[ruby-core:107730] [Ruby master Bug#18601] Invalid byte sequences in Big5 encodings

Issue #18601 has been updated by duerst (Martin D端rst).

Assignee set to duerst (Martin D端rst)

I'll try to take a closer look at this, but it will take a few days, sorry. Please ping me again if you don't hear back within a week or two.

----------------------------------------
Bug #18601: Invalid byte sequences in Big5 encodings
https://bugs.ruby-lang.org/issues/18601#change-96653

* Author: janosch-x (Janosch M端ller)
* Status: Open
* Priority: Normal
* Assignee: duerst (Martin D端rst)
* ruby -v: any
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
I encoded all unicode codepoints in all encodings:

```
full_string = ((0..0xD7FF).to_a + (0xE000..0x10FFFF).to_a).pack('U*'); 1

uniq_encodings =
  Encoding.name_list -
  Encoding.aliases.keys -
  %w[locale external filesystem internal]

encoded_strings = 
  uniq_encodings.map do |enc|
    full_string.encode(enc, invalid: :replace, undef: :replace, replace: '')
  rescue => e
    puts e
  end; 1
```

This prints about 10 "converter not found" errors, such as `code converter not found (UTF-8 to UTF-7)`, but I guess this is expected.

Some of the converters seem to output invalid strings, though:

```
encoded_strings.each do |str|
  str&.codepoints
rescue => e
  puts e
end; 1
```

This will print `invalid byte sequence in {Big5HKSCS,Big5-UAO,CP950,CP951}`.

Looking for example at the generated CP950 string, 8031 of its 25342 characters are invalid, spread across 2017 distinct ranges in the string. The invalid characters' codepoints are all in the range of 0x81..0xFE.

Is this a bug?

I would expect `String#encode` with `invalid: :replace, undef: :replace` not to create invalid byte sequences, but maybe I am misunderstanding these encodings and this is an unavoidable issue?

CC @duerst



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