/**
 * Alee Forth: A portable and concise Forth implementation in modern C++.
 * Copyright (C) 2023  Clyne Sullivan <clyne@bitgloo.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

#ifndef ALEEFORTH_STATE_HPP
#define ALEEFORTH_STATE_HPP

#include "dictionary.hpp"
#include "types.hpp"

#include <csetjmp>
#include <cstddef>
#include <tuple>

constexpr unsigned DataStackSize = 16;
constexpr unsigned ReturnStackSize = 16;

class State
{
    friend class CoreWords;

public:
    Addr ip = 0;
    Dictionary& dict;
    void (*input)(State&); // User-provided function to collect "stdin" input.

    constexpr State(Dictionary& d, void (*i)(State&)):
        dict(d), input(i) {}

    bool compiling() const;
    void compiling(bool);

    /**
     * Begins execution at the given execution token.
     * If the token is a CoreWord, this function exits after its execution.
     * Otherwise, execution continues until the word's execution completes.
     * Encountering an error will cause this function to exit immediately.
     */
    Error execute(Addr);

    /**
     * Clears the data and return stacks, sets ip to zero, and clears the
     * compiling flag.
     */
    void reset();

    std::size_t size() const noexcept;
    std::size_t rsize() const noexcept;

    inline void push(Cell value) {
        verify(dsp < dstack + DataStackSize, Error::push);
        *dsp++ = value;
    }

    inline Cell pop() {
        verify(dsp > dstack, Error::pop);
        return *--dsp;
    }

    inline void pushr(Cell value) {
        verify(rsp < rstack + ReturnStackSize, Error::pushr);
        *rsp++ = value;
    }

    inline Cell popr() {
        verify(rsp > rstack, Error::popr);
        return *--rsp;
    }

    inline Cell& top() {
        verify(dsp > dstack, Error::top);
        return *(dsp - 1);
    }

    inline Cell& pick(std::size_t i) {
        verify(dsp - i > dstack, Error::pick);
        return *(dsp - i - 1);
    }

    // Advances the instruction pointer and returns that cell's contents.
    inline Cell beyondip() {
        ip += sizeof(Cell);
        return dict.read(ip);
    }

private:
    Cell dstack[DataStackSize] = {};
    Cell rstack[ReturnStackSize] = {};
    Cell *dsp = dstack;
    Cell *rsp = rstack;
    std::jmp_buf jmpbuf = {}; // Used when catching execution errors.

    inline void verify(bool condition, Error error) {
        if (!condition)
            std::longjmp(jmpbuf, static_cast<int>(error));
    }

    /**
     * Saves execution state so that a new execution can begin.
     * Used for EVALUATE.
     */
    std::pair<Addr, std::jmp_buf> save();

    /**
     * Reloads the given execution state.
     */
    void load(const std::pair<Addr, std::jmp_buf>&);
};

#endif // ALEEFORTH_STATE_HPP