// SPDX-License-Identifier: Unlicense

package gigaparsec

//go:generate go run ./internal/bindgen -bindpath bind.go -seqpath seq.go -max 5 -pkg gigaparsec
import (
	"errors"
	"fmt"
	"io"
	"math"
	"slices"
	"strings"
)

type Result[In, Out any] struct {
	consumed, success bool
	value             Out
	next              State[In]
	message           Message
}

func Fail[In, Out any](consumed bool, msg Message) Result[In, Out] {
	return Result[In, Out]{
		consumed: consumed,
		success:  false,
		message:  msg,
	}
}

func Succeed[In, Out any](consumed bool, value Out, next State[In], msg Message) Result[In, Out] {
	return Result[In, Out]{
		success:  true,
		value:    value,
		consumed: consumed,
		next:     next,
		message:  msg,
	}
}

func (r Result[In, Out]) Status() (success bool, value Out, next State[In]) {
	success = r.success
	if success {
		value = r.value
		next = r.next
	}
	return
}

func (r Result[In, Out]) Consumed() bool {
	return r.consumed
}

func (r Result[In, Out]) Consume(consumed bool) Result[In, Out] {
	r.consumed = consumed
	return r
}

func (r Result[In, Out]) Message() Message {
	return r.message
}

func MakeMessage(pos uint64, got string, expected ...string) Message {
	return Message{
		pos:      pos,
		got:      got,
		expected: expected,
	}
}

type Message struct {
	pos      uint64
	got      string
	expected []string
}

func (m Message) Pos() uint64 {
	return m.pos
}

func (m Message) Got() string {
	return m.got
}

func (m Message) Expected() []string {
	return m.expected
}

func (m Message) expect(s string) Message {
	m.expected = []string{s}
	return m
}

func (m Message) String() string {
	s := fmt.Sprintf("bad parse at %d", m.pos)
	if m.got != "" || len(m.expected) > 0 {
		s += ":"
		if m.got != "" {
			s += fmt.Sprintf(" got %v", m.got)
			if len(m.expected) > 0 {
				s += ","
			}
		}
		if len(m.expected) > 0 {
			s += fmt.Sprintf(" expected %v", strings.Join(m.expected, " or "))
		}
	}
	return s
}

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

func MessageEnd(pos uint64, expected ...string) Message {
	return Message{pos: pos, got: "end of input", expected: expected}
}

type ReaderAt[T any] interface {
	ReadAt(p []T, off int64) (n int, err error)
}

type SliceReaderAt[T any] []T

func (s SliceReaderAt[T]) ReadAt(dst []T, off int64) (n int, err error) {
	if off < 0 {
		return 0, errors.New("SliceReaderAt.ReadAt: negative offset")
	}
	if off >= int64(len(s)) {
		return 0, io.EOF
	}
	n = copy(dst, s[off:])
	if n < len(dst) {
		err = io.EOF
	}
	return n, err
}

func MakeState[In any](r ReaderAt[In]) State[In] {
	return State[In]{r: r}
}

type State[In any] struct {
	r   ReaderAt[In]
	pos uint64
}

// Read fills dst with data from this State's position in the underlying source.
// It returns the number of data it read and a new State for the position at which
// the read ended, and an error if the read either (1) failed or (2) reached the
// end of the source before filling dst. All reads from a given State will return
// data from the same position the source.
// If the source had too few data left to fill dst, or if the State's position is
// at or past the end of the source, err will be io.EOF.
func (s State[In]) Read(dst []In) (n uint64, next State[In], err error) {
	if s.pos > math.MaxInt64 {
		return 0, s, io.EOF
	}
	nread, err := s.r.ReadAt(dst, int64(s.pos))
	if nread > 0 {
		s.pos += uint64(nread)
	}
	if nread == len(dst) && err == io.EOF {
		if nread == 0 {
			return 0, s, io.EOF
		}
		return uint64(nread), s, nil
	}
	return uint64(nread), s, err
}

// Pos returns this State's position.
func (s State[In]) Pos() uint64 {
	return s.pos
}

// At returns a State pointing at pos in the same data source.
func (s State[In]) At(pos uint64) State[In] {
	return State[In]{r: s.r, pos: pos}
}

type Parser[In, Out any] func(State[In]) (Result[In, Out], error)

// Label creates a parser identical to p, except that a failed result will
// include label as an expected parse.
func (p Parser[In, Out]) Label(label string) Parser[In, Out] {
	return func(input State[In]) (Result[In, Out], error) {
		result, err := p(input)
		if err != nil || result.Consumed() {
			return result, err
		}
		msg := result.Message()
		if success, value, next := result.Status(); success {
			return Succeed(false, value, next, msg.expect(label)), nil
		}
		return Fail[In, Out](false, msg.expect(label)), nil
	}
}

type ParseError Message

func (pe ParseError) Error() string {
	return Message(pe).String()
}

func Run[In, Out any](p Parser[In, Out], r ReaderAt[In]) (out Out, err error) {
	start := MakeState(r)
	result, err := p(start)
	if err != nil {
		err = fmt.Errorf("Run: %w", err)
		return
	}
	success, out, _ := result.Status()
	if !success {
		err = ParseError(result.Message())
		return
	}
	return
}

// Return creates a parser that always succeeds and returns value without consuming any input.
func Return[In, Out any](value Out) Parser[In, Out] {
	return func(state State[In]) (Result[In, Out], error) {
		return Succeed(false, value, state, MessageOK(state.Pos())), nil
	}
}

// Satisfy creates a parser that attempts to read an input value for which pred returns true.
// If Satisfy succeeds, it returns the matched input value.
func Satisfy[T any](pred func(T) bool) Parser[T, T] {
	return func(state State[T]) (Result[T, T], error) {
		token := make([]T, 1)
		n, next, err := state.Read(token)
		if errors.Is(err, io.EOF) {
			return Fail[T, T](false, MessageEnd(state.Pos())), nil
		}
		if err != nil {
			return Result[T, T]{}, err
		}
		if n != 1 {
			panic(fmt.Sprintf("expected 1 element from Read, but got %d", n))
		}
		if pred(token[0]) {
			return Succeed(true, token[0], next, MessageOK(state.Pos())), nil
		}
		return Fail[T, T](false, MakeMessage(state.Pos(), fmt.Sprint(token))), nil
	}
}

// Match creates a parser that attempts to read an input value equal to x.
// If Match succeeds, it returns the matched input value.
func Match[T comparable](x T) Parser[T, T] {
	expected := fmt.Sprint(x)
	return func(state State[T]) (Result[T, T], error) {
		token := make([]T, 1)
		_, next, err := state.Read(token)
		if errors.Is(err, io.EOF) {
			return Fail[T, T](false, MessageEnd(state.Pos())), nil
		}
		if err != nil {
			return Result[T, T]{}, err
		}
		if token[0] == x {
			return Succeed(true, token[0], next, MessageOK(state.Pos())), nil
		}
		return Fail[T, T](false, MakeMessage(state.Pos(), fmt.Sprint(token), expected)), nil
	}
}

// MatchSlice creates a parser that attempts to read the contents of s from the input.
// If MatchSlice succeeds, it returns a copy of the matched input values.
func MatchSlice[T comparable](s []T) Parser[T, []T] {
	expected := fmt.Sprint(s)
	return func(state State[T]) (Result[T, []T], error) {
		token := make([]T, len(s))
		_, next, err := state.Read(token)
		if errors.Is(err, io.EOF) {
			return Fail[T, []T](false, MessageEnd(state.Pos())), nil
		}
		if err != nil {
			return Result[T, []T]{}, err
		}
		if !slices.Equal(s, token) {
			return Fail[T, []T](false, MakeMessage(state.Pos(), fmt.Sprint(token), expected)), nil
		}
		return Succeed(true, token, next, MessageOK(state.Pos())), nil
	}
}

func Choose[In, Out any](p Parser[In, Out], ps ...Parser[In, Out]) Parser[In, Out] {
	// TODO Check this against the Parsec paper again, and simplify it.
	all := append([]Parser[In, Out]{p}, ps...)
	return func(input State[In]) (Result[In, Out], error) {
		expecteds := make([][]string, 0, len(all))
		var value Out
		var got string
		var failed bool
		for _, q := range all {
			result, err := q(input)
			if err != nil {
				return Result[In, Out]{}, err
			}
			if result.Consumed() {
				return result, nil
			}
			var qMsg Message
			msg := result.Message()
			success, qValue, _ := result.Status()
			if !success {
				qMsg = msg
				failed = true
			} else {
				if failed {
					value = qValue
					failed = false
				}
				qMsg = msg
			}
			if got == "" {
				got = qMsg.got
			}
		}
		msg := MakeMessage(input.Pos(), got, slices.Concat(expecteds...)...)
		if failed {
			return Fail[In, Out](false, msg), nil
		}
		return Succeed(false, value, input, msg), nil
	}
}

// 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 partially succeeded.
func Try[In, Out any](p Parser[In, Out]) Parser[In, Out] {
	return func(input State[In]) (Result[In, Out], error) {
		result, err := p(input)
		if err != nil {
			return result, err
		}
		success, _, _ := result.Status()
		if !success {
			return Fail[In, Out](false, result.Message()), nil
		}
		return result, nil
	}
}

// Map creates a parser that converts the output of p from Out1 to Out2.
func Map[In, Out1, Out2 any](p Parser[In, Out1], f func(Out1) Out2) Parser[In, Out2] {
	return Bind(p, func(out Out1) Parser[In, Out2] {
		return Return[In](f(out))
	})
}

func end[In any](s State[In]) (Result[In, struct{}], error) {
	_, _, err := s.Read([]In{})
	if errors.Is(err, io.EOF) {
		return Succeed(true, struct{}{}, s, MessageOK(s.Pos())), nil
	}
	if err != nil {
		return Result[In, struct{}]{}, fmt.Errorf("End: unexpected error: %w", err)
	}
	return Fail[In, struct{}](false, MakeMessage(s.Pos(), "", "end of input")), nil
}

// End creates a parser that succeeds at the end of the input and fails otherwise.
func End[In any]() Parser[In, struct{}] {
	return end
}

func Pipe[In, Ignore, Through any](p Parser[In, Ignore]) func(Through) Parser[In, Through] {
	return func(t Through) Parser[In, Through] {
		return Bind(p, func(Ignore) Parser[In, Through] {
			return Return[In](t)
		})
	}
}

// Repeat applies p until p fails, and returns the collected outputs.
// It succeeds if and only if p succeeds at least minCount times.
// It consumes if and only if at least one of the applications of p consumes.
func Repeat[In, Out any](minCount int, p Parser[In, Out]) Parser[In, []Out] {
	return func(s State[In]) (Result[In, []Out], error) {
		var values []Out
		var consumed bool
		next := s
		for {
			result, err := p(next)
			if err != nil {
				return Result[In, []Out]{}, fmt.Errorf("AtLeastN: %w", err)
			}
			consumed = consumed || result.Consumed()
			var value Out
			var success bool
			success, value, next = result.Status()
			if !success {
				if len(values) >= minCount {
					return Succeed(consumed, values, next, MessageOK(s.Pos())), nil
				}
				return Fail[In, []Out](consumed, result.Message()), nil
			}
			values = append(values, value)
		}
	}
}