Add DNS and UDP components (#58)

kronos-java/src/main/java/com/lyft/kronos/sntp/Dns.kt

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+package com.lyft.kronos.sntp
+
+import java.net.InetAddress
+import java.net.UnknownHostException
+
+internal interface Dns {
+
+    fun resolve(host: String): Array<out InetAddress>
+
+    class Impl : Dns {
+
+        override fun resolve(host: String) = try {
+            InetAddress.getAllByName(host).orEmpty()
+        } catch (e: SecurityException) {
+            emptyArray()
+        } catch (e: UnknownHostException) {
+            emptyArray()
+        }
+    }
+}

kronos-java/src/main/java/com/lyft/kronos/sntp/Udp.kt

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+package com.lyft.kronos.sntp
+
+import java.lang.Exception
+import java.net.DatagramPacket
+import java.net.DatagramSocket
+import java.net.SocketAddress
+import java.nio.ByteBuffer
+import java.util.concurrent.TimeUnit
+
+internal interface Udp {
+
+    enum class Result { Success, Failure }
+
+    fun request(address: SocketAddress, request: ByteBuffer, response: ByteBuffer): Result
+
+    class Impl : Udp {
+
+        companion object {
+            private val TIMEOUT_MILLIS = TimeUnit.SECONDS.toMillis(3).toInt()
+        }
+
+        override fun request(address: SocketAddress, request: ByteBuffer, response: ByteBuffer): Result {
+            val requestPacket = DatagramPacket(request.array(), request.capacity(), address)
+            val responsePacket = DatagramPacket(response.array(), response.capacity())
+
+            return try {
+                DatagramSocket().use { socket ->
+                    socket.soTimeout = TIMEOUT_MILLIS
+
+                    socket.send(requestPacket)
+                    socket.receive(responsePacket)
+                }
+
+                Result.Success
+            } catch (e: Exception) {
+                Result.Failure
+            }
+        }
+    }
+}

kronos-java/src/test/java/com/lyft/kronos/sntp/DnsTests.kt

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+package com.lyft.kronos.sntp
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+import java.net.InetAddress
+
+class DnsTests {
+
+    private val dns by lazy { Dns.Impl() }
+
+    @Test
+    fun resolveLocalhost() {
+        assertThat(dns.resolve("localhost")).allMatch { it.isLoopbackAddress }
+    }
+
+    @Test
+    fun resolveUnknown() {
+        assertThat(dns.resolve("unknown")).isEqualTo(emptyArray<InetAddress>())
+    }
+}

kronos-java/src/test/java/com/lyft/kronos/sntp/UdpTests.kt

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+package com.lyft.kronos.sntp
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+import java.net.InetAddress
+import java.net.InetSocketAddress
+import java.nio.ByteBuffer
+
+class UdpTests {
+
+    private val udp by lazy { Udp.Impl() }
+
+    @Test
+    fun requestUnknown() {
+        val address = InetSocketAddress(InetAddress.getLoopbackAddress(), 42)
+        val request = ByteBuffer.allocate(42)
+        val response = ByteBuffer.allocate(42)
+
+        assertThat(udp.request(address, request, response)).isEqualTo(Udp.Result.Failure)
+    }
+}