Day02

2025/gareth/day02/day02.go

@@ -0,0 +1,119 @@
+package day02
+
+import (
+	"math"
+	"strconv"
+	"strings"
+)
+
+// Very messy and not my finest work but not too bad for 6am
+
+func parseInput(input string) [][2]int {
+	var ranges [][2]int
+	lines := strings.Split(strings.TrimSpace(input), ",")
+	for _, part := range lines {
+		if part == "" {
+			continue
+		}
+		ab := strings.Split(part, "-")
+		if len(ab) != 2 {
+			continue
+		}
+
+		a, _ := strconv.Atoi(ab[0])
+		b, _ := strconv.Atoi(ab[1])
+		ranges = append(ranges, [2]int{a, b})
+	}
+	return ranges
+}
+
+func divisors(n int) []int {
+	out := make([]int, 0)
+	for i := 1; i <= n; i++ {
+		if n%i == 0 {
+			out = append(out, i)
+		}
+	}
+	return out
+}
+
+func Part1(input string) int {
+	ranges := parseInput(input)
+	invalidSum := 0
+
+	for _, r := range ranges {
+		low, high := r[0], r[1]
+
+		minLen := len(strconv.Itoa(low))
+		maxLen := len(strconv.Itoa(high))
+
+		for digits := minLen; digits <= maxLen; digits++ {
+			if digits%2 != 0 {
+				continue
+			}
+
+			half := digits / 2
+			start := int(math.Pow(10, float64(half-1)))
+			end := int(math.Pow(10, float64(half)))
+
+			for first := start; first < end; first++ {
+				s := strconv.Itoa(first)
+				selected, _ := strconv.Atoi(s + s)
+
+				if selected < low {
+					continue
+				}
+				if selected > high {
+					break
+				}
+
+				invalidSum += selected
+			}
+		}
+	}
+
+	return invalidSum
+}
+
+func Part2(input string) int {
+	ranges := parseInput(input)
+	total := 0
+
+	for _, r := range ranges {
+		low, high := r[0], r[1]
+		seen := make(map[int]bool)
+
+		minLen := len(strconv.Itoa(low))
+		maxLen := len(strconv.Itoa(high))
+
+		for d := minLen; d <= maxLen; d++ {
+			for _, L := range divisors(d) {
+				k := d / L
+				if k < 2 {
+					continue
+				}
+				start := int(math.Pow(10, float64(L-1)))
+				end := int(math.Pow(10, float64(L)))
+
+				for base := start; base < end; base++ {
+					s := strings.Repeat(strconv.Itoa(base), k)
+					selected, _ := strconv.Atoi(s)
+
+					if selected < low {
+						continue
+					}
+					if selected > high {
+						break
+					}
+
+					if !seen[selected] {
+						seen[selected] = true
+						total += selected
+					}
+				}
+			}
+		}
+	}
+
+	return total
+}