Improve animated webp detection (#1510)

Mlem/App/Views/Shared/Images/Helpers/NukeWebpBridgeDecoder.swift

@@ -11,30 +11,49 @@ import SDWebImageSwiftUI
 import SDWebImageWebPCoder
 import UIKit
 
-/// Raw values of "ANMF", which we can use to identify whether a webp is animated or not without decoding it
-/// https://stackoverflow.com/questions/45190469/how-to-identify-whether-webp-image-is-static-or-animated
-let animatedWebpHeader: [UInt8] = [65, 78, 77, 70]
-
 /// Custom Nuke decoder that processes the image using SDWebImage. The resulting ImageContainer will have the following properties:
 /// `image`: the first frame of the decoded webp
 /// `type`: `.webp`
 /// `data`: the raw webp data if the webp is animated, nil otherwise
 struct NukeWebpBridgeDecoder: ImageDecoding {
     public init?(context: ImageDecodingContext) {
-        guard let type = AssetType(context.data), type == .webp else { return nil }
+        guard
+            let type = AssetType(context.data),
+            type == .webp,
+            context.data.isAnimatedWebp() // only use this for animated webp, fall back on Nuke default for non-animated
+        else { return nil }
     }
 
     func decode(_ data: Data) throws -> ImageContainer {
         // decode the first frame to use as thumbnail
         let decoded = SDImageWebPCoder().decodedImage(with: data, options: [.decodeFirstFrameOnly: true])
 
-        // use magic numbers to check if animated
-        let animated = data.contains(animatedWebpHeader)
-
         if let ret = decoded?.cgImage {
-            return .init(image: .init(cgImage: ret), type: .webp, data: animated ? data : nil)
+            return .init(image: .init(cgImage: ret), type: .webp, data: data)
         } else {
             return .init(image: UIImage.blank)
         }
     }
 }
+
+/// Raw values of "ANIM", which we can use to identify whether a webp is animated or not without decoding it
+/// https://stackoverflow.com/questions/45190469/how-to-identify-whether-webp-image-is-static-or-animated
+private let animHeader: Data = Data([65, 78, 73, 77])
+
+private extension Data {
+    /// If the given data is a webp, returns true if that webp is animated and false otherwise.
+    /// - Warning: This function's behavior is undefined if the provided data is not an animated webp
+    func isAnimatedWebp() -> Bool {
+        // This function is built to run fast, banking on the fact that it's being passed in after Nuke checks that
+        // the image is a webp to guarantee safety. The check itself therefore only targets the bytes that, if the
+        // data is a webp, indicate an animated webp; it is assumed that the data is long enough and correctly formatted.
+
+        // Sanity checks that the data conforms to the webp spec
+        assert(self.count >= 33, "Invalid data (too short)")
+        assert(self[..<4] == Data([82, 73, 70, 70]), "Invalid data (no RIFF header)")
+        assert(self[8..<12] == Data([87, 69, 66, 80]), "Invalid data (no WEBP header)")
+        assert(self[12..<15] == Data([86, 80, 56]), "Invalid data (no VP8X header)")
+
+        return self[30..<34] == animHeader
+    }
+}