Add CIDR collapse functionality

internal/lang/funcs/cidr.go

@@ -6,6 +6,7 @@ package funcs
 import (
 	"fmt"
 	"math/big"
+	"sort"
 
 	"github.com/apparentlymart/go-cidr/cidr"
 	"github.com/hashicorp/terraform/internal/ipaddr"
@@ -14,6 +15,57 @@ import (
 	"github.com/zclconf/go-cty/cty/gocty"
 )
 
+// CidrCollapseFunc contructs a function that collapses input CIDRs
+var CidrCollapseFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "cidrs",
+			Type: cty.List(cty.String),
+		},
+	},
+	Type:         function.StaticReturnType(cty.List(cty.String)),
+	RefineResult: refineNotNull,
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		var cidrsInput []string
+		if err := gocty.FromCtyValue(args[0], &cidrsInput); err != nil {
+			return cty.UnknownVal(cty.String), err
+		}
+
+		// confirm proper cidr notation
+		cidrs := make(map[string]*ipaddr.IPNet)
+		for _, c := range cidrsInput {
+			parsedIP, parsedNet, err := ipaddr.ParseCIDR(c)
+			if err != nil {
+				return cty.UnknownVal(cty.String), err
+			}
+			cidrs[parsedIP.String()] = parsedNet
+		}
+
+		// collapse the cidr ranges
+		for iIP, iNet := range cidrs {
+			for jIP, jNet := range cidrs {
+				if iIP == jIP {
+					continue
+				}
+				if iNet.Contains(jNet.IP) {
+					delete(cidrs, jIP)
+					continue
+				}
+			}
+		}
+
+		retVals := []cty.Value{}
+		for _, net := range cidrs {
+			retVals = append(retVals, cty.StringVal(net.String()))
+		}
+		sort.Slice(retVals, func(i, j int) bool {
+			return i > j
+		})
+
+		return cty.ListVal(retVals), nil
+	},
+})
+
 // CidrHostFunc contructs a function that calculates a full host IP address
 // within a given IP network address prefix.
 var CidrHostFunc = function.New(&function.Spec{
@@ -197,6 +249,11 @@ var CidrSubnetsFunc = function.New(&function.Spec{
 	},
 })
 
+// CidrCollapse collapses input CIDRs
+func CidrCollapse(cidrs cty.Value) (cty.Value, error) {
+	return CidrCollapseFunc.Call([]cty.Value{cidrs})
+}
+
 // CidrHost calculates a full host IP address within a given IP network address prefix.
 func CidrHost(prefix, hostnum cty.Value) (cty.Value, error) {
 	return CidrHostFunc.Call([]cty.Value{prefix, hostnum})

internal/lang/funcs/cidr_test.go

@@ -396,3 +396,58 @@ func TestCidrSubnets(t *testing.T) {
 		})
 	}
 }
+
+func TestCidrCollapse(t *testing.T) {
+	tests := []struct {
+		Cidrs cty.Value
+		Want  cty.Value
+		Err   string
+	}{
+		{
+			Cidrs: cty.ListVal([]cty.Value{
+				cty.StringVal("192.168.0.0/16"),
+				cty.StringVal("192.168.0.56/32"),
+				cty.StringVal("192.0.0.0/8"),
+			}),
+			Want: cty.ListVal([]cty.Value{
+				cty.StringVal("192.0.0.0/8"),
+			}),
+			Err: "",
+		},
+		{
+			Cidrs: cty.ListVal([]cty.Value{
+				cty.StringVal("192.168.0.0/16"),
+				cty.StringVal("167.123.0.42/32"),
+				cty.StringVal("167.123.0.0/16"),
+			}),
+			Want: cty.ListVal([]cty.Value{
+				cty.StringVal("167.123.0.0/16"),
+				cty.StringVal("192.168.0.0/16"),
+			}),
+			Err: "",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("cidrcollapse(%#v)", test.Cidrs), func(t *testing.T) {
+			got, err := CidrCollapse(test.Cidrs)
+			wantErr := test.Err != ""
+
+			if wantErr {
+				if err == nil {
+					t.Fatal("succeeded; want error")
+				}
+				if err.Error() != test.Err {
+					t.Fatalf("wrong error\ngot:  %s\nwant: %s", err.Error(), test.Err)
+				}
+				return
+			} else if err != nil {
+				t.Fatalf("unexpected error: %s", err)
+			}
+
+			if !got.RawEquals(test.Want) {
+				t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, test.Want)
+			}
+		})
+	}
+}

website/data/language-nav-data.json

@@ -690,6 +690,10 @@
           {
             "title": "<code>cidrsubnets</code>",
             "href": "/language/functions/cidrsubnets"
+          },
+          {
+            "title": "<code>cidrcollapse</code>",
+            "href": "/language/functions/cidrcollapse"
           }
         ]
       },

website/docs/language/functions/cidrcollapse.mdx

@@ -0,0 +1,26 @@
+---
+page_title: cidrcollapse - Functions - Configuration Language
+description: |-
+  The cidrcollapse function takes a list of CIDR ranges and collapses them into a
+  list of strings of superset ranges
+---
+
+# `cidrcollapse` Function
+
+`cidrcollapse` takes a list of CIDR ranges and collapses them into a list
+  of superset ranges.
+
+```hcl
+cidrcollapse(cidrs)
+```
+
+`cidrs` must be given as a list of valid CIDR ranges.
+
+The result is a string list value of superset ranges.
+
+## Examples
+
+```
+> cidrcollapse(["192.168.0.0/16", "167.123.0.42/32", "167.123.0.0/16"])
+["167.123.0.0/16","192.168.0.0/16"]
+```