// This file Copyright © 2022-2023 Mnemosyne LLC.
// It may be used under GPLv2 (SPDX: GPL-2.0-only), GPLv3 (SPDX: GPL-3.0-only),
// or any future license endorsed by Mnemosyne LLC.
// License text can be found in the licenses/ folder.

#pragma once

#include <cstddef>
#include <iterator>
#include <limits>
#include <memory>
#include <string>
#include <string_view>

#include <event2/buffer.h>

#include "error.h"
#include "net.h" // tr_socket_t
#include "tr-assert.h"
#include "utils-ev.h"
#include "utils.h" // for tr_htonll(), tr_ntohll()

namespace libtransmission
{

template<typename T, typename ValueType>
class BufferWriter
{
public:
    BufferWriter(T* out)
        : out_{ out }
    {
        static_assert(sizeof(ValueType) == 1);
    }

    void add(void const* span_begin, size_t span_len)
    {
        auto const* const begin = reinterpret_cast<ValueType const*>(span_begin);
        auto const* const end = begin + span_len;
        out_->insert(std::end(*out_), begin, end);
    }

    template<typename ContiguousContainer>
    void add(ContiguousContainer const& container)
    {
        add(std::data(container), std::size(container));
    }

    template<typename OneByteType>
    void push_back(OneByteType ch)
    {
        add(&ch, 1);
    }

    void add_uint8(uint8_t uch)
    {
        add(&uch, 1);
    }

    void add_uint16(uint16_t hs)
    {
        uint16_t const ns = htons(hs);
        add(&ns, sizeof(ns));
    }

    void add_hton16(uint16_t hs)
    {
        add_uint16(hs);
    }

    void add_uint32(uint32_t hl)
    {
        uint32_t const nl = htonl(hl);
        add(&nl, sizeof(nl));
    }

    void eadd_hton32(uint32_t hl)
    {
        add_uint32(hl);
    }

    void add_uint64(uint64_t hll)
    {
        uint64_t const nll = tr_htonll(hll);
        add(&nll, sizeof(nll));
    }

    void add_hton64(uint64_t hll)
    {
        add_uint64(hll);
    }

    void add_port(tr_port const& port)
    {
        auto nport = port.network();
        add(&nport, sizeof(nport));
    }

private:
    T* out_;
};

class Buffer : public BufferWriter<Buffer, std::byte>
{
public:
    class Iterator
    {
    public:
        using difference_type = long;
        using value_type = std::byte;
        using pointer = value_type*;
        using reference = value_type&;
        using iterator_category = std::random_access_iterator_tag;

        Iterator(evbuffer* buf, size_t offset)
            : buf_{ buf }
        {
            set_offset(offset);
        }

        [[nodiscard]] constexpr value_type& operator*() noexcept
        {
            return static_cast<value_type*>(iov_.iov_base)[iov_offset_];
        }

        [[nodiscard]] constexpr value_type operator*() const noexcept
        {
            return static_cast<value_type*>(iov_.iov_base)[iov_offset_];
        }

        [[nodiscard]] Iterator operator+(size_t n_bytes)
        {
            return Iterator{ buf_, offset() + n_bytes };
        }

        [[nodiscard]] Iterator operator-(size_t n_bytes)
        {
            return Iterator{ buf_, offset() - n_bytes };
        }

        [[nodiscard]] constexpr auto operator-(Iterator const& that) const noexcept
        {
            return offset() - that.offset();
        }

        Iterator& operator++() noexcept
        {
            *this += 1U;
            return *this;
        }

        Iterator& operator+=(size_t n_bytes)
        {
            if (iov_offset_ + n_bytes < iov_.iov_len)
            {
                iov_offset_ += n_bytes;
            }
            else
            {
                inc_offset(n_bytes);
            }

            return *this;
        }

        Iterator& operator--() noexcept
        {
            if (iov_offset_ > 0)
            {
                --iov_offset_;
            }
            else
            {
                set_offset(offset() - 1);
            }
            return *this;
        }

        [[nodiscard]] constexpr bool operator==(Iterator const& that) const noexcept
        {
            return offset() == that.offset();
        }

        [[nodiscard]] constexpr bool operator!=(Iterator const& that) const noexcept
        {
            return offset() != that.offset();
        }

    private:
        [[nodiscard]] constexpr size_t offset() const noexcept
        {
            return ptr_.pos + iov_offset_;
        }

        void inc_offset(size_t increment)
        {
            evbuffer_ptr_set(buf_, &ptr_, iov_offset_ + increment, EVBUFFER_PTR_ADD);
            evbuffer_peek(buf_, std::numeric_limits<ev_ssize_t>::max(), &ptr_, &iov_, 1);
            iov_offset_ = 0;
        }

        void set_offset(size_t offset)
        {
            evbuffer_ptr_set(buf_, &ptr_, offset, EVBUFFER_PTR_SET);
            evbuffer_peek(buf_, std::numeric_limits<ev_ssize_t>::max(), &ptr_, &iov_, 1);
            iov_offset_ = 0;
        }

        evbuffer* buf_;
        evbuffer_ptr ptr_ = {};
        evbuffer_iovec iov_ = {};
        size_t iov_offset_ = 0;
    };

    Buffer(Buffer&&) = default;
    Buffer(Buffer const&) = delete;
    Buffer& operator=(Buffer const&) = delete;
    Buffer& operator=(Buffer&&) = default;

    Buffer()
        : BufferWriter<Buffer, std::byte>{ this }
    {
    }

    template<typename T>
    explicit Buffer(T const& data)
        : BufferWriter<Buffer, std::byte>{ this }
    {
        add(data);
    }

    [[nodiscard]] auto size() const noexcept
    {
        return evbuffer_get_length(buf_.get());
    }

    [[nodiscard]] auto empty() const noexcept
    {
        return evbuffer_get_length(buf_.get()) == 0;
    }

    [[nodiscard]] auto begin() noexcept
    {
        return Iterator{ buf_.get(), 0U };
    }

    [[nodiscard]] auto end() noexcept
    {
        return Iterator{ buf_.get(), size() };
    }

    [[nodiscard]] auto begin() const noexcept
    {
        return Iterator{ buf_.get(), 0U };
    }

    [[nodiscard]] auto end() const noexcept
    {
        return Iterator{ buf_.get(), size() };
    }

    template<typename T>
    [[nodiscard]] TR_CONSTEXPR20 bool starts_with(T const& needle) const
    {
        auto const n_bytes = std::size(needle);
        auto const needle_begin = reinterpret_cast<std::byte const*>(std::data(needle));
        auto const needle_end = needle_begin + n_bytes;
        return n_bytes <= size() && std::equal(needle_begin, needle_end, cbegin());
    }

    [[nodiscard]] std::string to_string() const
    {
        auto str = std::string{};
        str.resize(size());
        evbuffer_copyout(buf_.get(), std::data(str), std::size(str));
        return str;
    }

    auto to_buf(void* tgt, size_t n_bytes)
    {
        return evbuffer_remove(buf_.get(), tgt, n_bytes);
    }

    [[nodiscard]] auto to_uint8()
    {
        auto tmp = uint8_t{};
        to_buf(&tmp, sizeof(tmp));
        return tmp;
    }

    [[nodiscard]] uint16_t to_uint16()
    {
        auto tmp = uint16_t{};
        to_buf(&tmp, sizeof(tmp));
        return ntohs(tmp);
    }

    [[nodiscard]] uint32_t to_uint32()
    {
        auto tmp = uint32_t{};
        to_buf(&tmp, sizeof(tmp));
        return ntohl(tmp);
    }

    [[nodiscard]] uint64_t to_uint64()
    {
        auto tmp = uint64_t{};
        to_buf(&tmp, sizeof(tmp));
        return tr_ntohll(tmp);
    }

    void drain(size_t n_bytes)
    {
        evbuffer_drain(buf_.get(), n_bytes);
    }

    void clear()
    {
        drain(size());
    }

    // Returns the number of bytes written. Check `error` for error.
    size_t to_socket(tr_socket_t sockfd, size_t n_bytes, tr_error** error = nullptr)
    {
        EVUTIL_SET_SOCKET_ERROR(0);
        auto const res = evbuffer_write_atmost(buf_.get(), sockfd, n_bytes);
        auto const err = EVUTIL_SOCKET_ERROR();
        if (res >= 0)
        {
            return static_cast<size_t>(res);
        }
        tr_error_set(error, err, tr_net_strerror(err));
        return 0;
    }

    [[nodiscard]] std::pair<std::byte*, size_t> pullup()
    {
        return { reinterpret_cast<std::byte*>(evbuffer_pullup(buf_.get(), -1)), size() };
    }

    [[nodiscard]] std::byte const* data() const
    {
        return reinterpret_cast<std::byte*>(evbuffer_pullup(buf_.get(), -1));
    }

    [[nodiscard]] auto pullup_sv()
    {
        auto const [buf, buflen] = pullup();
        return std::string_view{ reinterpret_cast<char const*>(buf), buflen };
    }

    void reserve(size_t n_bytes)
    {
        evbuffer_expand(buf_.get(), n_bytes - size());
    }

    size_t add_socket(tr_socket_t sockfd, size_t n_bytes, tr_error** error = nullptr)
    {
        EVUTIL_SET_SOCKET_ERROR(0);
        auto const res = evbuffer_read(buf_.get(), sockfd, static_cast<int>(n_bytes));
        auto const err = EVUTIL_SOCKET_ERROR();

        if (res > 0)
        {
            return static_cast<size_t>(res);
        }

        if (res == 0)
        {
            tr_error_set_from_errno(error, ENOTCONN);
        }
        else
        {
            tr_error_set(error, err, tr_net_strerror(err));
        }

        return {};
    }

    template<typename T>
    void insert([[maybe_unused]] Iterator iter, T const* const begin, T const* const end)
    {
        TR_ASSERT(iter == this->end()); // tr_buffer only supports appending
        evbuffer_add(buf_.get(), begin, end - begin);
    }

private:
    evhelpers::evbuffer_unique_ptr buf_{ evbuffer_new() };

    [[nodiscard]] Iterator cbegin() const noexcept
    {
        return Iterator{ buf_.get(), 0U };
    }

    [[nodiscard]] Iterator cend() const noexcept
    {
        return Iterator{ buf_.get(), size() };
    }
};

} // namespace libtransmission