the6p4c · November 28, 2019 13:13 · Nov 28, 2019
diff --git a/decompress.py b/decompress.py
@@ -0,0 +1,60 @@
+import sys
+
+def main():
+    input_file = sys.argv[1]
+    output_file = sys.argv[2]
+
+    with open(input_file, 'rb') as f_in, open(output_file, 'wb') as f_out:
+        decompressed_length = int.from_bytes(f_in.read(4), 'little')
+        bytes_written = 0
+
+        output_buffer = [0] * 4096
+        output_buffer_pos = 0xFEE
+
+        def get_byte(offset):
+            nonlocal output_buffer
+
+            return output_buffer[offset % len(output_buffer)]
+
+        def write_byte(b):
+            nonlocal output_buffer, output_buffer_pos, bytes_written, f_out
+
+            output_buffer[output_buffer_pos] = b
+
+            output_buffer_pos += 1
+            output_buffer_pos %= len(output_buffer)
+
+            bytes_written += 1
+
+            f_out.write(bytes([b]))
+
+        while True:
+            # >= to catch an over-run, if we're unlucky enough for one to
+            # happen
+            if bytes_written >= decompressed_length:
+                return
+
+            control = ord(f_in.read(1))
+            control_bits = [(control >> i) & 1 for i in range(8)]
+
+            for control_bit in control_bits:
+                # same deal as above
+                if bytes_written >= decompressed_length:
+                    return
+
+                if control_bit == 1:
+                    # literal
+                    literal = ord(f_in.read(1))
+
+                    write_byte(literal)
+                else:
+                    # reference
+                    reference = int.from_bytes(f_in.read(2), 'big')
+                    offset = (((reference >> 4) & 0xF) << 8) | (reference >> 8)
+                    length = (reference & 0xF) + 3
+
+                    for i in range(length):
+                        write_byte(get_byte(offset + i))
+
+if __name__ == '__main__':
+    main()
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