Enable sorting of JSON object keys when encoding

* lisp/json.el (json-encoding-object-sort-predicate): New variable for
specifying a sorting predicate for JSON objects during encoding.
(json--plist-to-alist): New utility function.
(json-encode-hash-table): Re-use `json-encode-alist' when object keys
are to be sorted.
(json-encode-alist): Sort output by
`json-encoding-object-sort-predicate, when set.
(json-encode-plist): Re-use `json-encode-alist' when object keys are
to be sorted.
(json-pretty-print-buffer-ordered): New command to pretty print the
buffer with object keys sorted alphabetically.
(json-pretty-print-ordered): New command to pretty print the region with
object keys sorted alphabetically.

* test/automated/json-tests.el (test-json-plist-to-alist)
(test-json-encode-plist, test-json-encode-hash-table)
(test-json-encode-alist-with-sort-predicate)
(test-json-encode-plist-with-sort-predicate): New tests.

* etc/NEWS: Add an entry for the new commands.

1 files changed, 4 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index f3df92e51e5..46910b021c7 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -332,6 +332,10 @@ unlike `bookmark-set' which silently updates an existing bookmark.
 ---
 *** `json-pretty-print' and `json-pretty-print-buffer' now maintain
 the ordering of object keys by default.
+---
+*** New commands `json-pretty-print-ordered' and
+`json-pretty-print-buffer-ordered' pretty prints JSON objects with
+object keys sorted alphabetically.
 
 ** You can recompute the VC state of a file buffer with `M-x vc-refresh-state'
 ** Prog mode has some support for multi-mode indentation.