136 lines
4.8 KiB
Python
136 lines
4.8 KiB
Python
import sys
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import os
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# Compression function reverse-engineered from ALTTP's decompression routine at $00E7DE
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def compress(data):
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out = bytearray()
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i = 0
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while i < len(data):
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# Check for repeating byte pattern
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if i + 1 < len(data) and data[i] == data[i + 1]:
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length = 2
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while i + length < len(data) and data[i] == data[i + length] and length < 32:
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length += 1
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# Repeating byte: 0x20-0x3F
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out.append(0x20 | (length - 1))
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out.append(data[i])
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i += length
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continue
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# Check for incremental byte pattern
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if i + 2 < len(data) and data[i + 1] == data[i] + 1 and data[i + 2] == data[i] + 2:
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length = 3
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while i + length < len(data) and data[i + length] == data[i] + length and length < 32:
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length += 1
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# Incremental: 0x60-0x7F
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out.append(0x60 | (length - 1))
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out.append(data[i])
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i += length
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continue
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# Check for repeating word pattern (alternating two bytes)
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if i + 3 < len(data):
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# Check if we have an alternating pattern: A B A B...
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byte_a = data[i]
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byte_b = data[i + 1]
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length = 2
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while i + length < len(data) and length < 32:
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if length % 2 == 0:
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if data[i + length] != byte_a:
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break
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else:
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if data[i + length] != byte_b:
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break
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length += 1
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if length >= 4: # Need at least 4 bytes (2 alternations) to make it worthwhile
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# Repeating word: 0x40-0x5F
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out.append(0x40 | (length - 1))
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out.append(byte_a)
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out.append(byte_b)
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i += length
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continue
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# Check for copy from past (LZ with absolute offset)
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best_len = 0
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best_off = 0
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search_start = max(0, i - 65536) # Can reference anywhere in output
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for j in range(search_start, i):
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length = 0
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while i + length < len(data) and data[j + length] == data[i + length] and length < 1024:
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length += 1
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if length >= 2 and length > best_len:
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best_len = length
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best_off = j # Absolute offset, not relative!
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if best_len >= 2:
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# Copy from past: 0x80-0xDF or 0xE0-0xFE (extended)
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# Offset is ABSOLUTE position in the output buffer
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if best_len <= 32:
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# Standard copy: 0x80-0xDF (5 bits for length-1, 16 bits for absolute offset)
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out.append(0x80 | ((best_len - 1) & 0x1F))
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out.append(best_off & 0xFF)
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out.append((best_off >> 8) & 0xFF)
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else:
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# Extended copy: 0xE0-0xFE
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if best_len > 1024:
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best_len = 1024
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# Command byte: 111LLLLL where L is length bits
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cmd = 0xE0 | (((best_len - 1) >> 8) & 0x1F)
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out.append(cmd)
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out.append((best_len - 1) & 0xFF)
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out.append(best_off & 0xFF)
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out.append((best_off >> 8) & 0xFF)
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i += best_len
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continue
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# Raw copy (no pattern found)
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size = 1
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while size < 32 and i + size < len(data):
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# Don't extend raw copy if we find a better pattern ahead
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if i + size + 1 < len(data) and data[i + size] == data[i + size + 1]:
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break
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if i + size + 2 < len(data) and data[i + size + 1] == data[i + size] + 1:
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break
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# Check LZ
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found_lz = False
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for j in range(max(0, i + size - 2048), i + size):
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if i + size + 1 < len(data) and data[j] == data[i + size] and data[j + 1] == data[i + size + 1]:
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found_lz = True
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break
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if found_lz:
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break
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size += 1
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# Raw copy: 0x00-0x1F
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out.append(size - 1)
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out.extend(data[i:i + size])
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i += size
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# End marker
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out.append(0xFF)
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return out
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if __name__ == '__main__':
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if len(sys.argv) != 3:
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print("Usage: python compress.py <input_file> <output_file>")
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sys.exit(1)
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input_file_path = sys.argv[1]
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output_file_path = sys.argv[2]
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if not os.path.exists(input_file_path):
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print(f"Error: Input file not found at {input_file_path}")
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sys.exit(1)
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with open(input_file_path, 'rb') as f:
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input_data = f.read()
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compressed_data = compress(input_data)
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with open(output_file_path, 'wb') as f:
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f.write(compressed_data)
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print(f"Successfully compressed '{input_file_path}' to '{output_file_path}'")
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