From e8b6c9cbde2218515aea99dcfa9b5321165781e5 Mon Sep 17 00:00:00 2001
From: alexchao26 <alexchaoo@gmail.com>
Date: Sat, 26 Dec 2020 21:52:22 -0500
Subject: [PATCH] 2015-day17: recursive backtrack to valid combinations

---
 2015/day17/main.go      | 67 +++++++++++++++++++++++++++++++++++++++++
 2015/day17/main_test.go | 38 +++++++++++++++++++++++
 2 files changed, 105 insertions(+)
 create mode 100644 2015/day17/main.go
 create mode 100644 2015/day17/main_test.go

diff --git a/2015/day17/main.go b/2015/day17/main.go
new file mode 100644
index 0000000..bbf3a05
--- /dev/null
+++ b/2015/day17/main.go
@@ -0,0 +1,67 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"math"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/cast"
+	"github.com/alexchao26/advent-of-code-go/mathutil"
+	"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 := eggnogCombinations(util.ReadFile("./input.txt"), 150, part)
+	fmt.Println("Output:", ans)
+}
+
+func eggnogCombinations(input string, target int, part int) int {
+	var nums []int
+	for _, line := range strings.Split(input, "\n") {
+		nums = append(nums, cast.ToInt(line))
+	}
+
+	allIndexCombinations := backtrack(nums, 0, target, []int{})
+	// part 1, just return len
+	if part == 1 {
+		return len(allIndexCombinations)
+	}
+
+	// part 2, get the number of combinations w/ the lowest length
+	minLen := math.MaxInt32
+	for _, comb := range allIndexCombinations {
+		minLen = mathutil.MinInt(minLen, len(comb))
+	}
+
+	var count int
+	for _, comb := range allIndexCombinations {
+		if len(comb) == minLen {
+			count++
+		}
+	}
+
+	return count
+}
+
+func backtrack(nums []int, startingIndex, remaining int, usedIndices []int) [][]int {
+	if remaining == 0 {
+		return [][]int{append([]int{}, usedIndices...)}
+	}
+	if remaining < 0 {
+		return nil
+	}
+
+	var validReturns [][]int
+	for i := startingIndex; i < len(nums); i++ {
+		usedIndices = append(usedIndices, i)
+		validReturns = append(validReturns, backtrack(nums, i+1, remaining-nums[i], usedIndices)...)
+		usedIndices = usedIndices[:len(usedIndices)-1]
+	}
+	return validReturns
+}
diff --git a/2015/day17/main_test.go b/2015/day17/main_test.go
new file mode 100644
index 0000000..286ebde
--- /dev/null
+++ b/2015/day17/main_test.go
@@ -0,0 +1,38 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+var example = `20
+15
+10
+5
+5`
+
+func Test_eggnogCombinations(t *testing.T) {
+	type args struct {
+		input  string
+		target int
+		part   int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{"part1 example", args{example, 25, 1}, 4},
+		{"part1 actual", args{util.ReadFile("input.txt"), 150, 1}, 1304},
+		{"part2 example", args{example, 25, 2}, 3},
+		{"part2 actual", args{util.ReadFile("input.txt"), 150, 2}, 18},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := eggnogCombinations(tt.args.input, tt.args.target, tt.args.part); got != tt.want {
+				t.Errorf("eggnogCombinations() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}