From 44bb13882fd353507374e0ac1a4f1959e5ffd99f Mon Sep 17 00:00:00 2001
From: Dean Cowley <dean.w.cowley@gmail.com>
Date: Thu, 15 May 2025 09:35:12 +0100
Subject: [PATCH 1/3] Day 04: Security Through Obscurity (part 1)

---
 2016/src/Day04.kt | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 2016/src/Day04.kt

diff --git a/2016/src/Day04.kt b/2016/src/Day04.kt
new file mode 100644
index 0000000..29f0200
--- /dev/null
+++ b/2016/src/Day04.kt
@@ -0,0 +1,30 @@
+fun main() {
+    fun part1(input: List<String>): Int {
+        val regex = "([a-z-]+)+-(\\d+)\\[(\\w+)]".toRegex()
+
+        return input.sumOf {
+            val (name, id, checksum) = regex.find(it)!!.destructured
+
+            val counts = name
+                .replace("-", "")
+                .associateWith { char -> Regex("$char").findAll(name).count() }
+
+            val actualChecksum = counts.keys.sorted()
+                .sortedByDescending(counts::get)
+                .take(5)
+                .joinToString("")
+
+            if (actualChecksum == checksum) {
+                id.toInt()
+            } else {
+                0
+            }
+        }
+    }
+
+    check(part1(listOf("aaaaa-bbb-z-y-x-123[abxyz]")) == 123)
+    check(part1(listOf("a-b-c-d-e-f-g-h-987[abcde]")) == 987)
+    check(part1(listOf("not-a-real-room-404[oarel]")) == 404)
+    check(part1(listOf("totally-real-room-200[decoy]")) == 0)
+    println(part1(readInput("Day04")))
+}
\ No newline at end of file

From b2200cbd560b59b85e0b2a5fffdafb39f9610b02 Mon Sep 17 00:00:00 2001
From: Dean Cowley <dean.w.cowley@gmail.com>
Date: Fri, 16 May 2025 08:01:47 +0100
Subject: [PATCH 2/3] Day 04: Security Through Obscurity (part 2)

---
 2016/src/Day04.kt | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/2016/src/Day04.kt b/2016/src/Day04.kt
index 29f0200..08fa455 100644
--- a/2016/src/Day04.kt
+++ b/2016/src/Day04.kt
@@ -22,9 +22,26 @@ fun main() {
         }
     }
 
+    fun part2(input: List<String>) = input.map {
+        val regex = "([a-z-]+)+-(\\d+)\\[(\\w+)]".toRegex()
+
+        val (name, id, _) = regex.find(it)!!.destructured
+
+        val decrypted = name
+            .split("-")
+            .joinToString(" ") { word ->
+                word.map { c -> Char(((c.code - 'a'.code) + id.toInt()) % 26 + 'a'.code) }
+                    .joinToString("")
+            }
+
+        "$decrypted: $id"
+    }.find { it.contains("northpole object storage") }
+
     check(part1(listOf("aaaaa-bbb-z-y-x-123[abxyz]")) == 123)
     check(part1(listOf("a-b-c-d-e-f-g-h-987[abcde]")) == 987)
     check(part1(listOf("not-a-real-room-404[oarel]")) == 404)
     check(part1(listOf("totally-real-room-200[decoy]")) == 0)
     println(part1(readInput("Day04")))
+
+    println(part2(readInput("Day04")))
 }
\ No newline at end of file

From 2feb5295c19d13c2e826e22850e9a96117b5a929 Mon Sep 17 00:00:00 2001
From: Dean Cowley <dean.w.cowley@gmail.com>
Date: Fri, 16 May 2025 08:04:00 +0100
Subject: [PATCH 3/3] Day 04: Security Through Obscurity (refactor)

---
 2016/src/Day04.kt | 34 +++++++++++++++-------------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/2016/src/Day04.kt b/2016/src/Day04.kt
index 08fa455..67f41eb 100644
--- a/2016/src/Day04.kt
+++ b/2016/src/Day04.kt
@@ -1,30 +1,26 @@
 fun main() {
-    fun part1(input: List<String>): Int {
-        val regex = "([a-z-]+)+-(\\d+)\\[(\\w+)]".toRegex()
+    val regex = "([a-z-]+)+-(\\d+)\\[(\\w+)]".toRegex()
 
-        return input.sumOf {
-            val (name, id, checksum) = regex.find(it)!!.destructured
+    fun part1(input: List<String>) = input.sumOf {
+        val (name, id, checksum) = regex.find(it)!!.destructured
 
-            val counts = name
-                .replace("-", "")
-                .associateWith { char -> Regex("$char").findAll(name).count() }
+        val counts = name
+            .replace("-", "")
+            .associateWith { char -> Regex("$char").findAll(name).count() }
 
-            val actualChecksum = counts.keys.sorted()
-                .sortedByDescending(counts::get)
-                .take(5)
-                .joinToString("")
+        val actualChecksum = counts.keys.sorted()
+            .sortedByDescending(counts::get)
+            .take(5)
+            .joinToString("")
 
-            if (actualChecksum == checksum) {
-                id.toInt()
-            } else {
-                0
-            }
+        if (actualChecksum == checksum) {
+            id.toInt()
+        } else {
+            0
         }
     }
 
     fun part2(input: List<String>) = input.map {
-        val regex = "([a-z-]+)+-(\\d+)\\[(\\w+)]".toRegex()
-
         val (name, id, _) = regex.find(it)!!.destructured
 
         val decrypted = name
@@ -44,4 +40,4 @@ fun main() {
     println(part1(readInput("Day04")))
 
     println(part2(readInput("Day04")))
-}
\ No newline at end of file
+}