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AK: Define a traits helper for case-insensitive StringView hashing

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Currently, we define a CaseInsensitiveStringTraits structure for String.
Using this structure for StringView involves allocating a String from
that view, and a second string to convert that intermediate string to
lowercase.

This defines CaseInsensitiveStringViewTraits (and the underlying helper
case_insensitive_string_hash) to avoid allocations.
This commit is contained in:
Timothy Flynn 2022-01-10 11:47:23 -05:00 committed by Linus Groh
parent 7d9b6b41c3
commit 3dccaa39d8
3 changed files with 43 additions and 0 deletions
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21
AK/StringHash.h
View file

@ -24,6 +24,27 @@ constexpr u32 string_hash(char const* characters, size_t length, u32 seed = 0)
return hash;
}
constexpr u32 case_insensitive_string_hash(char const* characters, size_t length, u32 seed = 0)
{
// AK/CharacterTypes.h cannot be included from here.
auto to_lowercase = [](char ch) -> u32 {
if (ch >= 'A' && ch <= 'Z')
return static_cast<u32>(ch) + 0x20;
return static_cast<u32>(ch);
};
u32 hash = seed;
for (size_t i = 0; i < length; ++i) {
hash += to_lowercase(characters[i]);
hash += (hash << 10);
hash ^= (hash >> 6);
}
hash += hash << 3;
hash ^= hash >> 11;
hash += hash << 15;
return hash;
}
}
using AK::string_hash;

10
AK/StringView.h
View file

@ -221,6 +221,15 @@ struct Traits<StringView> : public GenericTraits<StringView> {
static unsigned hash(StringView s) { return s.hash(); }
};
struct CaseInsensitiveStringViewTraits : public Traits<StringView> {
static unsigned hash(StringView s)
{
if (s.is_empty())
return 0;
return case_insensitive_string_hash(s.characters_without_null_termination(), s.length());
}
};
}
[[nodiscard]] ALWAYS_INLINE constexpr AK::StringView operator"" sv(const char* cstring, size_t length)
@ -228,4 +237,5 @@ struct Traits<StringView> : public GenericTraits<StringView> {
return AK::StringView(cstring, length);
}
using AK::CaseInsensitiveStringViewTraits;
using AK::StringView;

12
Tests/AK/TestStringView.cpp
View file

@ -189,3 +189,15 @@ TEST_CASE(constexpr_stuff)
}
#undef do_test
}
TEST_CASE(case_insensitive_hash)
{
auto string1 = "abcdef"sv;
auto string2 = "ABCDEF"sv;
auto string3 = "aBcDeF"sv;
auto string4 = "foo"sv;
EXPECT_EQ(CaseInsensitiveStringViewTraits::hash(string1), CaseInsensitiveStringViewTraits::hash(string2));
EXPECT_EQ(CaseInsensitiveStringViewTraits::hash(string1), CaseInsensitiveStringViewTraits::hash(string3));
EXPECT_NE(CaseInsensitiveStringViewTraits::hash(string1), CaseInsensitiveStringViewTraits::hash(string4));
}