bug#21616: [PATCH] Enable sorting of JSON object keys when encoding

[Simen Heggestøyl]

When working with JSON data, it is often convenient to be able to
prettify the data, and having object keys sorted in a fixed order.
This patch adds the variable `json-encoding-object-sort-key' which
enables defining a sort key to be used when encoding JSON objects.
Additionally, the commands `json-pretty-print-ordered' and
`json-pretty-print-buffer-ordered' are added for convenience,
providing prettification with alphabetical ordering of object keys.

This gets rid of a lot of redundant code from `json-encode-hash-table'
and `json-encode-plist' by going via alists, and using the logic of
`json-encode-alist' commonly for all the structures.

I was in doubt whether to make `json-pretty-print-ordered' and
`json-pretty-print-buffer-ordered' their own commands, or if it would
be better to provide this functionality by having `json-pretty-print'
and `json-pretty-print-buffer' accept prefix arguments. I decided on
the former, because I think it makes the commands easier to discover.

A proposed patch follows!

-- Simen


>From bf32702520b17e9bb975c68adb79c0d84e2cbf41 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Simen=20Heggest=C3=B8yl?= <simenheg@gmail.com>
Date: Sun, 4 Oct 2015 12:21:57 +0200
Subject: [PATCH] Enable sorting of JSON object keys when encoding

* lisp/json.el (json-encoding-object-sort-key): New variable for
specifying a sort key for JSON objects during encoding.
(json--plist-to-alist): New utility function.
(json-encode-hash-table): Re-use `json-encode-alist'.
(json-encode-alist): Sort output by `json-encoding-object-sort-key, when
set.
(json-encode-plist): Re-use `json-encode-alist'.
(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-with-sort-key): New tests.

* etc/NEWS: Add an entry for the new commands.
---
 etc/NEWS                     |  4 +++
 lisp/json.el                 | 77 +++++++++++++++++++++-----------------------
 test/automated/json-tests.el | 22 +++++++++++++
 3 files changed, 63 insertions(+), 40 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index dbe0de3..062f17a 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -318,6 +318,10 @@ standards.
 ---
 *** `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.
diff --git a/lisp/json.el b/lisp/json.el
index e2c7cc7..33247bf 100644
--- a/lisp/json.el
+++ b/lisp/json.el
@@ -52,6 +52,8 @@
 
 ;;; Code:
 
+(require 'map)
+
 ;; Parameters
 
 (defvar json-object-type 'alist
@@ -100,6 +102,13 @@ this around your call to `json-read' instead of `setq'ing 
it.")
   "The default indentation level for encoding.
 Used only when `json-encoding-pretty-print' is non-nil.")
 
+(defvar json-encoding-object-sort-key nil
+  "Sort key for JSON object keys during encoding.
+If nil, no sorting is performed.  Else, JSON object keys are
+ordered by the specified sort key during encoding.  For instance,
+setting this to `string<' will have JSON object keys ordered
+alphabetically.")
+
 (defvar json--encoding-current-indentation "\n"
   "Internally used to keep track of the current indentation level of encoding.
 Used only when `json-encoding-pretty-print' is non-nil.")
@@ -148,6 +157,15 @@ Unlike `reverse', this keeps the property-value pairs 
intact."
         (push prop res)))
     res))
 
+(defun json--plist-to-alist (plist)
+  "Return an alist of the property-value pairs in PLIST."
+  (let (res)
+    (while plist
+      (let ((prop (pop plist))
+            (val (pop plist)))
+        (push (cons prop val) res)))
+    (nreverse res)))
+
 (defmacro json--with-indentation (body)
   `(let ((json--encoding-current-indentation
           (if json-encoding-pretty-print
@@ -421,32 +439,17 @@ Please see the documentation of `json-object-type' and 
`json-key-type'."
 
 (defun json-encode-hash-table (hash-table)
   "Return a JSON representation of HASH-TABLE."
-  (format "{%s%s}"
-          (json-join
-           (let (r)
-             (json--with-indentation
-              (maphash
-               (lambda (k v)
-                 (push (format
-                        (if json-encoding-pretty-print
-                            "%s%s: %s"
-                          "%s%s:%s")
-                        json--encoding-current-indentation
-                        (json-encode-key k)
-                        (json-encode v))
-                       r))
-               hash-table))
-             r)
-           json-encoding-separator)
-          (if (or (not json-encoding-pretty-print)
-                  json-encoding-lisp-style-closings)
-              ""
-            json--encoding-current-indentation)))
+  (json-encode-alist (map-into hash-table 'list)))
 
 ;; List encoding (including alists and plists)
 
 (defun json-encode-alist (alist)
   "Return a JSON representation of ALIST."
+  (when json-encoding-object-sort-key
+    (setq alist
+          (sort alist (lambda (a b)
+                        (funcall json-encoding-object-sort-key
+                                 (car a) (car b))))))
   (format "{%s%s}"
           (json-join
            (json--with-indentation
@@ -466,25 +469,7 @@ Please see the documentation of `json-object-type' and 
`json-key-type'."
 
 (defun json-encode-plist (plist)
   "Return a JSON representation of PLIST."
-  (let (result)
-    (json--with-indentation
-      (while plist
-        (push (concat
-               json--encoding-current-indentation
-               (json-encode-key (car plist))
-               (if json-encoding-pretty-print
-                   ": "
-                 ":")
-               (json-encode (cadr plist)))
-              result)
-        (setq plist (cddr plist))))
-    (concat "{"
-            (json-join (nreverse result) json-encoding-separator)
-            (if (and json-encoding-pretty-print
-                     (not json-encoding-lisp-style-closings))
-                json--encoding-current-indentation
-              "")
-            "}")))
+  (json-encode-alist (json--plist-to-alist plist)))
 
 (defun json-encode-list (list)
   "Return a JSON representation of LIST.
@@ -622,6 +607,18 @@ Advances point just past JSON object."
           (txt (delete-and-extract-region begin end)))
       (insert (json-encode (json-read-from-string txt))))))
 
+(defun json-pretty-print-buffer-ordered ()
+  "Pretty-print current buffer with object keys ordered."
+  (interactive)
+  (let ((json-encoding-object-sort-key 'string<))
+    (json-pretty-print-buffer)))
+
+(defun json-pretty-print-ordered (begin end)
+  "Pretty-print the region with object keys ordered."
+  (interactive "r")
+  (let ((json-encoding-object-sort-key 'string<))
+    (json-pretty-print begin end)))
+
 (provide 'json)
 
 ;;; json.el ends here
diff --git a/test/automated/json-tests.el b/test/automated/json-tests.el
index d1b7a2f..cdf85cc 100644
--- a/test/automated/json-tests.el
+++ b/test/automated/json-tests.el
@@ -28,11 +28,33 @@
   (should (equal (json--plist-reverse '(:a 1 :b 2 :c 3))
                  '(:c 3 :b 2 :a 1))))
 
+(ert-deftest test-json-plist-to-alist ()
+  (should (equal (json--plist-to-alist '()) '()))
+  (should (equal (json--plist-to-alist '(:a 1)) '((:a . 1))))
+  (should (equal (json--plist-to-alist '(:a 1 :b 2 :c 3))
+                 '((:a . 1) (:b . 2) (:c . 3)))))
+
+(ert-deftest test-json-encode-plist ()
+  (let ((plist '(:a 1 :b 2)))
+    (should (equal (json-encode plist) "{\"a\":1,\"b\":2}"))))
+
 (ert-deftest json-encode-simple-alist ()
   (should (equal (json-encode '((a . 1)
                                 (b . 2)))
                  "{\"a\":1,\"b\":2}")))
 
+(ert-deftest test-json-encode-hash-table ()
+  (let ((hash-table (make-hash-table))
+        (json-encoding-object-sort-key 'string<))
+    (puthash :a 1 hash-table)
+    (puthash :b 2 hash-table)
+    (should (equal (json-encode hash-table) "{\"a\":1,\"b\":2}"))))
+
+(ert-deftest test-json-encode-with-sort-key ()
+  (let ((alist '((:a . 1) (:b . 2) (:c . 3)))
+        (json-encoding-object-sort-key 'string>))
+    (should (equal (json-encode alist) "{\"c\":3,\"b\":2,\"a\":1}"))))
+
 (ert-deftest json-read-simple-alist ()
   (let ((json-object-type 'alist))
     (should (equal (json-read-from-string "{\"a\": 1, \"b\": 2}")
-- 
2.5.3