Fix scan method and add unit test

Author: Mordil

Sources/NIORedis/Commands/BasicCommands.swift

@@ -195,29 +195,33 @@ extension RedisCommandExecutor {
     ///
     /// [https://redis.io/commands/scan](https://redis.io/commands/scan)
     /// - Parameters:
+    ///     - startingFrom: The cursor position to start from.
     ///     - count: The number of elements to advance by. Redis default is 10.
     ///     - matching: A glob-style pattern to filter values to be selected from the result set.
     /// - Returns: A cursor position for additional invocations with a limited collection of keys stored in the database.
     @inlinable
     public func scan(
-        _ key: String,
-        atPosition pos: Int = 0,
+        startingFrom pos: Int = 0,
         count: Int? = nil,
         matching match: String? = nil) -> EventLoopFuture<(Int, [String])>
     {
-        return _scan(command: "SCAN", resultType: [String].self, key, pos, count, match)
+        return _scan(command: "SCAN", resultType: [String].self, nil, pos, count, match)
     }
 
     @inline(__always)
     @usableFromInline func _scan<T: RESPValueConvertible>(
         command: String,
         resultType: T.Type,
-        _ key: String,
+        _ key: String?,
         _ pos: Int,
         _ count: Int?,
         _ match: String?) -> EventLoopFuture<(Int, T)>
     {
-        var args: [RESPValueConvertible] = [key, pos]
+        var args: [RESPValueConvertible] = [pos]
+
+        if let k = key {
+            args.insert(k, at: 0)
+        }
 
         if let m = match {
             args.append("match")

Tests/NIORedisTests/Commands/BasicCommandsTests.swift

@@ -189,6 +189,32 @@ final class BasicCommandsTests: XCTestCase {
         XCTAssertEqual(values?[1], "2")
     }
 
+    func test_scan() throws {
+        var dataset: [String] = .init(repeating: "", count: 10)
+        for index in 1...15 {
+            let key = "key\(index)\(index % 2 == 0 ? "_even" : "_odd")"
+            dataset.append(key)
+            _ = try connection?.set(key, to: "\(index)").wait()
+        }
+
+        var (cursor, keys) = try connection?.scan(count: 5).wait() ?? (0, [])
+        XCTAssertGreaterThanOrEqual(cursor, 0)
+        XCTAssertGreaterThanOrEqual(keys.count, 5)
+
+        (_, keys) = try connection?.scan(startingFrom: cursor, count: 8).wait() ?? (0, [])
+        XCTAssertGreaterThanOrEqual(keys.count, 8)
+
+        (cursor, keys) = try connection?.scan(matching: "*_odd").wait() ?? (0, [])
+        XCTAssertGreaterThanOrEqual(cursor, 0)
+        XCTAssertGreaterThanOrEqual(keys.count, 1)
+        XCTAssertLessThanOrEqual(keys.count, 7)
+
+        (cursor, keys) = try connection?.scan(matching: "*_even*").wait() ?? (0, [])
+        XCTAssertGreaterThanOrEqual(cursor, 0)
+        XCTAssertGreaterThanOrEqual(keys.count, 1)
+        XCTAssertLessThanOrEqual(keys.count, 7)
+    }
+
     static var allTests = [
         ("test_select", test_select),
         ("test_set", test_set),
@@ -206,5 +232,6 @@ final class BasicCommandsTests: XCTestCase {
         ("test_mget", test_mget),
         ("test_mset", test_mset),
         ("test_msetnx", test_msetnx),
+        ("test_scan", test_scan),
     ]
 }