package gigaparsec

import (
	"errors"
	"fmt"
)

// TODO Everything. See https://smunix.github.io/dev.stephendiehl.com/fun/002_parsers.html.

var ErrNoParse = errors.New("no parse")

type Result[T any] struct {
	Value T
	State
	Message
}

type Message struct {
	Pos
	Msg      string
	Expected []string
}

type ParseError struct {
	Message
}

func (pe ParseError) Error() string {
	return fmt.Sprintf("parse error: %d: %s", pe.Message.Pos, pe.Message.Msg)
}

type Pos uint64

type State struct {
	Pos
	Input []byte
}

type Parser[T any] func(State) (consumed bool, reply Result[T], err error)

func Return[T any](value T) Parser[T] {
	return func(state State) (bool, Result[T], error) {
		return false, Result[T]{Value: value, State: state}, nil
	}
}

func Satisfy(pred func(byte) bool) Parser[byte] {
	return func(state State) (bool, Result[byte], error) {
		if len(state.Input) == 0 {
			return false, Result[byte]{}, ErrNoParse
		}
		b := state.Input[0]
		if pred(b) {
			return true, Result[byte]{Value: b, State: state.Input[1:]}, nil
		}
		return false, Result[byte]{}, ErrNoParse
	}
}

func Bind[A, B any](p Parser[A], f func(A) Parser[B]) Parser[B] {
	return func(input State) (bool, Result[B], error) {
		consumed, replyA, err := p(input)
		if err != nil {
			return false, Result[B]{}, err
		}
		consumed2, replyB, err := f(replyA.Value)(replyA.Rest)
		return consumed || consumed2, replyB, err
	}
}

func Choose[A any](p, q Parser[A]) Parser[A] {
	return func(input State) (bool, Result[A], error) {
		consumedP, replyP, errP := p(input)
		if consumedP {
			return consumedP, replyP, errP
		}
		if errP != nil {
			return q(input)
		}
		consumedQ, replyQ, errQ := q(input)
		if consumedQ {
			return consumedQ, replyQ, errQ
		}
		return consumedP, replyP, errP
	}
}

func Try[A any](p Parser[A]) Parser[A] {
	return func(input State) (bool, Result[A], error) {
		consumed, reply, err := p(input)
		if err != nil {
			return false, Result[A]{}, err
		}
		return consumed, reply, err
	}
}