Day03

2025/gareth/day03/day03.go

@@ -0,0 +1,69 @@
+package day03
+
+import (
+	"strconv"
+	"strings"
+)
+
+func Part1(input string) int {
+	lines := strings.Split(input, "\n")
+	output := 0
+
+	for _, bank := range lines {
+		best := 0
+		length := len(bank)
+		for i := 0; i < length; i++ {
+			tens, _ := strconv.Atoi(string(bank[i]))
+			for j := i + 1; j < length; j++ {
+				ones, _ := strconv.Atoi(string(bank[j]))
+				value := tens*10 + ones
+				if value > best {
+					best = value
+				}
+			}
+		}
+		output += best
+	}
+
+	return output
+}
+
+func Part2(input string) int {
+	lines := strings.Split(strings.TrimSpace(input), "\n")
+	total := 0
+	k := 12
+
+	for _, bank := range lines {
+		bank = strings.TrimSpace(bank)
+
+		n := len(bank)
+		start := 0
+		chosen := make([]byte, 0, k)
+
+		for picked := 0; picked < k; picked++ {
+			// leave enough digits to complete the subsequence
+			last := n - (k - picked)
+			bestChar := bank[start]
+			bestIdx := start
+
+			for i := start + 1; i <= last; i++ {
+				if bank[i] > bestChar {
+					bestChar = bank[i]
+					bestIdx = i
+
+					if bestChar == '9' {
+						break
+					}
+				}
+			}
+
+			chosen = append(chosen, bestChar)
+			start = bestIdx + 1
+		}
+
+		val, _ := strconv.Atoi(string(chosen))
+		total += val
+	}
+
+	return total
+}