Data commit

Task/Animate-a-pendulum/Kotlin/animate-a-pendulum.kotlin

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+import java.awt.*
+import java.util.concurrent.*
+import javax.swing.*
+
+class Pendulum(private val length: Int) : JPanel(), Runnable {
+    init {
+        val f = JFrame("Pendulum")
+        f.add(this)
+        f.defaultCloseOperation = JFrame.EXIT_ON_CLOSE
+        f.pack()
+        f.isVisible = true
+        isDoubleBuffered = true
+    }
+
+    override fun paint(g: Graphics) {
+        with(g) {
+            color = Color.WHITE
+            fillRect(0, 0, width, height)
+            color = Color.BLACK
+            val anchor = Element(width / 2, height / 4)
+            val ball = Element((anchor.x + Math.sin(angle) * length).toInt(), (anchor.y + Math.cos(angle) * length).toInt())
+            drawLine(anchor.x, anchor.y, ball.x, ball.y)
+            fillOval(anchor.x - 3, anchor.y - 4, 7, 7)
+            fillOval(ball.x - 7, ball.y - 7, 14, 14)
+        }
+    }
+
+    override fun run() {
+        angleVelocity += -9.81 / length * Math.sin(angle) * dt
+        angle += angleVelocity * dt
+        repaint()
+    }
+
+    override fun getPreferredSize() = Dimension(2 * length + 50, length / 2 * 3)
+
+    private data class Element(val x: Int, val y: Int)
+
+    private val dt = 0.1
+    private var angle = Math.PI / 2
+    private var angleVelocity = 0.0
+}
+
+fun main(a: Array<String>) {
+    val executor = Executors.newSingleThreadScheduledExecutor()
+    executor.scheduleAtFixedRate(Pendulum(200), 0, 15, TimeUnit.MILLISECONDS)
+}