// SPDX-License-Identifier: MPL-2.0
// Copyright © 2020 Skyline Team and Contributors (https://github.com/skyline-emu/)

#pragma once

#include <concepts>
#include <common.h>

namespace skyline::vfs {
    /**
     * @brief The Backing class provides abstract access to a storage device, all access can be done without using a specific backing
     */
    class Backing {
      protected:
        virtual size_t ReadImpl(span <u8> output, size_t offset) = 0;

        virtual size_t WriteImpl(span <u8> input, size_t offset) {
            throw exception("This backing does not support being written to");
        }

        virtual void ResizeImpl(size_t pSize) {
            throw exception("This backing does not support being resized");
        }

      public:
        union Mode {
            struct {
                bool read : 1; //!< The backing is readable
                bool write : 1; //!< The backing is writable
                bool append : 1; //!< The backing can be appended
            };
            u32 raw;
        } mode;
        static_assert(sizeof(Mode) == 0x4);

        size_t size; //!< The size of the backing in bytes

        /**
         * @param mode The mode to use for the backing
         * @param size The initial size of the backing
         */
        Backing(Mode mode = {true, false, false}, size_t size = 0) : mode(mode), size(size) {}

        /* Delete the move constructor to prevent multiple instances of the same backing */
        Backing(const Backing &) = delete;

        Backing &operator=(const Backing &) = delete;

        virtual ~Backing() = default;

        /**
         * @brief Read bytes from the backing at a particular offset to a buffer
         * @param output The object to write the data read to
         * @param offset The offset to start reading from
         * @return The amount of bytes read
         */
        size_t ReadUnchecked(span <u8> output, size_t offset = 0) {
            if (!mode.read)
                LOGW("Attempting to read a backing that is not readable");

            return ReadImpl(output, offset);
        };

        /**
         * @brief Read bytes from the backing at a particular offset to a buffer and check to ensure the full size was read
         * @param output The object to write the data read to
         * @param offset The offset to start reading from
         * @return The amount of bytes read
         */
        size_t Read(span <u8> output, size_t offset = 0) {
            if (offset > size)
                throw exception("Offset cannot be past the end of a backing");

            if ((size - offset) < output.size())
                throw exception("Trying to read past the end of a backing: 0x{:X}/0x{:X} (Offset: 0x{:X})", output.size(), size, offset);

            if (ReadUnchecked(output, offset) != output.size())
                LOGW("Failed to read the requested size from backing");

            return size;
        };

        /**
         * @brief Implicit casting for reading into spans of different types
         */
        template<typename T> requires (!std::same_as<T, u8>)
        size_t Read(span <T> output, size_t offset = 0) {
            return Read(output.template cast<u8>(), offset);
        }

        /**
         * @brief Read bytes from the backing at a particular offset into an object
         * @param offset The offset to start reading from
         * @return The object that was read
         */
        template<typename T>
        T Read(size_t offset = 0) {
            T object;
            Read(span(reinterpret_cast<u8 *>(&object), sizeof(T)), offset);
            return object;
        }

        /**
         * @brief Writes from a buffer to a particular offset in the backing
         * @param input The data to write to the backing
         * @param offset The offset where the input buffer should be written
         * @return The amount of bytes written
         */
        size_t Write(span <u8> input, size_t offset = 0) {
            if (!mode.write)
                LOGW("Attempting to write to a backing that is not writable");

            if (input.size() > (static_cast<ssize_t>(size) - static_cast<ssize_t>(offset))) {
                if (mode.append)
                    Resize(offset + input.size());
                else
                    LOGW("Trying to write past the end of a non-appendable backing: 0x{:X}/0x{:X} (Offset: 0x{:X})", input.size(), size, offset);
            }

            return WriteImpl(input, offset);
        }

        /**
         * @brief Writes from an object into a particular offset in the backing
         * @param object The object to write to the backing
         * @param offset The offset where the input should be written
         */
        template<typename T>
        void WriteObject(const T &object, size_t offset = 0) {
            size_t lSize;
            if ((lSize = Write(span(reinterpret_cast<u8 *>(&object), sizeof(T)), offset)) != sizeof(T))
                LOGW("Object wasn't written fully into output backing: {}/{}", lSize, sizeof(T));
        }

        /**
         * @brief Resizes a backing to the given size
         * @param pSize The new size for the backing
         */
        void Resize(size_t pSize) {
            ResizeImpl(pSize);
        }
    };
}