Solve day 1 part 2

day1/main.go

@@ -5,55 +5,147 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"slices"
 	"strconv"
 	"strings"
-	"unicode"
 )
 
-func filterChars(line string, criteria func(rune) bool) string {
-	validChars := []rune{}
-	for _, char := range line {
-		if criteria(char) {
-			validChars = append(validChars, char)
+// filterToMatches find all overlapping matches of the stringset "valid" in "line"
+func filterToMatches(line string, valid []string) []string {
+	type match struct {
+		idx   int
+		value string
+	}
+
+	matches := []match{}
+	start := 0
+	// While we _could_ advance by the length of the string this
+	// challenge allows for overlapping strings. We can be a bit
+	// naive by just advancing one char and working that way
+	for i := range line {
+		for _, matchCandidate := range valid {
+			if strings.HasPrefix(line[i:], matchCandidate) {
+				matches = append(matches, match{idx: start, value: matchCandidate})
+				break
+			}
 		}
 	}
 
-	return string(validChars)
+	slices.SortFunc(matches, func(a, b match) int {
+		return a.idx - b.idx
+	})
+
+	output := []string{}
+	for _, item := range matches {
+		output = append(output, item.value)
+	}
+
+	return output
 }
 
-func getCoordinate(line string) (int, error) {
-	candidates := filterChars(line, unicode.IsDigit)
+// smashToDigits will "smash" two digits together to form a two digit number
+func smashDigits(digit1, digit2 int) int {
+	return digit1*10 + digit2
+}
+
+// mapToDigits will convert a list of strings to an int, and return the aggregated results
+// or return an error immediately if one is encountered. In other words, it is assumed
+// every item in this list is a valid number according to the toDigit function
+func mapToDigits(items []string, toDigit func(string) (int, error)) ([]int, error) {
+	output := []int{}
+	for _, item := range items {
+		n, err := toDigit(item)
+		if err != nil {
+			return nil, fmt.Errorf("convert %s: %w", item, err)
+		}
+
+		output = append(output, n)
+	}
+
+	return output, nil
+}
+
+// getCoordinate gets a coordinate from the given calibration value
+func getCoordinate(line string, validNumbers []string, convertToNumber func(string) (int, error)) (int, error) {
+	candidates := filterToMatches(line, validNumbers)
 	if len(candidates) == 0 {
 		return 0, errors.New("invalid calibration value")
 	}
 
-	if len(candidates) == 1 {
-		value, err := strconv.Atoi(candidates)
-		if err != nil {
-			// Programmer error, given the above filter
-			panic("invalid numeric string " + candidates)
-		}
-
-		return value*10 + value, nil
+	digits, err := mapToDigits(candidates, convertToNumber)
+	if err != nil {
+		// Programmer error, given the above filter
+		panic(err)
 	}
 
-	value1 := int(candidates[0] - '0')
-	value2 := int(candidates[len(candidates)-1] - '0')
-	return value1*10 + value2, nil
+	if len(digits) == 1 {
+		return smashDigits(digits[0], digits[0]), nil
+	}
+
+	value1 := digits[0]
+	value2 := digits[len(candidates)-1]
+	return smashDigits(value1, value2), nil
 }
 
-func part1(input []string) int {
+func solve(input []string, validNumbers []string, convert func(string) (int, error)) (int, error) {
 	total := 0
 	for _, line := range input {
-		coordinate, err := getCoordinate(line)
+		coordinate, err := getCoordinate(line, validNumbers, convert)
 		if err != nil {
-			panic(fmt.Sprintf("No valid coordinate on line '%s': %s\n", line, err))
+			return 0, fmt.Errorf("no valid coordinate on line '%s': %s", line, err)
 		}
 
 		total += coordinate
 	}
 
-	return total
+	return total, nil
+}
+
+func part1(input []string) int {
+	digits := []string{
+		"0", "1", "2", "3", "4", "5", "6", "7", "8", "9",
+	}
+
+	result, err := solve(input, digits, strconv.Atoi)
+	if err != nil {
+		panic(err)
+	}
+
+	return result
+}
+
+func part2(input []string) int {
+	digits := []string{
+		"0", "1", "2", "3", "4", "5", "6", "7", "8", "9",
+	}
+
+	words := []string{
+		"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine",
+	}
+
+	allPossible := make([]string, len(digits), len(digits)+len(words))
+	copy(allPossible, digits)
+	allPossible = append(allPossible, words...)
+
+	result, err := solve(input, allPossible, func(s string) (int, error) {
+		n, err := strconv.Atoi(s)
+		if err == nil {
+			return n, nil
+		}
+
+		idx := slices.Index(words, s)
+		if idx != -1 {
+			return idx, nil
+		}
+
+		return 0, errors.New("invalid digit")
+	})
+
+	if err != nil {
+		panic(err)
+	}
+
+	return result
 }
 
 func main() {
@@ -79,4 +171,5 @@ func main() {
 	inputLines := strings.Split(strings.TrimSpace(input), "\n")
 
 	fmt.Printf("Part 1: %d\n", part1(inputLines))
+	fmt.Printf("Part 2: %d\n", part2(inputLines))
 }