Day 20

d20/input.txt

@@ -0,0 +1 @@
+33100000

d20/main.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"aoc2015/util"
+	"math"
+)
+
+// Brute force is my best friend.
+func part1(target int) int {
+	for houseNumber := range math.MaxInt32 {
+		giftCounter := 0
+
+		for _, factor := range util.FactorsOf(houseNumber) {
+			giftCounter += factor * 10
+		}
+
+		if giftCounter >= target {
+			return houseNumber
+		}
+	}
+
+	return -1
+}
+
+func part2(target int) int {
+	for houseNumber := range math.MaxInt32 {
+		giftCounter := 0
+
+		for _, factor := range util.FactorsOf(houseNumber) {
+			if houseNumber/factor <= 50 {
+				giftCounter += factor * 11
+			}
+		}
+
+		if giftCounter >= target {
+			return houseNumber
+		}
+	}
+
+	return -1
+}
+
+func main() {
+	reader := func(name string) int {
+		return util.StringToInt(util.ReadFile(name))
+	}
+
+	util.TestRuns("20", reader, part1, part2)
+	util.SolutionRuns("20", reader, part1, part2)
+}

util/math.go

@@ -1,9 +1,43 @@
 package util
 
 import (
+	"math"
+
+	mapset "github.com/deckarep/golang-set/v2"
 	"golang.org/x/exp/constraints"
 )
 
+func FactorsOf(value int) []int {
+	factors := mapset.NewSet[int]()
+
+	for i := 1; i <= int(math.Sqrt(float64(value))); i++ {
+		if value%i == 0 {
+			factors.Add(i)
+			factors.Add(value / i)
+		}
+	}
+
+	return factors.ToSlice()
+}
+
+func GreatestCommonDivisor(a, b int) int {
+	for b != 0 {
+		a, b = b, a%b
+	}
+
+	return a
+}
+
+func LeastCommonMultiple(numbers ...int) int {
+	lcm := numbers[0]
+
+	for i := 1; i < len(numbers); i++ {
+		lcm = ((numbers[i] * lcm) / GreatestCommonDivisor(numbers[i], lcm))
+	}
+
+	return lcm
+}
+
 func Sum[T constraints.Integer | constraints.Float](numbers []T) T {
 	sum := T(0)