fix: compress trailing zero run in toString when a zoneIndex is present

lib/ipaddr.js

@@ -773,7 +773,15 @@
         // in line with RFC 5952 (see https://tools.ietf.org/html/rfc5952#section-4)
         IPv6.prototype.toRFC5952String = function () {
             const regex = /((^|:)(0(:|$)){2,})/g;
-            const string = this.toNormalizedString();
+            // The zone identifier (RFC 4007) is not part of the address; match
+            // against the address alone so a trailing zero run right before the
+            // "%" suffix still gets compressed (RFC 5952, 4.2.2).
+            let suffix = '';
+            if (this.zoneId) {
+                suffix = `%${this.zoneId}`;
+            }
+            const normalized = this.toNormalizedString();
+            const string = normalized.slice(0, normalized.length - suffix.length);
             let bestMatchIndex = 0;
             let bestMatchLength = -1;
             let bestMatchGroups = -1;
@@ -793,10 +801,10 @@
             }
 
             if (bestMatchLength < 0) {
-                return string;
+                return string + suffix;
             }
 
-            return `${string.substring(0, bestMatchIndex)}::${string.substring(bestMatchIndex + bestMatchLength)}`;
+            return `${string.substring(0, bestMatchIndex)}::${string.substring(bestMatchIndex + bestMatchLength)}${suffix}`;
         };
 
         // Returns the address in compact, human-readable format like

test/ipaddr.test.js

@@ -275,6 +275,15 @@ describe('ipaddr', () => {
         );
     })
 
+    it('compresses a trailing zero run when a zoneIndex is present', () => {
+        // The zone identifier (RFC 4007) is not part of the address and must
+        // not prevent zero-run compression at the end of it (RFC 5952, 4.2.2).
+        assert.equal(new ipaddr.IPv6([0xfe80, 0, 0, 0, 0, 0, 0, 0], 'eth0').toString(), 'fe80::%eth0');
+        assert.equal(new ipaddr.IPv6([0x2001, 0xdb8, 0, 0, 0, 0, 0, 0], 'eth0').toRFC5952String(), '2001:db8::%eth0');
+        assert.equal(ipaddr.parse('fe80::%eth0').toString(), 'fe80::%eth0');
+        assert.equal(ipaddr.parse('::%eth0').toString(), '::%eth0');
+    })
+
     it('returns IPv6 zoneIndex for IPv4-mapped IPv6 addresses', () => {
         let addr = ipaddr.parse('::ffff:192.168.1.1%eth0');
         assert.equal(addr.toNormalizedString(), '0:0:0:0:0:ffff:c0a8:101%eth0');