From 57fff6d82985be335590556e9cd1320ae875c7a2 Mon Sep 17 00:00:00 2001
From: alexchao26 <alexchaoo@gmail.com>
Date: Sun, 27 Dec 2020 15:52:15 -0500
Subject: [PATCH] 2015-day20: slow-ish solution for infinite elves delivering
 to infinite houses

---
 2015/day20/main.go      | 54 +++++++++++++++++++++++++++++++++++++++++
 2015/day20/main_test.go | 26 ++++++++++++++++++++
 2 files changed, 80 insertions(+)
 create mode 100644 2015/day20/main.go
 create mode 100644 2015/day20/main_test.go

diff --git a/2015/day20/main.go b/2015/day20/main.go
new file mode 100644
index 0000000..fb44b81
--- /dev/null
+++ b/2015/day20/main.go
@@ -0,0 +1,54 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"math"
+
+	"github.com/alexchao26/advent-of-code-go/cast"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := infiniteElvesAndHouses(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+func infiniteElvesAndHouses(input string, part int) int {
+	targetNum := cast.ToInt(input)
+
+	for house := 1; house < math.MaxInt32; house++ {
+		var gifts int
+		for _, factor := range getFactors(house) {
+			if part == 1 {
+				gifts += factor * 10
+			} else if part == 2 && house/factor <= 50 {
+				// for part 2, ensure that this is the 50th or less house that
+				// the elf has visited before adding gifts
+				gifts += factor * 11
+			}
+		}
+		if gifts >= targetNum {
+			return house
+		}
+	}
+
+	panic("expect return from loop")
+}
+
+func getFactors(num int) []int {
+	var factors []int
+	sqrt := int(math.Sqrt(float64(num)))
+	for i := 1; i <= sqrt; i++ {
+		if num%i == 0 {
+			factors = append(factors, i, num/i)
+		}
+	}
+	return factors
+}
diff --git a/2015/day20/main_test.go b/2015/day20/main_test.go
new file mode 100644
index 0000000..e50f72a
--- /dev/null
+++ b/2015/day20/main_test.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func Test_infiniteElvesAndHouses(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{"part1 actual", util.ReadFile("input.txt"), 1, 776160},
+		{"part2 actual", util.ReadFile("input.txt"), 2, 786240},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := infiniteElvesAndHouses(tt.input, tt.part); got != tt.want {
+				t.Errorf("infiniteElvesAndHouses() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}