105 lines
4.5 KiB
Python
105 lines
4.5 KiB
Python
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from typing import List, Optional
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class Block:
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def __init__(self, bits=None, index=None, offset=None, chain=None):
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self.chain = chain
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self.bits = bits
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self.index = index
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self.offset = offset
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INITIAL_OFFSET = 1
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MAX_SCALE = 50
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def offset_ceiling(index, offset_limit):
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return offset_limit if index > offset_limit else (INITIAL_OFFSET if index < INITIAL_OFFSET else index)
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def elias_gamma_needed_bits(value):
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bits = 1
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while value > 1:
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value >>= 1
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bits += 2
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return bits
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def optimize(input_data, skip, offset_limit):
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# The algorithm has a floating window of size window_size describing the previous chain of matches
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input_size = len(input_data)
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window_size = offset_ceiling(input_size - 1, offset_limit) + 1
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last_literal: List[Optional[Block]] = [None] * window_size
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last_match: List[Optional[Block]] = [None] * window_size
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match_length = [0] * window_size
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# The algorithm is looking for the best match for each index of the input data
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optimal = [None] * input_size
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best_length = [0] * input_size
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if input_size > 2:
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best_length[2] = 2
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# Kickstart the algorithm by assigning a fake block
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last_match[INITIAL_OFFSET] = Block(-1, skip - 1, INITIAL_OFFSET, None)
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# The algorithm is checking for the best match for each index of the input data (skipping the skip part)
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for index in range(skip, input_size):
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best_length_size = 2 # It's useless to check for a match of length 1
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max_offset = offset_ceiling(index, offset_limit)
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for offset in range(1, max_offset + 1):
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# Checking for a match in the previous part of the input data, backwards
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if index != skip and index >= offset and input_data[index] == input_data[index - offset]:
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current_literal = last_literal[offset]
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if current_literal is not None:
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length = index - current_literal.index
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bits = current_literal.bits + 1 + elias_gamma_needed_bits(length)
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# Chain the current match to the previous literal
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last_match[offset] = Block(bits, index, offset, current_literal)
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# Update the best match
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if not optimal[index] or optimal[index].bits > bits:
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optimal[index] = last_match[offset]
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match_length[offset] += 1
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if match_length[offset] > 1:
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if best_length_size < match_length[offset]:
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bits = (optimal[index - best_length[best_length_size]].bits +
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elias_gamma_needed_bits(best_length[best_length_size] - 1))
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while True:
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best_length_size += 1
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bits2 = (optimal[index - best_length_size].bits +
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elias_gamma_needed_bits(best_length_size - 1))
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if bits2 <= bits:
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best_length[best_length_size] = best_length_size
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bits = bits2
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else:
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best_length[best_length_size] = best_length[best_length_size - 1]
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if best_length_size >= match_length[offset]:
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break
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length = best_length[match_length[offset]]
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bits = (optimal[index - length].bits + 8 +
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elias_gamma_needed_bits((offset - 1) // 128 + 1) +
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elias_gamma_needed_bits(length - 1))
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if not last_match[offset] or last_match[offset].index != index or last_match[offset].bits > bits:
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last_match[offset] = Block(bits, index, offset, optimal[index - length])
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if not optimal[index] or optimal[index].bits > bits:
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optimal[index] = last_match[offset]
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else:
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match_length[offset] = 0 # Resetting the match length
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if last_match[offset]:
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length = index - last_match[offset].index
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bits = last_match[offset].bits + 1 + elias_gamma_needed_bits(length) + length * 8
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last_literal[offset] = Block(bits, index, 0, last_match[offset])
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if not optimal[index] or optimal[index].bits > bits:
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optimal[index] = last_literal[offset]
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return optimal[input_size - 1]
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