> For the complete documentation index, see [llms.txt](https://trizen.gitbook.io/perl6-rosettacode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://trizen.gitbook.io/perl6-rosettacode/programming_tasks/l/lzw_compression.md).

# LZW compression

I just came across [this SO question](https://stackoverflow.com/questions/30531078/) by chance hence the update. Notably the ancestor Perl entry simply works without any further tweak.

```perl
# 20200421 Updated Raku programming solution ; add unicode support

sub compress(Str $uncompressed --> Seq)  { 
    my $dict-size = 256;
    my %dictionary = (.chr => .chr for ^$dict-size);
 
    my $w = "";
    gather {
  for $uncompressed.encode('utf8').list.chrs.comb -> $c {
      my $wc = $w ~ $c;
      if %dictionary{$wc}:exists { $w = $wc }
      else {
    take %dictionary{$w};
    %dictionary{$wc} = +%dictionary;
    $w = $c;
      }
  }
     
  take %dictionary{$w} if $w.chars;
    }
}
 
sub decompress(@compressed --> Str) { 
    my $dict-size = 256;
    my %dictionary = (.chr => .chr for ^$dict-size);
 
    my $w = shift @compressed;
    ( Blob.new: flat ( gather {
  take $w;
  for @compressed -> $k {
      my $entry;
      if %dictionary{$k}:exists { take $entry = %dictionary{$k} }
      elsif $k == $dict-size    { take $entry = $w ~ $w.substr(0,1) }
      else                      { die "Bad compressed k: $k" }
     
      %dictionary{$dict-size++} = $w ~ $entry.substr(0,1);
      $w = $entry;
  }
    }  )».ords ).decode('utf-8')
}
 
say my @compressed = compress('TOBEORNOTTOBEORTOBEORNOT');
say decompress(@compressed);

@compressed = compress('こんにちは𝒳𝒴𝒵こんにちは𝒳𝒴𝒵こんにちは𝒳𝒴𝒵');
say decompress(@compressed);
```

#### Output:

```
[T O B E O R N O T 256 258 260 265 259 261 263]
TOBEORNOTTOBEORTOBEORNOT
こんにちは𝒳𝒴𝒵こんにちは𝒳𝒴𝒵こんにちは𝒳𝒴𝒵
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://trizen.gitbook.io/perl6-rosettacode/programming_tasks/l/lzw_compression.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.