Add UpdateCheck and some supporting code

Sources/AppUpdater/Support/CaskFile.swift

@@ -0,0 +1,70 @@
+//
+//  Created by Nico Verbruggen on 30/05/2024.
+//  Copyright © 2024 Nico Verbruggen. All rights reserved.
+//
+
+import Foundation
+
+struct CaskFile {
+    var properties: [String: String]
+
+    var name: String { return self.properties["name"]! }
+    var url: String { return self.properties["url"]! }
+    var sha256: String { return self.properties["sha256"]! }
+    var version: String { return self.properties["version"]! }
+
+    static func from(url: URL) -> CaskFile? {
+        var string: String?
+
+        if url.scheme == "file" {
+            string = try? String(contentsOf: url)
+        } else {
+            string = system("curl -s --max-time 10 '\(url.absoluteString)'")
+        }
+
+        guard let string else {
+            Log.text("The content of the URL for the CaskFile could not be retrieved")
+            return nil
+        }
+
+        let lines = string.split(separator: "\n")
+            .map { line in
+                return line.trimmingCharacters(in: .whitespacesAndNewlines)
+            }
+            .filter { $0 != "" }
+
+        if lines.count < 4 {
+            Log.text("The CaskFile is <4 lines long, which is too short")
+            return nil
+        }
+
+        if !lines.first!.starts(with: "cask") || !lines.last!.starts(with: "end") {
+            Log.text("The CaskFile does not start with 'cask' or does not end with 'end'")
+            return nil
+        }
+
+        var props: [String: String] = [:]
+
+        let regex = try! NSRegularExpression(pattern: "(\\w+)\\s+'([^']+)'")
+
+        for line in lines {
+            if let match = regex.firstMatch(
+                in: String(line),
+                range: NSRange(location: 0, length: line.utf16.count)
+            ) {
+                let keyRange = match.range(at: 1)
+                let valueRange = match.range(at: 2)
+                let key = (line as NSString).substring(with: keyRange)
+                let value = (line as NSString).substring(with: valueRange)
+                props[key] = value
+            }
+        }
+
+        for required in ["version", "sha256", "url", "name"] where !props.keys.contains(required) {
+            Log.text("Property '\(required)' expected on CaskFile, assuming CaskFile is invalid")
+            return nil
+        }
+
+        return CaskFile(properties: props)
+    }
+}