From 705884d0807efeb4566cdde61d2aa5afb5435f51 Mon Sep 17 00:00:00 2001
From: alexchao26 <alexchaoo@gmail.com>
Date: Sat, 26 Dec 2020 02:08:25 -0500
Subject: [PATCH] 2015-day4: md5 hashing

---
 2015/day04/main.go      | 38 ++++++++++++++++++++++++++++++++++++++
 2015/day04/main_test.go | 26 ++++++++++++++++++++++++++
 2 files changed, 64 insertions(+)
 create mode 100644 2015/day04/main.go
 create mode 100644 2015/day04/main_test.go

diff --git a/2015/day04/main.go b/2015/day04/main.go
new file mode 100644
index 0000000..4aef2c7
--- /dev/null
+++ b/2015/day04/main.go
@@ -0,0 +1,38 @@
+package main
+
+import (
+	"crypto/md5"
+	"flag"
+	"fmt"
+	"math"
+	"strings"
+
+	"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 := md5StockingStuffer(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+func md5StockingStuffer(input string, part int) int {
+	prefixZeroes := 5
+	if part == 2 {
+		prefixZeroes = 6
+	}
+
+	for i := 0; i < math.MaxInt32; i++ {
+		toHash := fmt.Sprintf("%s%d", input, i)
+		hashed := fmt.Sprintf("%x", md5.Sum([]byte(toHash)))
+		if strings.HasPrefix(hashed, strings.Repeat("0", prefixZeroes)) {
+			return i
+		}
+	}
+
+	panic("no hash found")
+}
diff --git a/2015/day04/main_test.go b/2015/day04/main_test.go
new file mode 100644
index 0000000..8baa1cc
--- /dev/null
+++ b/2015/day04/main_test.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func Test_md5StockingStuffer(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{"part1 actual", util.ReadFile("input.txt"), 1, 254575},
+		{"part2 actual", util.ReadFile("input.txt"), 2, 1038736},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := md5StockingStuffer(tt.input, tt.part); got != tt.want {
+				t.Errorf("md5StockingStuffer() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}