Add EXISTS command

Motivation:

The EXISTS command was missing.

Modifications:

- Add 'EXISTS' to basic commands
- Add integration tests

Result:

The existence of a key can be checked.

Author: glbrntt

Sources/RediStack/Commands/BasicCommands.swift

@@ -101,6 +101,28 @@ extension RedisClient {
         return self.delete(keys)
     }
 
+    /// Checks the existence of the provided keys in the database.
+    ///
+    /// [https://redis.io/commands/exists](https://redis.io/commands/exists)
+    /// - Parameter keys: A list of keys whose existence will be checked for in the database.
+    /// - Returns: The number of provided keys which exist in the database.
+    public func exists(_ keys: [RedisKey]) -> EventLoopFuture<Int> {
+        let args: [RESPValue] = keys.map {
+            RESPValue(from: $0)
+        }
+        return self.send(command: "EXISTS", with: args)
+            .map(to: Int.self)
+    }
+
+    /// Checks the existence of the provided keys in the database.
+    ///
+    /// [https://redis.io/commands/exists](https://redis.io/commands/exists)
+    /// - Parameter keys: A list of keys whose existence will be checked for in the database.
+    /// - Returns: The number of provided keys which exist in the database.
+    public func exists(_ keys: RedisKey...) -> EventLoopFuture<Int> {
+        return self.exists(keys)
+    }
+
     /// Sets a timeout on key. After the timeout has expired, the key will automatically be deleted.
     /// - Note: A key with an associated timeout is often said to be "volatile" in Redis terminology.
     ///

Tests/RediStackIntegrationTests/Commands/BasicCommandsTests.swift

@@ -37,6 +37,22 @@ final class BasicCommandsTests: RediStackIntegrationTestCase {
         XCTAssertEqual(third, 2)
     }
 
+    func test_exists() throws {
+        try self.connection.set("first", to: "1").wait()
+        let first = try connection.exists("first").wait()
+        XCTAssertEqual(first, 1)
+
+        try self.connection.set("second", to: "2").wait()
+        let firstAndSecond = try connection.exists("first", "second").wait()
+        XCTAssertEqual(firstAndSecond, 2)
+
+        let secondAndThird = try connection.exists("second", "third").wait()
+        XCTAssertEqual(secondAndThird, 1)
+
+        let third = try connection.exists("third").wait()
+        XCTAssertEqual(third, 0)
+    }
+
     func test_expire() throws {
         try connection.set(#function, to: "value").wait()
         XCTAssertNotNil(try connection.get(#function).wait())

Tests/RediStackIntegrationTests/XCTestManifests.swift

@@ -8,6 +8,7 @@ extension BasicCommandsTests {
     static let __allTests__BasicCommandsTests = [
         ("test_delete", test_delete),
         ("test_echo", test_echo),
+        ("test_exists", test_exists),
         ("test_expire", test_expire),
         ("test_ping", test_ping),
         ("test_select", test_select),