From 7b59c510d8bac669861f24727eee867d8e744ec8 Mon Sep 17 00:00:00 2001
From: alexchao26 <alexchaoo@gmail.com>
Date: Sun, 13 Dec 2020 21:18:16 -0500
Subject: [PATCH] 2017-day6: cycling until state repeats itself

---
 2017/day06/main.go      | 77 +++++++++++++++++++++++++++++++++++++++++
 2017/day06/main_test.go | 27 +++++++++++++++
 2 files changed, 104 insertions(+)
 create mode 100644 2017/day06/main.go
 create mode 100644 2017/day06/main_test.go

diff --git a/2017/day06/main.go b/2017/day06/main.go
new file mode 100644
index 0000000..83e07ef
--- /dev/null
+++ b/2017/day06/main.go
@@ -0,0 +1,77 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+
+	"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)
+
+	if part == 1 {
+		ans := memoryReallocation(util.ReadFile("./input.txt"), 1)
+		fmt.Println("Output:", ans)
+	} else {
+		ans := memoryReallocation(util.ReadFile("./input.txt"), 2)
+		fmt.Println("Output:", ans)
+	}
+}
+
+func memoryReallocation(input string, part int) int {
+	banks := parseInput(input)
+
+	// [16]int arrays are comparable by values so can be used as map keys
+	seenBanks := map[[16]int]int{banks: 0}
+	var cycles int
+	for {
+		// find largest bank
+		var index, maxVal int
+		for i, val := range banks {
+			if val > maxVal {
+				index = i
+				maxVal = val
+			}
+		}
+
+		// run a cycle
+		blocksToDistribute := banks[index]
+		banks[index] = 0
+		// unoptimized but works just fine
+		for i := index + 1; blocksToDistribute > 0; i++ {
+			if blocksToDistribute == 0 {
+				break
+			}
+			banks[i%16]++
+			blocksToDistribute--
+		}
+		cycles++
+
+		// check if this set of banks has been seen before, if so return here
+		if val, ok := seenBanks[banks]; ok {
+			if part == 1 {
+				return cycles
+			}
+			// for part 2 take the difference with cycles when it was last seen
+			return cycles - val
+		}
+		// set the number of cycles that correspond to this state of banks
+		seenBanks[banks] = cycles
+	}
+
+	panic("should resolve in for loop")
+}
+
+func parseInput(input string) (ans [16]int) {
+	nums := strings.Split(input, "\t")
+	for i, num := range nums {
+		ans[i] = mathutil.StrToInt(num)
+	}
+	return ans
+}
diff --git a/2017/day06/main_test.go b/2017/day06/main_test.go
new file mode 100644
index 0000000..f0bd7d0
--- /dev/null
+++ b/2017/day06/main_test.go
@@ -0,0 +1,27 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func Test_memoryReallocation(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{"actual", util.ReadFile("input.txt"), 1, 6681},
+		{"actual", util.ReadFile("input.txt"), 2, 2392},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := memoryReallocation(tt.input, tt.part); got != tt.want {
+				t.Errorf("memoryReallocation() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+
+}