_ptr + literal_length] = data[enc_ptr:enc_ptr + literal_length]
enc_ptr += literal_length
dec_ptr += literal_length
literal_encountered = True
elif read_most_significant_nibble(data[enc_ptr:enc_ptr + 1]) > 0b0000 and literal_encountered == False:
# print(f'Encountered the initial literal symbol at position {enc_ptr}')
literal_length = read_byte(data[enc_ptr:enc_ptr + 1])
enc_ptr += 1
assert literal_length >= 18, f'Unexpected length of initial literal: {literal_length} (should be >= 18)'
literal_length -= 17
# print(f' length: {literal_length}')
decoded_data[dec_ptr:dec_ptr + literal_length] = data[enc_ptr:enc_ptr + literal_length]
enc_ptr += literal_length
dec_ptr += literal_length
literal_encountered = True
elif read_most_significant_nibble(data[enc_ptr:enc_ptr + 1]) >= 0b0100:
# print(f'Encountered {0b0100:04b} reference+literal symbol at position {enc_ptr}')
assert literal_encountered, f'No literal before a reference symbol {0b0100:04b}'
buffer = read_byte(data[enc_ptr:enc_ptr + 1])
enc_ptr += 1
length = (buffer >> 5) & 0b111
length += 1
distance = (buffer >> 2) & 0b111
following_literal_length = buffer & 0b0011
distance |= read_byte(data[enc_ptr:enc_ptr + 1]) << 3
distance += 1
enc_ptr += 1
# print(f' distance: {distance}, length: {length}, following literal length: {following_literal_length}')
for _ in range(length):
assert dec_ptr - distance > 0, f'Out of bounds access to decoded data at index {dec_ptr - distance} (data length: {len(decoded_data)})'
decoded_data[dec_ptr] = decoded_data[dec_ptr - distance]
dec_ptr += 1
decoded_data[dec_ptr:dec_ptr + following_literal_length] = data[enc_ptr:enc_ptr + following_literal_length]
enc_ptr += following_literal_length
dec_ptr += following_literal_length
elif read_most_significant_nibble(data[enc_ptr:enc_ptr + 1]) >= 0b0010:
# print(f'Encountered {0b0010:04b} reference+literal symbol at position {enc_ptr}')
assert literal_encountered, f'No literal before a reference symbol {0b0010:04b}'
length = read_byte(data[enc_ptr:enc_ptr + 1]) & 0b00011111
enc_ptr += 1
# print(f' base length: {length}')
if length == 0:
zero_bytes_count = 0
length += 31
while read_byte(data[enc_ptr:enc_ptr + 1]) == 0:
zero_bytes_count += 1
enc_ptr += 1
length += read_byte(data[enc_ptr:enc_ptr + 1])
enc_ptr += 1
length += zero_bytes_count * 255
# print(f' no. of zero bytes: {zero_bytes_count}')
length += 2
buffer = read_byte(data[enc_ptr:enc_ptr + 1])
enc_ptr += 1
distance = buffer >> 2
following_literal_length = buffer & 0b0011
distance |= read_byte(data[enc_ptr:enc_ptr + 1]) << 6
distance += 1
enc_ptr += 1
# print(f' distance: {distance}, length: {length}, following literal length: {following_literal_length}')
for _ in range(length):
assert dec_ptr - distance > 0, f'Out of bounds access to decoded data at index {dec_ptr - distance} (data length: {len(decoded_data)})'
decoded_data[dec_ptr] = decoded_data[dec_ptr - distance]
dec_ptr += 1
decoded_data[dec_ptr:dec_ptr + following_literal_length] = data[enc_ptr:enc_ptr + following_literal_length]
enc_ptr += following_literal_length
dec_ptr += following_literal_length
elif read_most_significant_nibble(data[enc_ptr:enc_ptr + 1]) == 0b0001:
# print(f'Encountered {0b0001:04b} reference+literal symbol at position {enc_ptr}')
assert literal_encountered, f'No literal before a reference symbol {0b0001:04b}'
buffer = read_byte(data[enc_ptr:enc_ptr + 1])
enc_ptr += 1
distance = ((buffer & 0b1000) >> 3) << 14
length = buffer & 0b111
# print(f' base length: {length}')
if length == 0:
zero_bytes_count = 0
length += 7
while read_byte(data[enc_ptr:enc_ptr + 1]) == 0:
zero_bytes_count += 1
enc_ptr += 1
length += read_byte(data[enc_ptr:enc_ptr + 1])
enc_ptr += 1
length += zero_bytes_count * 255
# print(f' no. of zero bytes: {zero_bytes_count}')
length += 2
buffer = read_byte(data[enc_ptr:enc_ptr + 1])
enc_ptr += 1
distance |= buffer >> 2
following_literal_length = buffer & 0b0011
distance |= read_byte(data[enc_ptr:enc_ptr + 1]) << 6
distance += 16384
enc_ptr += 1
# print(f' distance: {distance}, length: {length}, following literal length: {following_literal_length}')
for _ in range(length):
assert dec_ptr - distance > 0, f'Out of bounds access to decoded data at index {dec_ptr - distance} (data length: {len(decoded_data)})'
decoded_data[dec_ptr] = decoded_data[dec_ptr - distance]
dec_ptr += 1
decoded_data[dec_ptr:dec_ptr + following_literal_length] = data[enc_ptr:enc_ptr + following_literal_length]
enc_ptr += following_literal_length
dec_ptr += following_literal_length
assert dec_ptr == decoded_length, f'Length of decoded data {len(decoded_data)} does not match length from the header {decoded_length}'
return decoded_data
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