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Challenges Run-length encode a byte sequence

2 answers  ·  posted 12mo ago by celtschk‭  ·  last activity 12mo ago by Moshi‭

Question code-golf encoding
#1: Initial revision by user avatar celtschk‭ · 2021-10-06T08:09:12Z (12 months ago)
Run-length encode a byte sequence
Run-length encoding is a simple compression technique which compresses sequences of repeating identical bytes.

The encoding rules for this task are as follows:

Any sequence of $n$ identical bytes ($3 \leq n \leq 63$) is replaced by a byte with value `n+0xc0` followed by only one copy of that byte. If any of the original bytes has value `0xc0` or above, such encoding also happens if that byte occurs only one or two times in a row (thus the encoded sequence may actually be longer if such bytes occur).

The task is to write a run-length encoder. The input is a sequence of bytes. It may be a sequence of actual bytes, or a sequence of numbers in the range 0 to 255 (inclusive), whichever is more convenient. The output is again a sequence of bytes (again, in any suitable form), which is the proper run-length encoding of the input sequence.

Examples (byte values are given in 2-digit hex):
```
00 01 02 03 04        → 00 01 02 03 04
00 00 00 01 01 00     → c3 00 01 01 00
be bf c0 c1 c2        → be bf c1 c0 c1 c1 c1 c2
00 00 00 … (63 bytes) → ff 00
00 00 00 … (64 bytes) → ff 00 00
00 00 00 … (65 bytes) → ff 00 00 00
00 00 00 … (66 bytes) → ff 00 c3 00
ff ff ff … (63 bytes) → ff ff
ff ff ff … (64 bytes) → ff ff c1 ff
ff ff ff … (65 bytes) → ff ff c2 ff
ff ff ff … (66 bytes) → ff ff c3 ff
```

This is a <span class="badge is-tag">code-golf</span> challenge; the shortest program for a language wins.