From 63bd4cd95f0b1a66948584823acc4eabb37b9244 Mon Sep 17 00:00:00 2001
From: David Gardiner <david@gardiner.net.au>
Date: Thu, 30 Jan 2025 23:07:37 +1030
Subject: [PATCH] Handle when VNet is empty

Fixes #34
---
 Find-FreeSubnets-NoExisting.Expected.json | 12 +++++
 VNet.Tests.ps1                            | 56 +++++++++++++++++++++++
 VNet.psm1                                 | 11 ++++-
 3 files changed, 77 insertions(+), 2 deletions(-)
 create mode 100644 Find-FreeSubnets-NoExisting.Expected.json

diff --git a/Find-FreeSubnets-NoExisting.Expected.json b/Find-FreeSubnets-NoExisting.Expected.json
new file mode 100644
index 0000000..1ab96ed
--- /dev/null
+++ b/Find-FreeSubnets-NoExisting.Expected.json
@@ -0,0 +1,12 @@
+{
+    "VNetStart": "10.1.0.0",
+    "VNetEnd": "10.1.255.255",
+    "Available": [
+        {
+            "Start": "10.1.0.0",
+            "End": "10.1.255.255",
+            "Size": 65536,
+            "CIDRAvailable": "{10.1.0.0/25, 10.1.0.0/26, 10.1.0.0/27, 10.1.0.0/28…}"
+        }
+    ]
+}
\ No newline at end of file
diff --git a/VNet.Tests.ps1 b/VNet.Tests.ps1
index 9fa7e93..33a82a2 100644
--- a/VNet.Tests.ps1
+++ b/VNet.Tests.ps1
@@ -153,4 +153,60 @@ Describe "Find-FreeSubnets" {
         $expected = [VNetSummary](Get-content .\Find-FreeSubnets.Expected.json | ConvertFrom-Json)
         $result | Should -BeVNetSummary -ExpectedValue $expected
     }
+
+    It "Returns expected output with no existing subnets" {
+
+        $vnetWithNoSubnets = @"
+{
+  "AddressSpace": {
+    "AddressPrefixes": [
+      "10.1.0.0/16"
+    ]
+  },
+  "DhcpOptions": {
+    "DnsServers": null
+  },
+  "FlowTimeoutInMinutes": null,
+  "Subnets": [],
+  "BgpCommunities": null,
+  "Encryption": {
+    "Enabled": "False",
+    "Enforcement": "AllowUnencrypted"
+  },
+  "VirtualNetworkPeerings": [],
+  "ProvisioningState": "Succeeded",
+  "EnableDdosProtection": false,
+  "DdosProtectionPlan": null,
+  "IpAllocations": [],
+  "ExtendedLocation": null,
+  "ResourceGroupName": "rg-freesubnet-australiaeast",
+  "Location": "australiaeast",
+  "ResourceGuid": "6dac3c19-4583-4a63-bbb3-38746d1f6aab",
+  "Type": "Microsoft.Network/virtualNetworks",
+  "Tag": {},
+  "TagsTable": null,
+  "Name": "vnet-freesubnet2-australiaeast",
+  "Etag": "W/\"28d22421-aaea-47e1-a456-f6c3b413e7a8\"",
+  "Id": "/subscriptions/b4b2e7e9-66e7-46b5-a56b-cce2b50011d4/resourceGroups/rg-freesubnet-australiaeast/providers/Microsoft.Network/virtualNetworks/vnet-freesubnet2-australiaeast",
+  "AddressSpaceText": "{\r\n  \"AddressPrefixes\": [\r\n    \"10.1.0.0/16\"\r\n  ]\r\n}",
+  "DhcpOptionsText": "{}",
+  "FlowTimeoutInMinutesText": "null",
+  "SubnetsText": "[]",
+  "BgpCommunitiesText": "null",
+  "EncryptionText": "{\r\n  \"Enabled\": \"False\",\r\n  \"Enforcement\": \"AllowUnencrypted\"\r\n}",
+  "VirtualNetworkPeeringsText": "[]",
+  "EnableDdosProtectionText": "false",
+  "DdosProtectionPlanText": "null",
+  "IpAllocationsText": "[]",
+  "ExtendedLocationText": "null"
+}
+"@ | ConvertFrom-Json
+
+        Mock -ModuleName VNet -CommandName Get-AzVirtualNetwork { return $vnetWithNoSubnets }
+
+        $result = Find-FreeSubnets -ResourceGroup rg-freesubnet-australiaeast -VNetName vnet-freesubnet2-australiaeast
+
+        $expected = [VNetSummary](Get-content .\Find-FreeSubnets-NoExisting.Expected.json | ConvertFrom-Json)
+        $result | Should -BeVNetSummary -ExpectedValue $expected
+    }
 }
diff --git a/VNet.psm1 b/VNet.psm1
index fc7c941..eac1966 100644
--- a/VNet.psm1
+++ b/VNet.psm1
@@ -145,7 +145,14 @@ function Find-FreeSubnets {
 
             $afterLastAvailableCidr = "$afterLastAvailable/31"
 
-            $sorted = @($vnet.Subnets.AddressPrefix) + $afterLastAvailableCidr | Sort-Object -Property {
+            # create fake subnet immediately before the start of the VNet
+            $ipNum = [Net.IPAddress]::NetworkToHostOrder([BitConverter]::ToInt32($vnetStart.GetAddressBytes(), 0)) - 1
+            $bytes = [BitConverter]::GetBytes([Net.IPAddress]::HostToNetworkOrder($ipNum))
+            [Net.IPAddress] $beforeFirstAvailable = New-Object Net.IPAddress -ArgumentList (, $bytes)
+
+            $beforeFirstAvailableCidr = "$beforeFirstAvailable/31"
+
+            $sorted = @($beforeFirstAvailableCidr) + @($vnet.Subnets.AddressPrefix) + @($afterLastAvailableCidr) | Sort-Object -Property {
                 $addr, $maskLength = $_ -split '/'
 
                 $ip = ([Net.IPAddress] $addr)
@@ -207,7 +214,7 @@ function Find-FreeSubnets {
 
                 $notFirst = $true
 
-                if ($cidr -ne $afterLastAvailableCidr) {
+                if ($cidr -ne $afterLastAvailableCidr -and $cidr -ne $beforeFirstAvailableCidr) {
                     $subnet = [Subnet]::new()
                     $subnet.CIDR = $cidr
                     $subnet.Start = $start