Added jitter strategy that jitters after a specified threshold.

src/Jitter/JitterAfterThreshold.php

@@ -0,0 +1,28 @@
+<?php
+declare(strict_types=1);
+
+namespace EventSauce\BackOff\Jitter;
+
+class JitterAfterThreshold implements Jitter
+{
+    private int $threshold;
+    private Jitter $strategy;
+
+    public function __construct(
+        int $threshold,
+        Jitter $strategy = null
+    ) {
+
+        $this->threshold = $threshold;
+        $this->strategy = $strategy ?: new FullJitter();
+    }
+
+    public function jitter(int $sleep): int
+    {
+        if ($sleep <= $this->threshold) {
+            return $sleep;
+        }
+
+        return $this->threshold + $this->strategy->jitter($sleep - $this->threshold);
+    }
+}

src/Jitter/JitterAfterThresholdTest.php

@@ -0,0 +1,37 @@
+<?php
+declare(strict_types=1);
+
+namespace EventSauce\BackOff\Jitter;
+
+use PHPUnit\Framework\TestCase;
+use function array_sum;
+use function count;
+use function max;
+use function min;
+
+class JitterAfterThresholdTest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function jitting_results_in_random_values_across_a_full_range_but_not_below_threshold(): void
+    {
+        $jitter = new JitterAfterThreshold(250);
+        $jittedSleeps = [];
+
+        for ($i = 0; $i < 100000; $i++) {
+            $jittedSleeps[] = $jitter->jitter(1000);
+        }
+
+        $average = (int) (array_sum($jittedSleeps) / count($jittedSleeps));
+        $max = max(...$jittedSleeps);
+        $min = min(...$jittedSleeps);
+
+        self::assertTrue($average > 525);
+        self::assertTrue($average < 725);
+        self::assertTrue($max > 900);
+        self::assertTrue($max <= 1000);
+        self::assertTrue($min < 350);
+        self::assertTrue($min >= 250);
+    }
+}