package gigaparsec

import (
	"errors"
	"fmt"
	"io"

	"git.codemonkeysoftware.net/b/gigaparsec/cursor"
)

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

type Result[In, Out any] struct {
	Value Out
	State[In]
	Message
}

type Message struct {
	Pos      uint64
	Got      string
	Expected []string
}

func MessageOK(pos uint64) Message { return Message{Pos: pos} }

func MessageEnd(pos uint64) Message { return Message{Pos: pos, Got: "end of input"} }

type ParseError Message

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

type State[T any] cursor.Cursor[T]

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

func Return[In, Out any](value Out) Parser[In, Out] {
	return func(state State[In]) (bool, Result[In, Out], error) {
		return false, Result[In, Out]{
			Value:   value,
			State:   state,
			Message: MessageOK(state.Pos()),
		}, nil
	}
}

func Satisfy[T any](pred func(T) bool) Parser[T, T] {
	return func(state State[T]) (bool, Result[T, T], error) {
		token := make([]T, 1)
		n, next, err := state.Read(token)
		if errors.Is(err, io.EOF) {
			return false, Result[T, T]{}, ParseError(MessageEnd(state.Pos()))
		}
		if n != 1 {
			panic(fmt.Sprintf("expected 1 element from Read, but got %d", n))
		}
		if pred(token[0]) {
			return true, Result[T, T]{
				Value:   token[0],
				State:   next,
				Message: MessageOK(state.Pos()),
			}, nil
		}
		return false, Result[T, T]{}, ParseError{
			Pos: state.Pos(),
			Got: fmt.Sprint(token),
		}
	}
}

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

// TODO Add error propagation
func Choose[In, Out any](p, q Parser[In, Out]) Parser[In, Out] {
	return func(input State[In]) (bool, Result[In, Out], 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
	}
}

// Try behaves identically to p, except that if p returns an error,
// Try will pretend that no input was consumed. This allows infinite
// lookahead: Since Choose only calls another parser when the previous
// parser consumed nothing, Try will allow backing out of a complex
// parser that did partially succeeded.
func Try[In, Out any](p Parser[In, Out]) Parser[In, Out] {
	return func(input State[In]) (bool, Result[In, Out], error) {
		consumed, reply, err := p(input)
		if err != nil {
			return false, Result[In, Out]{}, err
		}
		return consumed, reply, err
	}
}