aoc2023/day-1/two.go

package main

import (
	"fmt"
	"regexp"
	"strconv"
)

var replaceMap = map[string]string{
	"one":   "1",
	"two":   "2",
	"three": "3",
	"four":  "4",
	"five":  "5",
	"six":   "6",
	"seven": "7",
	"eight": "8",
	"nine":  "9",
}

var r = regexp.MustCompile(`\d|one|two|three|four|five|six|seven|eight|nine`)
var rr = regexp.MustCompile(`\d|eno|owt|eerht|ruof|evif|xis|neves|thgie|enin`)

// forward parses a single line from start to last
// Match all substrings matching `\d|one|two|three|four|five|six|seven|eight|nine`
// Returns the first match
func forward(s string) string {
	m_all := r.FindAllString(s, -1)
	match := m_all[0]
	match, ok := replaceMap[match]
	if !ok {
		match = m_all[0]
	}

	return match
}

// backward parses a single line in reverse order
// Match all substrings matching `\d|eno|owt|eerht|ruof|evif|xis|neves|thgie|enin`
// Returns the first match
func backward(s string) string {
	m_all := rr.FindAllString(s, -1)
	match := m_all[0]
	match, ok := replaceMap[reverse(match)]
	if !ok {
		match = m_all[0]
	}

	return match
}

// two takes a string array and for each line:
// Search by regex all substrings matching numeric (and word representations of numeric) values.
// It then combines the first digit and the last digit (in that order) to form a single two-digit number.
// Returns the sum of all calibrated values.
func two(lines []string) (int, error) {
	sum := 0
	for _, line := range lines {
		m1 := forward(line)
		m2 := backward(reverse(line))
		n, err := strconv.Atoi(fmt.Sprintf("%s%s", m1, m2))
		if err != nil {
			return 0, err
		}
		sum += n
	}
	return sum, nil
}