ladybird/AK/StringBase.h
Andreas Kling a88799c032 AK: Remove excessive hashing caused by FlyString table
Before this change, the global FlyString table looked like this:

    HashMap<StringView, Detail::StringBase>

After this change, we have:

    HashTable<Detail::StringData const*, FlyStringTableHashTraits>

The custom hash traits are used to extract the stored hash from
StringData which avoids having to rehash the StringView repeatedly like
we did before.

This necessitated a handful of smaller changes to make it work.
2024-03-24 13:28:24 +01:00

131 lines
3.8 KiB
C++

/*
* Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <AK/Badge.h>
#include <AK/Endian.h>
#include <AK/Forward.h>
namespace AK::Detail {
class StringData;
static constexpr size_t MAX_SHORT_STRING_BYTE_COUNT = sizeof(StringData*) - 1;
struct ShortString {
ReadonlyBytes bytes() const;
size_t byte_count() const;
// NOTE: This is the byte count shifted left 1 step and or'ed with a 1 (the SHORT_STRING_FLAG)
u8 byte_count_and_short_string_flag { 0 };
u8 storage[MAX_SHORT_STRING_BYTE_COUNT] = { 0 };
};
static_assert(HostIsLittleEndian, "Order of fields in ShortString assumes LE.");
static_assert(sizeof(ShortString) >= sizeof(StringData*));
static_assert(__builtin_offsetof(ShortString, byte_count_and_short_string_flag) == 0);
class StringBase {
public:
// Creates an empty (zero-length) String.
constexpr StringBase()
: StringBase(ShortString { SHORT_STRING_FLAG, {} })
{
}
StringBase(StringBase const&);
constexpr StringBase(StringBase&& other)
: m_short_string(other.m_short_string)
{
other.m_short_string = ShortString {};
other.m_short_string.byte_count_and_short_string_flag = SHORT_STRING_FLAG;
}
StringBase& operator=(StringBase&&);
StringBase& operator=(StringBase const&);
constexpr ~StringBase()
{
if (!is_constant_evaluated())
destroy_string();
}
// NOTE: This is primarily interesting to unit tests.
[[nodiscard]] constexpr bool is_short_string() const
{
return (m_short_string.byte_count_and_short_string_flag & SHORT_STRING_FLAG) != 0;
}
// Returns the underlying UTF-8 encoded bytes.
// NOTE: There is no guarantee about null-termination.
[[nodiscard]] ReadonlyBytes bytes() const;
[[nodiscard]] u32 hash() const;
[[nodiscard]] size_t byte_count() const;
[[nodiscard]] bool operator==(StringBase const&) const;
[[nodiscard]] ALWAYS_INLINE FlatPtr raw(Badge<FlyString>) const { return bit_cast<FlatPtr>(m_data); }
protected:
template<typename Func>
ErrorOr<void> replace_with_new_string(size_t byte_count, Func&& callback)
{
Bytes buffer = TRY(replace_with_uninitialized_buffer(byte_count));
if (byte_count != 0)
TRY(callback(buffer));
return {};
}
template<typename Func>
constexpr void replace_with_new_short_string(size_t byte_count, Func&& callback)
{
Bytes buffer = replace_with_uninitialized_short_string(byte_count);
if (byte_count != 0)
callback(buffer);
}
// This is not a trivial operation with storage, so it does not belong here. Unfortunately, it
// is impossible to implement it without access to StringData.
ErrorOr<StringBase> substring_from_byte_offset_with_shared_superstring(size_t start, size_t byte_count) const;
private:
friend class ::AK::String;
friend class ::AK::FlyString;
// NOTE: If the least significant bit of the pointer is set, this is a short string.
static constexpr uintptr_t SHORT_STRING_FLAG = 1;
explicit StringBase(NonnullRefPtr<Detail::StringData const>);
explicit constexpr StringBase(ShortString short_string)
: m_short_string(short_string)
{
}
ErrorOr<Bytes> replace_with_uninitialized_buffer(size_t byte_count);
constexpr Bytes replace_with_uninitialized_short_string(size_t byte_count)
{
VERIFY(is_short_string());
VERIFY(byte_count <= MAX_SHORT_STRING_BYTE_COUNT);
m_short_string = ShortString {};
m_short_string.byte_count_and_short_string_flag = (byte_count << 1) | SHORT_STRING_FLAG;
return { m_short_string.storage, byte_count };
}
void destroy_string();
union {
ShortString m_short_string;
Detail::StringData const* m_data { nullptr };
};
};
}