Add conversion to reverse format and back for ipv4

lib/common.ts

@@ -1,6 +1,10 @@
 import { Address4 } from './ipv4';
 import { Address6 } from './ipv6';
 
+export interface ReverseFormOptions {
+  omitSuffix?: boolean;
+}
+
 export function isInSubnet(this: Address4 | Address6, address: Address4 | Address6) {
   if (this.subnetMask < address.subnetMask) {
     return false;

lib/ipv4.ts

@@ -117,6 +117,29 @@ export class Address4 {
     return Address4.fromHex(integer.toString(16));
   }
 
+
+  /**
+   * Return an address from in-addr.arpa form
+   * @memberof Address4
+   * @static
+   * @param {string} arpaFormAddress - an 'in-addr.arpa' form ipv4 address
+   * @returns {Adress4}
+   * @example
+   * var address = Address4.fromArpa(42.2.0.192.in-addr.arpa.)
+   * address.correctForm(); // '192.0.2.42'
+   */
+  static fromArpa(arpaFormAddress: string): Address4 {
+    // remove ending ".in-addr.arpa." or just "."
+    const leader = arpaFormAddress.replace(/(\.in-addr\.arpa)?\.$/, '');
+
+    const address = leader.split('.')
+      .reverse()
+      .join('.');
+
+    return new Address4(address);
+  }
+
+
   /**
    * Converts an IPv4 address object to a hex string
    * @memberof Address4
@@ -275,6 +298,31 @@ export class Address4 {
     return this.binaryZeroPad().slice(start, end);
   }
 
+  /**
+   * Return the reversed ip6.arpa form of the address
+   * @memberof Address4
+   * @param {Object} options
+   * @param {boolean} options.omitSuffix - omit the "in-addr.arpa" suffix
+   * @instance
+   * @returns {String}
+   */
+  reverseForm(options?: common.ReverseFormOptions): string {
+    if (!options) {
+      options = {};
+    }
+
+    const reversed = this.correctForm()
+      .split('.')
+      .reverse()
+      .join('.');
+
+    if (options.omitSuffix) {
+      return reversed;
+    } else {
+      return sprintf('%s.in-addr.arpa.', reversed);
+    }
+  }
+
   /**
    * Returns true if the given address is in the subnet of the current address
    * @memberof Address4

lib/ipv6.ts

@@ -66,9 +66,6 @@ function unsignByte(b: number) {
   return b & 0xff;
 }
 
-interface ReverseFormOptions {
-  omitSuffix?: boolean;
-}
 
 interface SixToFourProperties {
   prefix: string;
@@ -506,7 +503,7 @@ export class Address6 {
    * @instance
    * @returns {String}
    */
-  reverseForm(options?: ReverseFormOptions): string {
+  reverseForm(options?: common.ReverseFormOptions): string {
     if (!options) {
       options = {};
     }

test/address-test.ts

@@ -30,6 +30,14 @@ function addressIs(addressString: string, descriptors: string[]) {
           address4.subnetMask.should.be.at.least(0);
           address4.subnetMask.should.be.at.most(128);
         });
+        it('converts to arpa format and back', () => {
+          const arpa = address4.reverseForm();
+          arpa.length.should.be.at.most(29);
+
+          const converted = Address4.fromArpa(arpa);
+
+          address4.correctForm().should.equal(converted.correctForm());
+        });
       }
 
       if (descriptor === 'valid-ipv6') {