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Page: Rle8182Algorithm
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1 Rle8182Algorithm
Paul Taykalo edited this page 2018-03-09 17:30:59 +02:00
Table of Contents

Description

This is a simple RLE algorithm used by some old Mac archivers such as Compact Pro. It uses a double-byte escape code to encode repeated runs of characters.

  • The two bytes 0x81 0x82 followed by a a single non-zero byte N start a run of N-1 repetitions of the last byte previously output.
  • 0x81 0x82 0x00 outputs the byte sequence "0x81 0x82".
  • 0x81 followed by any other byte is treated as a literal byte 0x81 (and whatever follows it).
  • Any other byte sequence is a literal string.

As a consequence,

  • 0x81 0x81 0x82 N encodes a run of 0x81 bytes.
  • 0x81 0x82 0x00 0x81 0x82 N encodes the bytes "0x81 0x82" followed by a run of 0x82 bytes.
  • 0x81 0x82 0x01 appears to be a no-op or illegal.