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string.c: read String#chop! back from the last byte #7122

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Merged
matz merged 2 commits into from
Aug 12, 2026
Merged

string.c: read String#chop! back from the last byte #7122

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5f186e9
test: ask String#chop! where a broken sequence ends a string
takumin Aug 12, 2026
d1433cd
string.c: read String#chop! back from the last byte
takumin Aug 12, 2026
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4 changes: 2 additions & 2 deletions mrbgems/mruby-string-ext/test/string.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -891,8 +891,8 @@ def assert_upto(exp, receiver, *args)
end if UTF8STRING

assert('String#chop! on a binary string removes one byte') do
# `chop!` walks to the last character, and a byte-indexed string ends in a
# byte rather than in a character. Walking it as UTF-8 took the whole of a
# `chop!` cuts at the last character, and a byte-indexed string ends in a
# byte rather than in a character. Reading it as UTF-8 took the whole of a
# multi-byte sequence off, or all of a string that held only one.
if UTF8STRING
s = "\u{1F600}".b # F0 9F 98 80: four bytes, one character
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14 changes: 6 additions & 8 deletions src/string.c
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Expand Up @@ -2045,14 +2045,12 @@ mrb_str_chop_bang(mrb_state *mrb, mrb_value str)
len = RSTR_LEN(s) - 1;
}
else {
const char* t = RSTR_PTR(s), *p = t;
const char* e = p + RSTR_LEN(s);
while (p<e) {
mrb_int clen = mrb_utf8len(p, e);
if (p + clen>=e) break;
p += clen;
}
len = p - t;
/* The last character starts at the head of the one covering the last
byte, which is read backwards from there rather than by walking the
whole string. */
const char* t = RSTR_PTR(s);
const char* e = t + RSTR_LEN(s);
len = mrb_utf8_char_head(t, e-1, e) - t;
}
#else
len = RSTR_LEN(s) - 1;
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16 changes: 16 additions & 0 deletions test/t/string.rb
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Expand Up @@ -403,6 +403,22 @@ def [](*args)
assert_equal c, 'あいう'
end if UTF8STRING

assert('String#chop! cuts where String#length counts a character') do
# The last character starts where the character covering the last byte
# starts, and a byte that no lead byte reaches is a character of its own.
assert_equal "あ", "あ\x82".chop
assert_equal "\x80\x80", "\x80\x80\x80".chop
# A sequence RFC 3629 forbids spells no character, so its bytes stand alone.
assert_equal "\xC0", "\xC0\x80".chop
assert_equal "\xED\xA0", "\xED\xA0\x80".chop
# A lead byte the string end cuts short reaches none of the bytes after it.
assert_equal "a\xE3", "a\xE3\x81".chop
# a whole character still goes at once, however many bytes it spells
assert_equal "", "\u{1F600}".chop
# and the \r\n pair is still taken together after one
assert_equal "あ", "あ\r\n".chop
end if UTF8STRING

assert('String#downcase', '15.2.10.5.13') do
a = 'ABC'.downcase
b = 'ABC'
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