Implement arc4random_buf (#43)

Sources/PlaydateKit/Core/System.swift

@@ -200,6 +200,18 @@ public enum System {
         }
     }
 
+    /// Returns the number of seconds elapsed since midnight (hour 0), January 1, 2000.
+    public static var secondsSinceEpoch: CUnsignedInt {
+        system.getSecondsSinceEpoch.unsafelyUnwrapped(nil)
+    }
+
+    /// Returns the number of milliseconds elapsed since midnight (hour 0), January 1, 2000.
+    public static var millisecondsSinceEpoch: CUnsignedInt {
+        var ms: CUnsignedInt = 0
+        _ = system.getSecondsSinceEpoch.unsafelyUnwrapped(&ms)
+        return ms
+    }
+
     // MARK: - Memory allocation
 
     /// Allocates heap space if `pointer` is nil, else reallocates the given pointer. If `size` is zero, frees the given pointer.
@@ -436,11 +448,6 @@ public enum System {
         menuItems = []
     }
 
-    /// Returns the number of seconds (and sets milliseconds if not NULL) elapsed since midnight (hour 0), January 1, 2000.
-    public static func getSecondsSinceEpoch(milliseconds: UnsafeMutablePointer<CUnsignedInt>? = nil) -> CUnsignedInt {
-        system.getSecondsSinceEpoch.unsafelyUnwrapped(milliseconds)
-    }
-
     /// Resets the high-resolution timer.
     public static func resetElapsedTime() {
         system.resetElapsedTime.unsafelyUnwrapped()

Sources/PlaydateKit/Playdate.swift

@@ -22,6 +22,7 @@ public enum Playdate {
 
     public static func initialize(with pointer: UnsafeMutableRawPointer) {
         _playdateAPI = pointer.bindMemory(to: PlaydateAPI.self, capacity: 1).pointee
+        arc4random_seed = System.millisecondsSinceEpoch
         System.setUpdateCallback(update: { _ in
             (System.updateCallback?() ?? false) ? 1 : 0
         }, userdata: nil)
@@ -32,7 +33,7 @@ public enum Playdate {
     private nonisolated(unsafe) static var _playdateAPI: PlaydateAPI?
 }
 
-/// Implement `posix_memalign(3)`, which is required by the Swift runtime but is
+/// Implement `posix_memalign(3)`, which is required by the Embedded Swift runtime but is
 /// not provided by the Playdate C library.
 @_documentation(visibility: internal)
 @_cdecl("posix_memalign") public func posix_memalign(
@@ -46,3 +47,22 @@ public enum Playdate {
     memptr.pointee = allocation
     return 0
 }
+
+nonisolated(unsafe) var arc4random_seed: CUnsignedInt = 0
+@_cdecl("arc4random") public func arc4random() -> UInt32 {
+    arc4random_seed = 1664525 &* arc4random_seed &+ 1013904223
+    return arc4random_seed
+}
+
+/// Implement `arc4random_buf` which is required by the Embedded Swift runtime for Hashable, Set, Dictionary,
+/// and random-number generating APIs but is not provided by the Playdate C library.
+@_cdecl("arc4random_buf") public func arc4random_buf(buf: UnsafeMutableRawPointer, nbytes: Int) {
+    var r = arc4random()
+    for i in 0..<nbytes {
+        if i % 4 == 0 {
+            r = arc4random()
+        }
+        buf.advanced(by: i).assumingMemoryBound(to: UInt8.self).pointee = UInt8(r & 0xff)
+        r >>= 8
+    }
+}