lib: add exponential backoff policy

lib/android/src/main/kotlin/im/molly/monero/BackoffPolicy.kt

@@ -0,0 +1,49 @@
+package im.molly.monero
+
+import kotlin.math.pow
+import kotlin.random.Random
+import kotlin.time.Duration
+import kotlin.time.Duration.Companion.minutes
+import kotlin.time.Duration.Companion.seconds
+import kotlin.time.DurationUnit
+import kotlin.time.toDuration
+
+interface BackoffPolicy {
+    /**
+     * Returns the amount of time to wait before performing a retry or a reconnect, based on the current [retryCount].
+     */
+    fun waitTime(retryCount: Int): Duration
+}
+
+/**
+ * A [BackoffPolicy] based on exponential backoff and jitter.
+ *
+ * @param minBackoff Set the minimum [Duration] for the first backoff.
+ * @param maxBackoff Set a hard maximum [Duration] for exponential backoff.
+ */
+class ExponentialBackoff(
+    private val minBackoff: Duration = 1.seconds,
+    private val maxBackoff: Duration = 20.seconds,
+    private val multiplier: Double = 1.6,
+    private val jitter: Double = 0.2,
+) : BackoffPolicy {
+    init {
+        require(minBackoff.isPositive())
+        require(maxBackoff.isPositive())
+        require(multiplier > 1.0)
+        require(jitter < 1.0)
+    }
+
+    override fun waitTime(retryCount: Int): Duration =
+        if (retryCount > 0) {
+            addJitter((minBackoff * multiplier.pow(retryCount)).coerceAtMost(maxBackoff))
+        } else {
+            Duration.ZERO
+        }
+
+    private fun addJitter(waitTime: Duration): Duration {
+        val jitterAmount = waitTime.inWholeMilliseconds * jitter
+        val jitter = Random.nextDouble(-jitterAmount, jitterAmount)
+        return waitTime + jitter.toDuration(DurationUnit.MILLISECONDS)
+    }
+}