From 8cefae503fba61805e59ab5cb16acafb2c49b706 Mon Sep 17 00:00:00 2001
From: William Dillon <wmdillon@me.com>
Date: Thu, 26 Feb 2026 19:55:48 -0500
Subject: [PATCH] debugging DefaltApplyJitter, adding tests

---
 fibonacci/fibonacci.go      |  2 ++
 fibonacci/fibonacci_test.go |  8 -----
 utilities/jitter.go         | 59 +++++++++++++++++++++++++++++++++----
 utilities/jitter_test.go    | 23 +++++++++++++++
 4 files changed, 79 insertions(+), 13 deletions(-)
 create mode 100644 utilities/jitter_test.go

diff --git a/fibonacci/fibonacci.go b/fibonacci/fibonacci.go
index d94c0fe..b790bfe 100644
--- a/fibonacci/fibonacci.go
+++ b/fibonacci/fibonacci.go
@@ -93,6 +93,8 @@ func Fibonacci(n int64) int64 {
 	case n >= int64(len(TheFibonacciSequence)):
 		panic(fmt.Sprintf("invalid input: %d overflows int64\n",
 			n))
+	case n < 0:
+		panic(fmt.Sprintf("invalid input: %d would cause index out of range panic\n", n))
 	default:
 		return TheFibonacciSequence[n]
 	}
diff --git a/fibonacci/fibonacci_test.go b/fibonacci/fibonacci_test.go
index 6b731f5..7d03840 100644
--- a/fibonacci/fibonacci_test.go
+++ b/fibonacci/fibonacci_test.go
@@ -2,19 +2,12 @@ package fibonacci
 
 import (
 	"context"
-	"fmt"
-	"math/big"
 	"testing"
 	"time"
 
 	"code.wmdillon.com/wmdillon/backoff/utilities"
 )
 
-func wouldOverflow(a, b int64) (product int64, overflows bool) {
-	results := new(big.Int).Mul(big.NewInt(a), big.NewInt(b))
-	return results.Int64(), results.IsInt64()
-}
-
 func fibonacci(n int64) int64 {
 	switch {
 	case n <= 0:
@@ -129,7 +122,6 @@ func TestFibonacciPauseLimit(t *testing.T) {
 			t.Fatalf("error for %d: expected value in range %s - %s; got %s\n", input, min, max, got)
 		}
 	}
-	fmt.Printf("finished\n")
 }
 
 func TestFibonacciBackoffAfter(t *testing.T) {
diff --git a/utilities/jitter.go b/utilities/jitter.go
index 2d0d923..40a409d 100644
--- a/utilities/jitter.go
+++ b/utilities/jitter.go
@@ -10,15 +10,64 @@ var (
 	ApplyJitter = DefaultApplyJitter
 )
 
+const (
+	maxInt64 = math.MaxInt64
+	minInt64 = math.MinInt64
+)
+
+func satAdd(a, b int64) int64 {
+	switch {
+	case b > 0 && a > maxInt64-b:
+		return maxInt64
+	case b < 0 && a < minInt64-b:
+		return minInt64
+	default:
+		return a + b
+	}
+}
+
+func satSub(a, b int64) int64 {
+	switch {
+	case b > 0 && a < minInt64+b:
+		return minInt64
+	case b < 0 && a > maxInt64+b:
+		return maxInt64
+	default:
+		return a - b
+	}
+}
+
 func DefaultApplyJitter(pauseDuration time.Duration, jitter time.Duration) time.Duration {
 	switch {
-	case jitter == 0, pauseDuration == 0:
+	case pauseDuration <= 0:
+		return 0
+	case jitter == 0:
 		return pauseDuration
+	case jitter < 0:
+		jitter = -jitter
 	case jitter > pauseDuration:
 		pauseDuration, jitter = jitter, pauseDuration
 	}
-	min := pauseDuration.Nanoseconds() - jitter.Nanoseconds()
-	max := pauseDuration.Nanoseconds() + jitter.Nanoseconds()
-	rnd := int64(math.Max(float64(rand.Int64N(max-min)+min), 0))
-	return time.Duration(rnd)
+	min := satSub(pauseDuration.Nanoseconds(), jitter.Nanoseconds())
+	if min < 0 {
+		min = 0
+	}
+	max := satAdd(pauseDuration.Nanoseconds(), jitter.Nanoseconds())
+	if max <= min {
+		return time.Duration(min)
+	}
+
+	var maxExcl int64
+	if max == math.MaxInt64 {
+		span := max - min
+		if span == math.MaxInt64 {
+			return time.Duration(rand.Int64N(math.MaxInt64) + min)
+		}
+		n := rand.Int64N(span + 1)
+		return time.Duration(min + n)
+	}
+
+	maxExcl = max + 1
+	return time.Duration(min + rand.Int64N(maxExcl-min))
+
 }
diff --git a/utilities/jitter_test.go b/utilities/jitter_test.go
new file mode 100644
index 0000000..95059e7
--- /dev/null
+++ b/utilities/jitter_test.go
@@ -0,0 +1,23 @@
+package utilities
+
+import (
+	"testing"
+	"time"
+)
+
+func TestDefaultApplyJitterIsAlwaysGEZero(t *testing.T) {
+	if got := DefaultApplyJitter(maxInt64, time.Nanosecond); got < 0 {
+		t.Fatalf("error: wanted >= 0; got %s\n", got)
+	}
+	if got := DefaultApplyJitter(maxInt64, maxInt64); got < 0 {
+		t.Fatalf("error: wanted >= 0; got %s\n", got)
+	}
+}
+
+func TestDefaultApplyJitterWithZeroPauseAlwaysZero(t *testing.T) {
+	for i := 0; i < 10; i++ {
+		if got := DefaultApplyJitter(0, time.Duration(i)); got != 0 {
+			t.Fatalf("error: wanted 0; got %s\n", got)
+		}
+	}
+}