Fix panic on piring.Buffer.PointerTo(-Cap())

Function panic when parameter is equal to -Cap().

Update the ring buffer’s index logic so negative and positive offsets always wrap correctly within bounds

Author: elgopher

piring/piring.go

@@ -42,10 +42,10 @@ func (b *Buffer[E]) Cap() int {
 func (b *Buffer[E]) PointerTo(index int) *E {
 	idx := index + b.start
 
-	if idx < 0 {
-		idx = len(b.data) + idx%len(b.data)
-	} else if idx >= len(b.data) {
-		idx %= len(b.data)
+	// wrap the index so that both positive and negative values fall
+	// into the [0, Cap()-1] range
+	if capacity := len(b.data); capacity > 0 {
+		idx = ((idx % capacity) + capacity) % capacity
 	}
 
 	return &b.data[idx]

piring/piring_test.go

@@ -18,7 +18,7 @@ func TestBuffer(t *testing.T) {
 		assert.Zero(t, *buffer.PointerTo(0))
 	})
 
-	t.Run("no circle", func(t *testing.T) {
+	t.Run("no cycle", func(t *testing.T) {
 		buffer := piring.NewBuffer[int](3)
 		*buffer.NextWritePointer() = 0
 		*buffer.NextWritePointer() = 1
@@ -31,7 +31,7 @@ func TestBuffer(t *testing.T) {
 		assertHas(t, buffer, 2, 2)
 	})
 
-	t.Run("circle", func(t *testing.T) {
+	t.Run("cycle", func(t *testing.T) {
 		buffer := piring.NewBuffer[int](2)
 		*buffer.NextWritePointer() = 0
 		*buffer.NextWritePointer() = 1
@@ -43,7 +43,7 @@ func TestBuffer(t *testing.T) {
 		assertHas(t, buffer, 1, 2)
 	})
 
-	t.Run("two circles", func(t *testing.T) {
+	t.Run("two cycles", func(t *testing.T) {
 		buffer := piring.NewBuffer[int](2)
 		*buffer.NextWritePointer() = 0
 		*buffer.NextWritePointer() = 1
@@ -95,6 +95,14 @@ func TestBuffer(t *testing.T) {
 		assertHas(t, buffer, 5, *buffer.PointerTo(1))
 		assertHas(t, buffer, -5, *buffer.PointerTo(1))
 	})
+
+	t.Run("negative index equal to -Cap()", func(t *testing.T) {
+		buffer := piring.NewBuffer[int](2)
+		*buffer.NextWritePointer() = 0
+		*buffer.NextWritePointer() = 1
+
+		assertHas(t, buffer, -buffer.Cap(), *buffer.PointerTo(0))
+	})
 }
 
 func assertHas[T any](t *testing.T, buffer *piring.Buffer[T], index int, value T) {