package saccharine

import (
	"fmt"
	"unicode"

	"git.maximhutz.com/max/lambda/pkg/iterator"
	"git.maximhutz.com/max/lambda/pkg/token"
)

// Pulls the next token from an iterator over runes. If it cannot, it will
// return nil. If an error occurs, it will return that.
func scanToken(i *iterator.Iterator[rune]) (*Token, error) {
	index := i.Index()

	if i.Done() {
		return nil, nil
	}

	letter, err := i.Next()
	if err != nil {
		return nil, fmt.Errorf("cannot produce next token: %w", err)
	}

	switch {
	case letter == '(':
		return token.New(TokenOpenParen, index), nil
	case letter == ')':
		return token.New(TokenCloseParen, index), nil
	case letter == '.':
		return token.New(TokenDot, index), nil
	case letter == '\\':
		return token.New(TokenSlash, index), nil
	case letter == '\n':
		return token.New(TokenSoftBreak, index), nil
	case letter == '{':
		return token.New(TokenOpenBrace, index), nil
	case letter == '}':
		return token.New(TokenCloseBrace, index), nil
	case letter == ':':
		if _, err := token.ScanCharacter(i, '='); err != nil {
			return nil, err
		} else {
			return token.New(TokenAssign, index), nil
		}
	case letter == ';':
		return token.New(TokenHardBreak, index), nil
	case letter == '#':
		// Skip everything until the next newline or EOF.
		i.While(func(r rune) bool { return r != '\n' })
		return nil, nil
	case unicode.IsSpace(letter):
		return nil, nil
	case token.IsVariable(letter):
		return token.ScanAtom(i, letter, TokenAtom, index), nil
	}

	return nil, fmt.Errorf("unknown character '%v'", string(letter))
}

// scan a string into tokens.
func scan(input string) ([]Token, error) {
	return token.Scan(input, scanToken)
}