Fix negative shift amount during prefix usage calc (#114)

prefix.go

@@ -638,8 +638,12 @@ func (p *Prefix) availablePrefixes() (uint64, []string) {
 	totalAvailable := uint64(0)
 	availablePrefixes := []string{}
 	for _, pfx := range pfxs {
+		bits := maxBits - pfx.Bits()
+		if bits < 0 {
+			continue
+		}
 		// same as: totalAvailable += uint64(math.Pow(float64(2), float64(maxBits-pfx.Bits)))
-		totalAvailable += 1 << (maxBits - pfx.Bits())
+		totalAvailable += 1 << bits
 		availablePrefixes = append(availablePrefixes, pfx.String())
 	}
 	// we are not reporting more that 2^31 available prefixes

prefix_test.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/sync/errgroup"
 )
@@ -1644,3 +1645,83 @@ func TestNamespaceFromContext(t *testing.T) {
 		})
 	}
 }
+func TestAvailablePrefixes(t *testing.T) {
+	testCases := []struct {
+		name                 string
+		cidr                 string
+		expectedTotal        uint64
+		expectedAvailablePfx []string
+	}{
+		{
+			name:                 "192.168.0.0/32",
+			cidr:                 "192.168.0.0/32",
+			expectedTotal:        0,
+			expectedAvailablePfx: []string{},
+		},
+		{
+			name:                 "192.168.0.0/31",
+			cidr:                 "192.168.0.0/31",
+			expectedTotal:        0,
+			expectedAvailablePfx: []string{},
+		},
+		{
+			name:                 "192.168.0.0/30",
+			cidr:                 "192.168.0.0/30",
+			expectedTotal:        1,
+			expectedAvailablePfx: []string{"192.168.0.0/30"},
+		},
+		{
+			name:                 "192.168.0.0/24",
+			cidr:                 "192.168.0.0/24",
+			expectedTotal:        64,
+			expectedAvailablePfx: []string{"192.168.0.0/24"},
+		},
+		{
+			name:                 "2001:0db8:85a3::/128",
+			cidr:                 "2001:0db8:85a3::/128",
+			expectedTotal:        0,
+			expectedAvailablePfx: []string{},
+		},
+		{
+			name:                 "2001:0db8:85a3::/127",
+			cidr:                 "2001:0db8:85a3::/127",
+			expectedTotal:        0,
+			expectedAvailablePfx: []string{},
+		},
+		{
+			name:                 "2001:0db8:85a3::/126",
+			cidr:                 "2001:0db8:85a3::/126",
+			expectedTotal:        1,
+			expectedAvailablePfx: []string{"2001:db8:85a3::/126"},
+		},
+		{
+			name:                 "Invalid CIDR",
+			cidr:                 "Invalid CIDR",
+			expectedTotal:        0,
+			expectedAvailablePfx: []string{},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			prefix := &Prefix{
+				Cidr:                   tc.cidr,
+				isParent:               false,
+				availableChildPrefixes: make(map[string]bool),
+			}
+
+			totalAvailable, availablePrefixes := prefix.availablePrefixes()
+
+			assert.Equal(
+				t, tc.expectedTotal, totalAvailable,
+				"Expected totalAvailable: %d, got: %d",
+				tc.expectedTotal, totalAvailable,
+			)
+			assert.ElementsMatchf(
+				t, availablePrefixes, tc.expectedAvailablePfx,
+				"Expected availablePrefixes: %v, got: %v",
+				tc.expectedAvailablePfx, availablePrefixes,
+			)
+		})
+	}
+}