diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
new file mode 100644
index 0000000..cba40a2
--- /dev/null
+++ b/.github/workflows/build.yml
@@ -0,0 +1,20 @@
+name: Build
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+env:
+  DEVELOPER_DIR: /Applications/Xcode_15.0.1.app/Contents/Developer
+
+jobs:
+  build:
+    runs-on: macos-13
+    steps:
+    - uses: actions/checkout@v4
+    - name: Build
+      run: swift build
+    - name: Run tests
+      run: swift test
diff --git a/Sources/Day02/Day02.swift b/Sources/Day02/Day02.swift
index 2b700d7..e47951b 100644
--- a/Sources/Day02/Day02.swift
+++ b/Sources/Day02/Day02.swift
@@ -6,7 +6,7 @@
 
 import AoCTools
 
-struct Game {
+private struct Game {
     let id: Int
     let sets: [Set]
 
@@ -63,7 +63,7 @@ struct Game {
 }
 
 final class Day02: AOCDay {
-    let games: [Game]
+    private let games: [Game]
 
     init(input: String) {
         games = input.lines.map { Game($0) }
diff --git a/Sources/Day03/Day03.swift b/Sources/Day03/Day03.swift
index a691d72..4639e7e 100644
--- a/Sources/Day03/Day03.swift
+++ b/Sources/Day03/Day03.swift
@@ -6,7 +6,7 @@
 
 import AoCTools
 
-struct Number: Hashable {
+private struct Number: Hashable {
     let value: Int
     let start: Point
     let length: Int
@@ -27,9 +27,9 @@ struct Number: Hashable {
 }
 
 final class Day03: AOCDay {
-    let symbols: Set<Point>
-    let gears: Set<Point>
-    let numbers: Set<Number>
+    private let symbols: Set<Point>
+    private let gears: Set<Point>
+    private let numbers: Set<Number>
 
     init(input: String) {
         var symbols = Set<Point>()
diff --git a/Sources/Day04/Day04.swift b/Sources/Day04/Day04.swift
index 77b520e..d18f1cb 100644
--- a/Sources/Day04/Day04.swift
+++ b/Sources/Day04/Day04.swift
@@ -5,24 +5,19 @@
 //
 
 import AoCTools
-import Foundation
 
-struct Card {
+private struct Card {
     let id: Int
-    let winners: Set<Int>
-    let numbers: Set<Int>
+    let matches: Int
 
     init(_ string: String) {
         // Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
         let parts = string.components(separatedBy: ":")
         self.id = parts[0].allInts().first!
-        let numbers = parts[1].components(separatedBy: "|")
-        self.winners = Set(numbers[0].allInts())
-        self.numbers = Set(numbers[1].allInts())
-    }
-
-    var matches: Int {
-        numbers.intersection(winners).count
+        let nums = parts[1].components(separatedBy: "|")
+        let winners = Set(nums[0].allInts())
+        let numbers = Set(nums[1].allInts())
+        matches = numbers.intersection(winners).count
     }
 
     var points: Int {
@@ -31,7 +26,7 @@ struct Card {
 }
 
 final class Day04: AOCDay {
-    let cards: [Card]
+    private let cards: [Card]
 
     init(input: String) {
         cards = input.lines.map { Card($0) }
@@ -46,7 +41,7 @@ final class Day04: AOCDay {
         copies[0] = 0
 
         for card in cards where card.matches > 0 {
-            for i in card.id + 1 ... card.id + card.matches where i < copies.count + 1 {
+            for i in card.id + 1 ... card.id + card.matches {
                 copies[i] += copies[card.id]
             }
         }