From a583097bbce25e2938e76f89b42e5fdabc9d6c60 Mon Sep 17 00:00:00 2001
From: Alex Kocharin <alex@kocharin.ru>
Date: Mon, 28 Dec 2020 19:05:08 +0300
Subject: [PATCH] Shorthand tags with !! whenever possible

fix https://github.com/nodeca/js-yaml/issues/258
---
 CHANGELOG.md        |  1 +
 lib/dumper.js       |  2 ++
 test/issues/0258.js | 26 ++++++++++++++++++++++++++
 3 files changed, 29 insertions(+)
 create mode 100644 test/issues/0258.js

diff --git a/CHANGELOG.md b/CHANGELOG.md
index edaa3f9d..f9c7c8e0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `undefined` in mappings, #571.
 - `dump()` with `skipInvalid=true` now serializes invalid items in collections as null.
 - Custom tags starting with `!` are now dumped as `!tag` instead of `!<!tag>`, #576.
+- Custom tags starting with `tag:yaml.org,2002:` are now shorthanded using `!!`, #258.
 
 ### Added
 - Added `.mjs` (es modules) support.
diff --git a/lib/dumper.js b/lib/dumper.js
index 25a92433..f357a6ae 100644
--- a/lib/dumper.js
+++ b/lib/dumper.js
@@ -888,6 +888,8 @@ function writeNode(state, level, object, block, compact, iskey, isblockseq) {
 
       if (state.tag[0] === '!') {
         tagStr = '!' + tagStr;
+      } else if (tagStr.slice(0, 18) === 'tag:yaml.org,2002:') {
+        tagStr = '!!' + tagStr.slice(18);
       } else {
         tagStr = '!<' + tagStr + '>';
       }
diff --git a/test/issues/0258.js b/test/issues/0258.js
new file mode 100644
index 00000000..ef89d880
--- /dev/null
+++ b/test/issues/0258.js
@@ -0,0 +1,26 @@
+'use strict';
+
+
+const assert = require('assert');
+const yaml = require('../../');
+
+
+it('should shorthand tags with !! whenever possible', function () {
+  let regexp = new yaml.Type('tag:yaml.org,2002:js/regexp', {
+    kind: 'scalar',
+    resolve: () => true,
+    construct: str => new RegExp(str),
+    instanceOf: RegExp,
+    represent: object => object.source
+  });
+
+  let schema = yaml.DEFAULT_SCHEMA.extend(regexp);
+
+  let source = 're: !!js/regexp .*\n';
+
+  let object = yaml.load(source, { schema });
+  assert(object.re instanceof RegExp);
+
+  let str = yaml.dump(object, { schema });
+  assert.strictEqual(str, source);
+});