From 1e269d7988d059f3a54e865a672c99606865ebf5 Mon Sep 17 00:00:00 2001
From: alexchao26 <alexchaoo@gmail.com>
Date: Fri, 25 Dec 2020 00:48:10 -0500
Subject: [PATCH] 2020-day25: 2020 finished! "breaking" mock public key
 cryptography

---
 2020/day25/main.go      | 70 +++++++++++++++++++++++------------------
 2020/day25/main_test.go | 22 +++----------
 2 files changed, 43 insertions(+), 49 deletions(-)

diff --git a/2020/day25/main.go b/2020/day25/main.go
index bde80ba..dc62139 100644
--- a/2020/day25/main.go
+++ b/2020/day25/main.go
@@ -1,46 +1,54 @@
 package main
 
 import (
-	"flag"
 	"fmt"
 	"strings"
 
-	"github.com/alexchao26/advent-of-code-go/mathutil"
+	"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)
-
-	if part == 1 {
-		ans := part1(util.ReadFile("./input.txt"))
-		util.CopyToClipboard(fmt.Sprintf("%v", ans))
-		fmt.Println("Output:", ans)
-	} else {
-		ans := part2(util.ReadFile("./input.txt"))
-		util.CopyToClipboard(fmt.Sprintf("%v", ans))
-		fmt.Println("Output:", ans)
-	}
+	ans := part1(util.ReadFile("./input.txt"))
+	fmt.Println("Output:", ans)
 }
 
+// ~1900..
 func part1(input string) int {
-	parsed := parseInput(input)
-	_ = parsed
-
-	return 0
-}
-
-func part2(input string) int {
-	return 0
-}
-
-func parseInput(input string) (ans []int) {
-	lines := strings.Split(input, "\n")
-	for _, l := range lines {
-		ans = append(ans, mathutil.StrToInt(l))
+	var publicKeys []int
+	for _, l := range strings.Split(input, "\n") {
+		publicKeys = append(publicKeys, cast.ToInt(l))
 	}
-	return ans
+
+	var loopSizes [2]int
+	val := 1
+	// calculate both loop sizes in one pass to avoid recalculating over and over...
+	for loops := 1; loopSizes[0] == 0 || loopSizes[1] == 0; loops++ {
+		val *= 7
+		val %= 20201227
+		if val == publicKeys[0] {
+			loopSizes[0] = loops
+		}
+		if val == publicKeys[1] {
+			loopSizes[1] = loops
+		}
+	}
+
+	// ensure both loop sizes are correct by reaching the same encryption key
+	key1 := runTranformations(publicKeys[0], loopSizes[1])
+	key2 := runTranformations(publicKeys[1], loopSizes[0])
+	if key1 != key2 {
+		panic(fmt.Sprintf("encryption keys should be the same, got %d and %d", key1, key2))
+	}
+
+	return key1
+}
+
+func runTranformations(subject, loops int) int {
+	val := 1
+	for i := 0; i < loops; i++ {
+		val *= subject
+		val %= 20201227
+	}
+	return val
 }
diff --git a/2020/day25/main_test.go b/2020/day25/main_test.go
index 22a4487..c8f0f4a 100644
--- a/2020/day25/main_test.go
+++ b/2020/day25/main_test.go
@@ -2,6 +2,8 @@ package main
 
 import (
 	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
 )
 
 func Test_part1(t *testing.T) {
@@ -10,7 +12,8 @@ func Test_part1(t *testing.T) {
 		input string
 		want  int
 	}{
-		// {"actual", util.ReadFile("input.txt"), ACTUAL_ANSWER},
+		{"example", "5764801\n17807724", 14897079},
+		{"actual", util.ReadFile("input.txt"), 18608573},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -20,20 +23,3 @@ func Test_part1(t *testing.T) {
 		})
 	}
 }
-
-func Test_part2(t *testing.T) {
-	tests := []struct {
-		name  string
-		input string
-		want  int
-	}{
-		// {"actual", util.ReadFile("input.txt"), ACTUAL_ANSWER},
-	}
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			if got := part2(tt.input); got != tt.want {
-				t.Errorf("part2() = %v, want %v", got, tt.want)
-			}
-		})
-	}
-}