lexer/lexer.go

package lexer

import (
	"fmt"
	"matheval/token"
)

// Tokenize converts an input string into a slice of tokens.
// Returns an error if the input contains invalid characters.
func Tokenize(input string) ([]token.Token, error) {
	var tokens []token.Token
	i := 0

	for i < len(input) {
		ch := input[i]

		// Skip whitespace.
		if ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r' {
			i++
			continue
		}

		// Number: digits and dots.
		if isDigit(ch) || ch == '.' {
			start := i
			hasDot := false
			for i < len(input) && (isDigit(input[i]) || input[i] == '.') {
				if input[i] == '.' {
					if hasDot {
						break // second dot ends this number
					}
					hasDot = true
				}
				i++
			}
			tokens = append(tokens, token.Token{
				Type:    token.Number,
				Literal: input[start:i],
				Pos:     start,
			})
			continue
		}

		// Single-character tokens.
		var typ token.Type
		switch ch {
		case '+':
			typ = token.Plus
		case '-':
			typ = token.Minus
		case '*':
			typ = token.Star
		case '/':
			typ = token.Slash
		case '(':
			typ = token.LParen
		case ')':
			typ = token.RParen
		default:
			return nil, fmt.Errorf("unexpected character %q at position %d", string(ch), i)
		}

		tokens = append(tokens, token.Token{
			Type:    typ,
			Literal: string(ch),
			Pos:     i,
		})
		i++
	}

	// Always append EOF.
	tokens = append(tokens, token.Token{
		Type:    token.EOF,
		Literal: "",
		Pos:     i,
	})

	return tokens, nil
}

func isDigit(ch byte) bool {
	return ch >= '0' && ch <= '9'
}