serenity/AK/StringBase.h

/*
 * 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 };
    };
};

}