Solve day 3 part 2

day3/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -35,10 +36,11 @@ 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))
 }
 
 func part1(inputLines []string) int {
-	symbolPositions := findSymbols(inputLines)
+	symbolPositions := findParts(inputLines, isSymbol)
 	partNumberCandidates := []Coordinate{}
 	for _, symbolPos := range symbolPositions {
 		candidates := findNumbersAdjacentTo(inputLines, symbolPos)
@@ -46,42 +48,21 @@ func part1(inputLines []string) int {
 	}
 
 	total := 0
+	// keep track of digits we've already scanned; it's possible one number
+	// is adjacent to two parts, and we don't wanna double count
 	scanned := map[Coordinate]struct{}{}
 	for _, candidate := range partNumberCandidates {
 		if _, ok := scanned[candidate]; ok {
 			continue
 		}
 
-		line := inputLines[candidate.row]
+		scannedNumber, scannedDigits, err := scanPartNumber(inputLines, candidate)
-
-		backwardsBuf := strings.Builder{}
-		// Scan backward
-		for i := candidate.col - 1; i >= 0; i-- {
-			char := line[i]
-			if !unicode.IsDigit(rune(char)) {
-				break
-			}
-
-			scanned[Coordinate{row: candidate.row, col: i}] = struct{}{}
-			backwardsBuf.WriteByte(char)
-		}
-
-		forwardsBuf := strings.Builder{}
-		// Scan forward
-		for i := candidate.col; i < len(line); i++ {
-			char := line[i]
-			if !unicode.IsDigit(rune(char)) {
-				break
-			}
-
-			scanned[Coordinate{row: candidate.row, col: i}] = struct{}{}
-			forwardsBuf.WriteByte(char)
-		}
-
-		fullNumber := reverseString(backwardsBuf.String()) + forwardsBuf.String()
-		scannedNumber, err := strconv.Atoi(fullNumber)
 		if err != nil {
-			panic(fmt.Sprintf("%s was not a number", fullNumber))
+			panic(fmt.Sprintf("candidate was invalid: %s", err))
+		}
+
+		for _, scannedDigit := range scannedDigits {
+			scanned[scannedDigit] = struct{}{}
 		}
 
 		total += scannedNumber
@@ -90,11 +71,30 @@ func part1(inputLines []string) int {
 	return total
 }
 
-func findSymbols(inputLines []string) []Coordinate {
+func part2(inputLines []string) int {
+	gearPositions := findParts(inputLines, isGear)
+	totalRatio := 0
+	for _, gearPos := range gearPositions {
+		partNumbers, err := scanForGearPartNumbers(inputLines, gearPos)
+		if err != nil {
+			panic(fmt.Sprintf("gear scan failed: %s", err))
+		}
+
+		if len(partNumbers) == 2 {
+			totalRatio += partNumbers[0] * partNumbers[1]
+		}
+	}
+
+	return totalRatio
+}
+
+// findParts will find all parts in the input, using the given isPart function to tell
+// if a character is a valid part.
+func findParts(inputLines []string, isPart func(rune) bool) []Coordinate {
 	coords := []Coordinate{}
 	for row, line := range inputLines {
 		for col, char := range line {
-			if isSymbol(char) {
+			if isPart(char) {
 				coord := Coordinate{
 					row: row,
 					col: col,
@@ -108,6 +108,7 @@ func findSymbols(inputLines []string) []Coordinate {
 	return coords
 }
 
+// findNumbersAdjacentTo will find all the digit characters adjacent to a given coordinate
 func findNumbersAdjacentTo(inputLines []string, coordinate Coordinate) []Coordinate {
 	coords := []Coordinate{}
 	for dRow := -1; dRow <= 1; dRow++ {
@@ -132,10 +133,91 @@ func findNumbersAdjacentTo(inputLines []string, coordinate Coordinate) []Coordin
 	return coords
 }
 
+// scanPartNumber takes an initial digit character and attempts to complete it by scanning left and right from that
+// position. Returns the found part number and the positions scanned.
+func scanPartNumber(inputLines []string, knownDigitPosition Coordinate) (int, []Coordinate, error) {
+	if !unicode.IsDigit(rune(inputLines[knownDigitPosition.row][knownDigitPosition.col])) {
+		return 0, nil, errors.New("position was not a numeric char")
+	}
+
+	line := inputLines[knownDigitPosition.row]
+
+	scanned := []Coordinate{}
+	backwardsBuf := strings.Builder{}
+	// Scan backward
+	for i := knownDigitPosition.col - 1; i >= 0; i-- {
+		char := line[i]
+		if !unicode.IsDigit(rune(char)) {
+			break
+		}
+
+		scanned = append(scanned, Coordinate{row: knownDigitPosition.row, col: i})
+		backwardsBuf.WriteByte(char)
+	}
+
+	forwardsBuf := strings.Builder{}
+	// Scan forward
+	for i := knownDigitPosition.col; i < len(line); i++ {
+		char := line[i]
+		if !unicode.IsDigit(rune(char)) {
+			break
+		}
+
+		scanned = append(scanned, Coordinate{row: knownDigitPosition.row, col: i})
+		forwardsBuf.WriteByte(char)
+	}
+
+	fullNumber := reverseString(backwardsBuf.String()) + forwardsBuf.String()
+	scannedNumber, err := strconv.Atoi(fullNumber)
+	if err != nil {
+		panic(fmt.Sprintf("%s was not a number", fullNumber))
+	}
+
+	return scannedNumber, scanned, nil
+}
+
+// scanForGearPartNumbers will find all part numbers adjacent to the given gear character position
+func scanForGearPartNumbers(inputLines []string, gearPosition Coordinate) ([]int, error) {
+	if !isGear(rune(inputLines[gearPosition.row][gearPosition.col])) {
+		return nil, errors.New("position is not gear")
+	}
+
+	candidates := findNumbersAdjacentTo(inputLines, gearPosition)
+	// Keep track of the digits scanned, as it is possible for a single part number to be adjacent
+	// to the gear in multiple places
+	scanned := map[Coordinate]struct{}{}
+
+	adjacentPartNumbers := []int{}
+	for _, candidate := range candidates {
+		if _, ok := scanned[candidate]; ok {
+			continue
+		}
+
+		scannedNumber, scannedDigits, err := scanPartNumber(inputLines, candidate)
+		if err != nil {
+			// Strictly a programmer error, as this would imply that findNumbersAdjacentTo
+			// didn't find us a number
+			panic(fmt.Sprintf("candidate was invalid: %s", err))
+		}
+
+		for _, scannedDigit := range scannedDigits {
+			scanned[scannedDigit] = struct{}{}
+		}
+
+		adjacentPartNumbers = append(adjacentPartNumbers, scannedNumber)
+	}
+
+	return adjacentPartNumbers, nil
+}
+
 func isSymbol(r rune) bool {
 	return r != '.' && !unicode.IsDigit(r)
 }
 
+func isGear(r rune) bool {
+	return r == '*'
+}
+
 func reverseString(s string) string {
 	buffer := strings.Builder{}
 	for i := len(s) - 1; i >= 0; i-- {