package source

import (
	"bufio"
	"errors"
	"io"
)

type Walker struct {
	source *bufio.Reader

	row int
	col int
}

func NewWalker(source io.Reader) *Walker {
	return &Walker{
		source: bufio.NewReader(source),
		row:    0,
		col:    0,
	}
}

func (w *Walker) Loc() Loc {
	return Loc{
		Row: w.row,
		Col: w.col,
	}
}

func (w *Walker) Next() (rune, bool, error) {
	r, _, err := w.source.ReadRune()
	if err != nil {
		if errors.Is(err, io.EOF) {
			return 0, true, nil
		}

		return 0, false, err
	}

	if r == '\n' {
		w.row++
		w.col = 0
	} else {
		w.col++
	}

	return r, false, nil
}

func (w *Walker) Consume(want rune) (bool, error) {
	c, _, err := w.Next()
	if err != nil {
		return false, err
	}

	if c != want {
		return true, ErrUnexpectedChar{
			Expected: want,
			Actual:   c,
		}
	}

	return true, nil
}

func (w *Walker) Peek() (rune, bool, error) {
	r, _, err := w.source.ReadRune()
	defer w.source.UnreadRune()

	if err != nil {
		if errors.Is(err, io.EOF) {
			return 0, true, nil
		}

		return 0, false, err
	}

	return r, false, nil
}