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string.c: ask a broken string again after String#reverse! #7226

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Merged
matz merged 1 commit into from
Aug 17, 2026
Merged

string.c: ask a broken string again after String#reverse! #7226

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17 changes: 15 additions & 2 deletions mrbgems/mruby-encoding/test/string.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -217,14 +217,20 @@

assert('String#reverse! leaves what the bytes read as standing') do
# Reversing puts the same bytes back with every character whole, so a string
# that read as UTF-8 still does, and one that did not still does not. The
# reversal is where that is decided; asking afterwards is how it is seen.
# that read as UTF-8 still does. A string that did not is the one case the
# reversal can settle either way, since bytes that spell nothing where they
# stood can spell a character once they are turned around. The reversal is
# where all of that is decided; asking afterwards is how it is seen.
if UTF8STRING
a = "あいうz"
a.reverse!
assert_equal "zういあ", a
assert_equal 4, a.length
assert_true a.valid_encoding?
a.reverse!
assert_equal "あいうz", a
assert_equal 4, a.length
assert_true a.valid_encoding?

b = "abc"
b.reverse!
Expand All @@ -235,6 +241,13 @@
c.reverse!
assert_equal "\x81\xE3a".b, c.b
assert_false c.valid_encoding?

d = "\x80\xC2" # a trailing byte and then a lead byte, spelling nothing
assert_false d.valid_encoding? # asked here, so the answer is on the string
d.reverse! # and the same bytes now spell U+0080
assert_equal "\xC2\x80".b, d.b
assert_equal 1, d.length
assert_true d.valid_encoding?
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end
end

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7 changes: 5 additions & 2 deletions src/string.c
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Expand Up @@ -2954,8 +2954,11 @@ mrb_str_reverse_bang(mrb_state *mrb, mrb_value str)
char *p, *e;

/* Reversing writes the string's own bytes back in another order, and both
paths below leave every character whole, so what the bytes read as is
still what they read as: both write through str_modify_keep_cr(). */
paths below leave every character whole, so a string that read as UTF-8
still does: both write through str_modify_keep_cr(). A string already
read as broken is the one this cannot answer for, since bytes that spell
nothing where they stand can spell a character turned around, and that is
the string the helper asks again on its own. */

#ifdef MRB_UTF8_STRING
/* mrb_str_char_len() walks the string and records what it finds. The
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