Improve 'ipSliceDifference' function memory footprint (vmware#567)

.changes/v2.20.0/532-features.md

@@ -4,9 +4,9 @@
 * Added `NsxtEdgeGateway.GetUsedIpAddressSlice` method to fetch used IP addresses in a slice
   [GH-532]
 * Added `NsxtEdgeGateway.GetUnusedExternalIPAddresses` method that can help to find an unused
-  IP address in an Edge Gateway by given constraints [GH-532]
+  IP address in an Edge Gateway by given constraints [GH-532,GH-567]
 * Added `NsxtEdgeGateway.GetAllUnusedExternalIPAddresses` method that can return all unused IP
-  addresses in an Edge Gateway [GH-532]
+  addresses in an Edge Gateway [GH-532,GH-567]
 * Added `NsxtEdgeGateway.GetAllocatedIpCount` method that sums up `TotalIPCount` fields in all
   subnets [GH-532]
 * Added `NsxtEdgeGateway.QuickDeallocateIpCount` and `NsxtEdgeGateway.DeallocateIpCount`

govcd/nsxt_edgegateway.go

@@ -755,6 +755,9 @@ func flattenEdgeGatewayUplinkToIpSlice(uplinks []types.EdgeGatewayUplinks) ([]ne
 // Special behavior:
 // * Passing nil minuend results in nil
 // * Passing nil subtrahend will return minuendSlice
+//
+// NOTE. This function will mutate minuendSlice to save memory and avoid having a copy of all values
+// which can become expensive if there are a lot of items
 func ipSliceDifference(minuendSlice, subtrahendSlice []netip.Addr) []netip.Addr {
 	if minuendSlice == nil {
 		return nil
@@ -773,31 +776,38 @@ func ipSliceDifference(minuendSlice, subtrahendSlice []netip.Addr) []netip.Addr
 		return minuendSlice
 	}
 
-	var difference []netip.Addr
+	resultIpCount := 0 // count of IPs after removing items from subtrahendSlice
 
 	// Loop over minuend IPs
 	for _, minuendIp := range minuendSlice {
 
 		// Check if subtrahend has minuend element listed
 		var foundSubtrahend bool
-
 		for _, subtrahendIp := range subtrahendSlice {
 			if subtrahendIp == minuendIp {
 				// IP found in subtrahend, therefore breaking inner loop early
 				foundSubtrahend = true
 				break
 			}
-
 		}
 
-		// Store the IP in difference when subtrahend does not contain IP of minuend
+		// Store the IP in `minuendSlice` at `resultIpCount` index and increment the index itself
 		if !foundSubtrahend {
-			// Add IP to the resulting difference slice
-			difference = append(difference, minuendIp)
+			// Add IP to the 'resultIpCount' index position
+			minuendSlice[resultIpCount] = minuendIp
+			resultIpCount++
 		}
 	}
 
-	return difference
+	// if all elements are removed - return nil
+	if resultIpCount == 0 {
+		return nil
+	}
+
+	// cut off all values, greater than `resultIpCount`
+	minuendSlice = minuendSlice[:resultIpCount]
+
+	return minuendSlice
 }
 
 // filterIpSlicesBySubnet accepts 'ipRange' and returns a slice of IPs only that fall into given