zircon/system/ulib/fbl/include/fbl/string.h

// Copyright 2017 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef FBL_STRING_H_
#define FBL_STRING_H_

#include <atomic>
#include <fbl/alloc_checker.h>
#include <fbl/string_piece.h>
#include <fbl/string_traits.h>
#include <initializer_list>
#include <type_traits>
#include <zircon/compiler.h>

namespace fbl {
namespace tests {
struct StringTestHelper;
} // namespace tests

// A string with immutable contents.
//
// fbl::String is designed to resemble std::string except that its content
// is immutable.  This makes it easy to share string buffers so that copying
// strings does not incur any allocation cost.
//
// Empty string objects do not incur any allocation.  Non-empty strings are
// stored on the heap.  Note that fbl::String does not have a null state
// distinct from the empty state.
//
// The content of a fbl::String object is always stored with a null terminator
// so that |c_str()| is fast.  However, be aware that the string may also contain
// embedded null characters (this is not checked by the implementation).
class String {
public:
    // Creates an empty string.
    // Does not allocate heap memory.
    String() { InitWithEmpty(); }

    // Creates a copy of another string.
    // Does not allocate heap memory.
    String(const String& other)
        : data_(other.data_) {
        AcquireRef(data_);
    }

    // Move constructs from another string.
    // The other string is set to empty.
    // Does not allocate heap memory.
    String(String&& other)
        : data_(other.data_) {
        other.InitWithEmpty();
    }

    // Creates a string from the contents of a null-terminated C string.
    // Allocates heap memory only if |data| is non-empty.
    // |data| must not be null.
    String(const char* data) {
        Init(data, constexpr_strlen(data));
    }

    // Creates a string from the contents of a null-terminated C string.
    // Allocates heap memory only if |data| is non-empty.
    // |data| and |ac| must not be null.
    String(const char* data, AllocChecker* ac) {
        Init(data, constexpr_strlen(data), ac);
    }

    // Creates a string from the contents of a character array of given length.
    // Allocates heap memory only if |length| is non-zero.
    // |data| must not be null.
    String(const char* data, size_t length) {
        Init(data, length);
    }

    // Creates a string from the contents of a character array of given length.
    // Allocates heap memory only if |length| is non-zero.
    // |data| and |ac| must not be null.
    String(const char* data, size_t length, AllocChecker* ac) {
        Init(data, length, ac);
    }

    // Creates a string with |count| copies of |ch|.
    // Allocates heap memory only if |count| is non-zero.
    String(size_t count, char ch) {
        Init(count, ch);
    }

    // Creates a string with |count| copies of |ch|.
    // Allocates heap memory only if |count| is non-zero.
    // |ac| must not be null.
    String(size_t count, char ch, AllocChecker* ac) {
        Init(count, ch, ac);
    }

    // Creates a string from the contents of a string piece.
    // Allocates heap memory only if |piece.length()| is non-zero.
    String(const StringPiece& piece)
        : String(piece.data(), piece.length()) {}

    // Creates a string from the contents of a string piece.
    // Allocates heap memory only if |piece.length()| is non-zero.
    // |ac| must not be null.
    String(const StringPiece& piece, AllocChecker* ac)
        : String(piece.data(), piece.length(), ac) {}

    // Creates a string from a string-like object.
    // Allocates heap memory only if the length of |value| is non-zero.
    //
    // Works with various string types including fbl::String, fbl::StringView,
    // std::string, and std::string_view.
    template <typename T, typename = typename std::enable_if<is_string_like<T>::value>::type>
    constexpr String(const T& value)
        : String(GetStringData(value), GetStringLength(value)) {}

    // Destroys the string.
    ~String() { ReleaseRef(data_); }

    // Returns a pointer to the null-terminated contents of the string.
    const char* data() const { return data_; }
    const char* c_str() const { return data(); }

    // Returns the length of the string, excluding its null terminator.
    size_t length() const { return *length_field_of(data_); }
    size_t size() const { return length(); }

    // Returns true if the string's length is zero.
    bool empty() const { return length() == 0U; }

    // Character iterators, excluding the null terminator.
    const char* begin() const { return data(); }
    const char* cbegin() const { return data(); }
    const char* end() const { return data() + length(); }
    const char* cend() const { return data() + length(); }

    // Gets the character at the specified index.
    // Position must be greater than or equal to 0 and less than |length()|.
    const char& operator[](size_t pos) const { return data()[pos]; }

    // Performs a lexicographical character by character comparison.
    // Returns a negative value if |*this| comes before |other| in lexicographical order.
    // Returns zero if the strings are equivalent.
    // Returns a positive value if |*this| comes after |other| in lexicographical order.
    int compare(const String& other) const;

    // Sets this string to empty.
    // Does not allocate heap memory.
    void clear();

    // Swaps the contents of this string with another string.
    // Does not allocate heap memory.
    void swap(String& other);

    // Assigns this string to a copy of another string.
    // Does not allocate heap memory.
    String& operator=(const String& other);

    // Move assigns from another string.
    // The other string is set to empty.
    // Does not allocate heap memory.
    String& operator=(String&& other) noexcept;

    // Assigns this string from the contents of a null-terminated C string.
    // Allocates heap memory only if |data| is non-empty.
    // |data| must not be null.
    String& operator=(const char* data) {
        Set(data);
        return *this;
    }

    // Assigns this string from the contents of a string piece.
    // Allocates heap memory only if |piece.length()| is non-zero.
    String& operator=(const StringPiece& piece) {
        Set(piece);
        return *this;
    }

    // Assigns this string from the contents of a string-like object.
    // Allocates heap memory only if the length of |value| is non-zero.
    //
    // Works with various string types including fbl::String, fbl::StringView,
    // std::string, and std::string_view.
    template <typename T, typename = typename std::enable_if<is_string_like<T>::value>::type>
    String& operator=(const T& value) {
        Set(GetStringData(value), GetStringLength(value));
        return *this;
    }

    // Assigns this string from the contents of a null-terminated C string.
    // Allocates heap memory only if |data| is non-empty.
    // |data| must not be null.
    void Set(const char* data) {
        Set(data, constexpr_strlen(data));
    }

    // Assigns this string from the contents of a null-terminated C string.
    // Allocates heap memory only if |data| is non-empty.
    // |data| and |ac| must not be null.
    void Set(const char* data, AllocChecker* ac) {
        Set(data, constexpr_strlen(data), ac);
    }

    // Assigns this string from the contents of a character array of given length.
    // Allocates heap memory only if |length| is non-zero.
    // |data| must not be null.
    void Set(const char* data, size_t length);

    // Assigns this string from the contents of a character array of given length.
    // Allocates heap memory only if |length| is non-zero.
    // |data| and |ac| must not be null.
    void Set(const char* data, size_t length, AllocChecker* ac);

    // Assigns this string with |count| copies of |ch|.
    // Allocates heap memory only if |count| is non-zero.
    void Set(size_t count, char ch) {
        ReleaseRef(data_);
        Init(count, ch);
    }

    // Assigns this string with |count| copies of |ch|.
    // Allocates heap memory only if |count| is non-zero.
    // |ac| must not be null.
    void Set(size_t count, char ch, AllocChecker* ac) {
        ReleaseRef(data_);
        Init(count, ch, ac);
    }

    // Assigns this string from the contents of a string piece.
    // Allocates heap memory only if |piece.length()| is non-zero.
    void Set(const StringPiece& piece) {
        Set(piece.data(), piece.length());
    }

    // Assigns this string from the contents of a string piece.
    // Allocates heap memory only if |piece.length()| is non-zero.
    // |ac| must not be null.
    void Set(const StringPiece& piece, AllocChecker* ac) {
        Set(piece.data(), piece.length(), ac);
    }

    // Creates a string piece backed by the string.
    // The string piece does not take ownership of the data so the string
    // must outlast the string piece.
    StringPiece ToStringPiece() const {
        return StringPiece(data(), length());
    }

    // Concatenates the specified strings.
    static String Concat(std::initializer_list<String> strings);

    // Concatenates the specified strings.
    // |ac| must not be null.
    static String Concat(std::initializer_list<String> strings,
                         AllocChecker* ac);

private:
    friend struct fbl::tests::StringTestHelper;

    explicit String(char* data, decltype(nullptr) /*overload disambiguation*/)
        : data_(data) {}

    // A string buffer consists of a length followed by a reference count
    // followed by a null-terminated string.  To make access faster, we offset
    // the |data_| pointer to point at the first byte of the content instead of
    // at the beginning of the string buffer itself.
    static constexpr size_t kLengthFieldOffset = 0U;
    static constexpr size_t kRefCountFieldOffset = sizeof(size_t);
    static constexpr size_t kDataFieldOffset = sizeof(size_t) + sizeof(std::atomic_uint);

    static size_t* length_field_of(char* data) {
        return reinterpret_cast<size_t*>(data - kDataFieldOffset + kLengthFieldOffset);
    }
    static std::atomic_uint* ref_count_field_of(char* data) {
        return reinterpret_cast<std::atomic_uint*>(data - kDataFieldOffset + kRefCountFieldOffset);
    }
    static constexpr size_t buffer_size(size_t length) {
        return kDataFieldOffset + length + 1U;
    }

    // For use by test code only.
    unsigned int ref_count() const {
        return ref_count_field_of(data_)->load(std::memory_order_relaxed);
    }

    // Storage for an empty string.
    struct EmptyBuffer {
        size_t length{0U};
        std::atomic_uint ref_count{1U};
        char nul{0};
    };
    static_assert(offsetof(EmptyBuffer, length) == kLengthFieldOffset, "");
    static_assert(offsetof(EmptyBuffer, ref_count) == kRefCountFieldOffset, "");
    static_assert(offsetof(EmptyBuffer, nul) == kDataFieldOffset, "");

    static EmptyBuffer gEmpty;

    void Init(const char* data, size_t length);
    void Init(const char* data, size_t length, AllocChecker* ac);
    void Init(size_t count, char ch);
    void Init(size_t count, char ch, AllocChecker* ac);
    void InitWithEmpty();

    static char* AllocData(size_t length);
    static char* AllocData(size_t length, AllocChecker* ac);
    static char* InitData(void* buffer, size_t length);
    static void AcquireRef(char* data);
    static void ReleaseRef(char* data);

    char* data_;
};

bool operator==(const String& lhs, const String& rhs);

inline bool operator!=(const String& lhs, const String& rhs) {
    return !(lhs == rhs);
}

inline bool operator<(const String& lhs, const String& rhs) {
    return lhs.compare(rhs) < 0;
}

inline bool operator>(const String& lhs, const String& rhs) {
    return lhs.compare(rhs) > 0;
}

inline bool operator<=(const String& lhs, const String& rhs) {
    return lhs.compare(rhs) <= 0;
}

inline bool operator>=(const String& lhs, const String& rhs) {
    return lhs.compare(rhs) >= 0;
}

} // namespace fbl

#endif  // FBL_STRING_H_