Add factorial handler

Sources/D2Handlers/D2Receiver.swift

@@ -87,6 +87,7 @@ public class D2Receiver: Receiver {
             UniversalSummoningHandler(hostInfo: hostInfo),
             HaikuHandler($configuration: $haikuConfiguration, inventoryManager: inventoryManager),
             LuckyNumberHandler(luckyNumbers: [69, 42, 1337], acceptPowerOfTenMultiples: true, minimumNumberCount: 2),
+            FactorialHandler(),
             MessageDatabaseHandler(messageDB: messageDB) // Below other handlers so as to not pick up on commands
         ]
         reactionHandlers = [

Sources/D2Handlers/Message/Handler/FactorialHandler.swift

@@ -0,0 +1,32 @@
+import Utils
+import D2MessageIO
+
+fileprivate let factorialPattern = #/\b(?<operand>\d+)!\b/#
+
+public struct FactorialHandler: MessageHandler {
+    private let operandRange: Range<Int>
+
+    public init(operandRange: Range<Int> = 3..<171) {
+        self.operandRange = operandRange
+    }
+
+    public func handle(message: Message, sink: any Sink) -> Bool {
+        if let channelId = message.channelId,
+           let author = message.author,
+           !author.bot {
+            let matches = message.content.matches(of: factorialPattern)
+            if matches.count == 1,
+               let match = matches.first,
+               let operand = Int(match.operand),
+               operandRange.contains(operand) {
+                let result = factorial(operand)
+                sink.sendMessage("\(operand)! = \(result.isLessThanOrEqualTo(Double(Int.max)) ? String(Int(result)) : String(result))", to: channelId)
+            }
+        }
+        return false
+    }
+
+    private func factorial(_ n: Int) -> Double {
+        (1...n).map(Double.init).reduce(1, *)
+    }
+}