fix: make LinearRetry behave linearly for all inputs

gateway/src/main/kotlin/retry/LinearRetry.kt

@@ -5,6 +5,7 @@ import kotlinx.atomicfu.update
 import kotlinx.coroutines.delay
 import mu.KotlinLogging
 import kotlin.time.Duration
+import kotlin.time.times
 
 private val linearRetryLogger = KotlinLogging.logger { }
 
@@ -27,7 +28,7 @@ public class LinearRetry(
         require(
             maxBackoff.minus(firstBackoff).isPositive()
         ) { "maxBackoff ${maxBackoff.inWholeMilliseconds} ms needs to be bigger than firstBackoff ${firstBackoff.inWholeMilliseconds} ms" }
-        require(maxTries > 0) { "maxTries needs to be positive but was $maxTries" }
+        require(maxTries > 1) { "maxTries needs to be greater than 1 but was $maxTries" }
     }
 
     private val tries = atomic(0)
@@ -42,10 +43,12 @@ public class LinearRetry(
     override suspend fun retry() {
         if (!hasNext) error("max retries exceeded")
 
-        tries.incrementAndGet()
-        var diff = (maxBackoff - firstBackoff).inWholeMilliseconds / maxTries
-        diff *= tries.value
-        linearRetryLogger.trace { "retry attempt ${tries.value}, delaying for $diff ms" }
+        // tries/maxTries ratio * (backOffDiff) = retryProgress
+        val ratio = tries.getAndIncrement() / (maxTries - 1).toDouble()
+        val retryProgress = ratio * (maxBackoff - firstBackoff)
+        val diff = firstBackoff + retryProgress
+
+        linearRetryLogger.trace { "retry attempt ${tries.value}, delaying for $diff" }
         delay(diff)
     }
 

gateway/src/test/kotlin/helper/LinearRetryTest.kt

@@ -0,0 +1,46 @@
+package helper
+
+import dev.kord.gateway.retry.LinearRetry
+import kotlinx.coroutines.test.currentTime
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+import kotlin.time.Duration.Companion.milliseconds
+import kotlin.time.Duration.Companion.seconds
+
+class LinearRetryTest {
+
+    @Test
+    fun testLinearity() = runTest {
+        val linearRetry = LinearRetry(1.seconds, 10.seconds, 10)
+        var i = 0
+        val start = currentTime
+
+        while (linearRetry.hasNext) {
+            linearRetry.retry()
+            i++
+        }
+
+        val end = currentTime
+        val elapsed = (end - start).milliseconds
+
+        assert(elapsed == 55.seconds)
+        assert(i == 10)
+    }
+
+    @Test
+    fun testExtreme() = runTest {
+        val linearRetry = LinearRetry(1.seconds, 60.seconds, Int.MAX_VALUE)
+        var i = 0
+        val start = currentTime
+
+        while (linearRetry.hasNext && i < 1000) {
+            linearRetry.retry()
+            i++
+        }
+
+        val end = currentTime
+        val elapsed = (end - start).milliseconds
+
+        assert(elapsed == 1000.seconds)
+    }
+}