Bug #21559: Unicode normalization nfd -> nfc -> nfd is not reversible

• Status: Closed
• Assignee: duerst (Martin Dürst)
• Target version: -
• Backport: 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED

[tompng (tomoya ishida)]

I expect nfd(nfc(str)) == nfd(str) but found a string that doesn't.

# Ruby 3.1 - 3.5
str = "s\u{11930}\u{323}\u{11930}\u{307}"
p str.unicode_normalize(:nfd) == str.unicode_normalize(:nfc).unicode_normalize(:nfd)
#=> false

# ruby 3.5.0dev
str = "s\u{1611e}\u{323}\u{1611e}\u{307}\u{1611f}"
p str.unicode_normalize(:nfd) == str.unicode_normalize(:nfc).unicode_normalize(:nfd)
#=> false

[nobu (Nobuyoshi Nakada)]

"s\u{11930 323 11930 307}".unicode_normalize(:nfc).dump #=> "\u1E69\u{11930}\u{11930}"
"s\u{323 307}".unicode_normalize(:nfc).dump  #=> "\u1E69"

Are U+0323 and U+0307 composed to s jumping over U+11930?

[ima1zumi (Mari Imaizumi)]

• Assignee set to ima1zumi (Mari Imaizumi)

This looks like a bug. Per Unicode TR15, the identity toNFD(x) == toNFD(toNFC(x)) must be maintained. https://unicode.org/reports/tr15/#Design_Goals
It seems the NFC process is combining characters across U+11930, even though its CCC is 0.

CC: @duerst (Martin Dürst)

[duerst (Martin Dürst)]

• Assignee changed from ima1zumi (Mari Imaizumi) to duerst (Martin Dürst)

@ima1zumi (Mari Imaizumi) Not sure this is even allowed, but I'm sure I'm responsible for this behavior, and want to fix it myself, so I change the Assignee to myself.

[ima1zumi (Mari Imaizumi)]

@duerst (Martin Dürst) Thank you, I appreciate you taking care of it.

[duerst (Martin Dürst)]

• Status changed from Open to Closed

Added regression test at https://github.com/ruby/ruby/commit/a122d7a58e91ed6cd531e906cb398688d7cc8b17
and fix at https://github.com/ruby/ruby/commit/e4c8e3544237b8c0efba6b945173dc66552d641c.
Many thanks to Tomoya Ishida for finding this bug.

[duerst (Martin Dürst)]

• Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN to 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED

Backport would only be needed if the upgrade to Unicode 16.0.0 (see https://bugs.ruby-lang.org/issues/20724) is backported.

[duerst (Martin Dürst)]

Note to potential backporters: https://github.com/ruby/ruby/commit/bd51b20c50 should also be backported.