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AK: Improve performance of StringUtils::find_last

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The current algorithm is currently O(N^2) because we forward-search an
ever-increasing substring of the haystack. This implementation reduces
the search time of a 500,000-length string (where the desired needle is
at index 0) from 72 seconds to 2-3 milliseconds.
This commit is contained in:
Timothy Flynn 2024-01-03 16:48:42 -05:00 committed by Tim Flynn
parent 8064c9fc4d
commit cae184d7cf
Notes: sideshowbarker 2024-07-17 08:35:21 +09:00 
Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/cae184d7cf
Pull-request: https://github.com/SerenityOS/serenity/pull/22562
Reviewed-by: https://github.com/nico ✅
1 changed files with 9 additions and 4 deletions
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13
AK/StringUtils.cpp
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@ -412,10 +412,15 @@ Optional<size_t> find_last(StringView haystack, char needle)
Optional<size_t> find_last(StringView haystack, StringView needle)
{
for (size_t i = haystack.length(); i > 0; --i) {
auto value = StringUtils::find(haystack, needle, i - 1);
if (value.has_value())
return value;
if (needle.length() > haystack.length())
return {};
for (size_t i = haystack.length() - needle.length();; --i) {
if (haystack.substring_view(i, needle.length()) == needle)
return i;
if (i == 0)
break;
}
return {};