Support plists in json.c

* doc/lispref/text.texi (Parsing JSON): Mention plists and
json-serialize-use-plists.

* src/json.c (lisp_to_json_toplevel_1): Decide with
Vjson_serialize_use_plists.
(Fjson_serialize): Update docstring.
(enum json_object_type): Add json_object_plist.
(json_to_lisp): Can build plists.
(json_parse_object_type): Accept plists.
(Fjson_parse_string): Update docstring.
(json-serialize-use-plist): New DEFVAR_LISP.
(Qplist): New sym_of_json

* test/src/json-tests.el (json-serialize/object): Do some tests
with json-serialize-use-plists to t.
(json-parse-string/object): Parse something as a plist.

3 files changed, 110 insertions, 32 deletions

diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index da09b4ae1c6..3995102237a 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -5025,16 +5025,18 @@ represented using Lisp vectors.
 
 @item
 JSON has only one map type, the object.  JSON objects are represented
-using Lisp hashtables or alists.  When an alist contains several
-elements with the same key, Emacs uses only the first element for
-serialization, in accordance with the behavior of @code{assq}.
+using Lisp hashtables, alists or plists.  When an alist or plist
+contains several elements with the same key, Emacs uses only the first
+element for serialization, in accordance with the behavior of
+@code{assq}.
 
 @end itemize
 
 @noindent
-Note that @code{nil} is a valid alist and represents the empty JSON
-object, @code{@{@}}, not @code{null}, @code{false}, or an empty array,
-all of which are different JSON values.
+Note that @code{nil} is both a valid alist and a valid plist and
+represents the empty JSON object, @code{@{@}}, not @code{null},
+@code{false}, or an empty array, all of which are different JSON
+values.
 
   If some Lisp object can't be represented in JSON, the serialization
 functions will signal an error of type @code{wrong-type-argument}.
@@ -5057,12 +5059,21 @@ The parsing functions will signal the following errors:
   Only top-level values (arrays and objects) can be serialized to
 JSON.  The subobjects within these top-level values can be of any
 type.  Likewise, the parsing functions will only return vectors,
-hashtables, and alists.
+hashtables, alists and plists.
 
   The parsing functions accept keyword arguments.  Currently only one
 keyword argument, @code{:object-type}, is recognized; its value can be
 either @code{hash-table} to parse JSON objects as hashtables with
-string keys (the default) or @code{alist} to parse them as alists.
+string keys (the default), @code{alist} to parse them as alists or
+@code{plist} to parse them as plists.
+
+@vindex json-serialize-use-plists
+@cindex serializing plists as json
+  For the serialization function, the variable
+@var{json-serialize-use-plists} controls the converse process,
+resolving the ambiguity when a list is found in the Lisp object to
+serialize. If @code{nil}, its default, the list is interpreted as an
+alist, otherwise it is interpreted as a plist.
 
 @defun json-serialize object
 This function returns a new Lisp string which contains the JSON
diff --git a/src/json.c b/src/json.c
index b046d34f667..ccd58c047e5 100644
--- a/src/json.c
+++ b/src/json.c
@@ -395,16 +395,31 @@ lisp_to_json_toplevel_1 (Lisp_Object lisp, json_t **json)
       record_unwind_protect_ptr (json_release_object, *json);
       FOR_EACH_TAIL (tail)
         {
-          Lisp_Object pair = XCAR (tail);
-          CHECK_CONS (pair);
-          Lisp_Object key_symbol = XCAR (pair);
-          Lisp_Object value = XCDR (pair);
+          const char *key_str;
+          Lisp_Object value;
+          Lisp_Object key_symbol;
+          if ( EQ (Vjson_serialize_use_plists, Qt) ) {
+            key_symbol = XCAR (tail);
+            tail = XCDR(tail);
+            CHECK_CONS (tail);
+            value = XCAR (tail);
+            if (EQ (tail, li.tortoise)) circular_list (lisp);
+          } else {
+            Lisp_Object pair = XCAR (tail);
+            CHECK_CONS (pair);
+            key_symbol = XCAR (pair);
+            value = XCDR (pair);
+          }
           CHECK_SYMBOL (key_symbol);
           Lisp_Object key = SYMBOL_NAME (key_symbol);
           /* We can't specify the length, so the string must be
              null-terminated.  */
           check_string_without_embedded_nulls (key);
-          const char *key_str = SSDATA (key);
+          key_str = SSDATA (key);
+          /* If using plists, maybe strip the ":" from symbol-name */
+          if (EQ (Vjson_serialize_use_plists, Qt) &&
+              ':' == key_str[0] &&
+              key_str[1] ) key_str = &key_str[1];
           /* Only add element if key is not already present.  */
           if (json_object_get (*json, key_str) == NULL)
             {
@@ -476,13 +491,17 @@ lisp_to_json (Lisp_Object lisp)
 
 DEFUN ("json-serialize", Fjson_serialize, Sjson_serialize, 1, 1, NULL,
        doc: /* Return the JSON representation of OBJECT as a string.
-OBJECT must be a vector, hashtable, or alist, and its elements can
-recursively contain `:null', `:false', t, numbers, strings, or other
-vectors hashtables, and alist.  `:null', `:false', and t will be
-converted to JSON null, false, and true values, respectively.  Vectors
-will be converted to JSON arrays, and hashtables and alists to JSON
-objects.  Hashtable keys must be strings without embedded null
-characters and must be unique within each object.  Alist keys must be
+
+OBJECT must be a vector of values or a key-value map.  Hashtables,
+alists and plists are accepted as maps, the variable
+`json-serialize-use-plists' controlling which one of the latter two to
+use.  In any of these cases, values can be `:null', `:false', t,
+numbers, strings, or, recursively, other vectors, hashtables, alists
+or plists.  `:null', `:false', and t will be converted to JSON null,
+false, and true values, respectively.  Vectors will be converted to
+JSON arrays, and hashtables, alists and plists to JSON objects.
+Hashtable keys must be strings without embedded null characters and
+must be unique within each object.  Alist or plist keys must be
 symbols; if a key is duplicate, the first instance is used.  */)
   (Lisp_Object object)
 {
@@ -605,6 +624,7 @@ OBJECT.  */)
 enum json_object_type {
   json_object_hashtable,
   json_object_alist,
+  json_object_plist
 };
 
 /* Convert a JSON object to a Lisp object.  */
@@ -692,6 +712,30 @@ json_to_lisp (json_t *json, enum json_object_type object_type)
               result = Fnreverse (result);
               break;
             }
+          case json_object_plist:
+            {
+              result = Qnil;
+              const char *key_str;
+              json_t *value;
+              json_object_foreach (json, key_str, value)
+                {
+                  /* No idea if using AUTO_STRING and Fconcat for
+                     making keywords is idiomatic, but seems to work
+                     nicely */
+                  AUTO_STRING (colon, ":");
+                  Lisp_Object key =
+                    Fintern (CALLN (Fconcat, colon, json_build_string (key_str))
+                             , Qnil);
+                  result = Fcons (key, result); /* build the plist as
+                                                   value-key since
+                                                   we're going to
+                                                   reverse it in the
+                                                   end.*/
+                  result = Fcons (json_to_lisp (value, object_type), result);
+                }
+              result = Fnreverse (result);
+              break;
+            }
           default:
             /* Can't get here.  */
             emacs_abort ();
@@ -721,8 +765,10 @@ json_parse_object_type (ptrdiff_t nargs, Lisp_Object *args)
           return json_object_hashtable;
         else if (EQ (value, Qalist))
           return json_object_alist;
+        else if (EQ (value, Qplist))
+          return json_object_plist;
         else
-          wrong_choice (list2 (Qhash_table, Qalist), value);
+          wrong_choice (list3 (Qhash_table, Qalist, Qplist), value);
       }
     default:
       wrong_type_argument (Qplistp, Flist (nargs, args));
@@ -733,15 +779,16 @@ DEFUN ("json-parse-string", Fjson_parse_string, Sjson_parse_string, 1, MANY,
        NULL,
        doc: /* Parse the JSON STRING into a Lisp object.
 This is essentially the reverse operation of `json-serialize', which
-see.  The returned object will be a vector, hashtable, or alist.  Its
-elements will be `:null', `:false', t, numbers, strings, or further
-vectors, hashtables, and alists.  If there are duplicate keys in an
-object, all but the last one are ignored.  If STRING doesn't contain a
-valid JSON object, an error of type `json-parse-error' is signaled.
-The keyword argument `:object-type' specifies which Lisp type is used
-to represent objects; it can be `hash-table' or `alist'.
-usage: (json-parse-string STRING &key (OBJECT-TYPE \\='hash-table))  */)
-  (ptrdiff_t nargs, Lisp_Object *args)
+see.  The returned object will be a vector, hashtable, alist, or
+plist.  Its elements will be `:null', `:false', t, numbers, strings,
+or further vectors, hashtables, alists, or plists.  If there are
+duplicate keys in an object, all but the last one are ignored.  If
+STRING doesn't contain a valid JSON object, an error of type
+`json-parse-error' is signaled.  The keyword argument `:object-type'
+specifies which Lisp type is used to represent objects; it can be
+`hash-table', `alist' or `plist'.
+usage: (json-parse-string STRING &key (OBJECT-TYPE \\='hash-table)) */)
+     (ptrdiff_t nargs, Lisp_Object *args)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
 
@@ -903,6 +950,11 @@ syms_of_json (void)
   DEFSYM (Qpure, "pure");
   DEFSYM (Qside_effect_free, "side-effect-free");
 
+  DEFVAR_LISP ("json-serialize-use-plists", Vjson_serialize_use_plists,
+               doc:
+               /* If non-nil use plists instead of alists in json-serialize.*/);
+  Vjson_serialize_use_plists = Qnil;
+
   DEFSYM (Qjson_serialize, "json-serialize");
   DEFSYM (Qjson_parse_string, "json-parse-string");
   Fput (Qjson_serialize, Qpure, Qt);
@@ -912,6 +964,7 @@ syms_of_json (void)
 
   DEFSYM (QCobject_type, ":object-type");
   DEFSYM (Qalist, "alist");
+  DEFSYM (Qplist, "plist");
 
   defsubr (&Sjson_serialize);
   defsubr (&Sjson_insert);
diff --git a/test/src/json-tests.el b/test/src/json-tests.el
index 09067bad8c8..5c9be20e957 100644
--- a/test/src/json-tests.el
+++ b/test/src/json-tests.el
@@ -69,7 +69,19 @@
   (should-error (json-serialize '((1 . 2))) :type 'wrong-type-argument)
   (should-error (json-serialize '((a . 1) . b)) :type 'wrong-type-argument)
   (should-error (json-serialize '#1=((a . 1) . #1#)) :type 'circular-list)
-  (should-error (json-serialize '(#1=(a #1#)))))
+  (should-error (json-serialize '(#1=(a #1#))))
+
+  (let ((json-serialize-use-plists t))
+    (should (equal (json-serialize '(:abc [1 2 t] :def :null))
+                   "{\"abc\":[1,2,true],\"def\":null}"))
+    (should (equal (json-serialize '(abc [1 2 t] :def :null))
+                   "{\"abc\":[1,2,true],\"def\":null}"))
+    (should-error (json-serialize '#1=(:a 1 . #1#)) :type 'circular-list)
+    (should-error (json-serialize '((abc . 1))) :type 'wrong-type-argument)
+    (should-error (json-serialize '(:foo bar (abc . 1)))
+                  :type 'wrong-type-argument)
+    (should-error (json-serialize '(:foo bar :odd-numbered))
+                  :type 'wrong-type-argument)))
 
 (ert-deftest json-serialize/object-with-duplicate-keys ()
   (skip-unless (fboundp 'json-serialize))
@@ -89,7 +101,9 @@
       (should (equal (cl-sort (map-pairs actual) #'string< :key #'car)
                      '(("abc" . [9 :false]) ("def" . :null)))))
     (should (equal (json-parse-string input :object-type 'alist)
-                   '((abc . [9 :false]) (def . :null))))))
+                   '((abc . [9 :false]) (def . :null))))
+    (should (equal (json-parse-string input :object-type 'plist)
+                   '(:abc [9 :false] :def :null)))))
 
 (ert-deftest json-parse-string/string ()
   (skip-unless (fboundp 'json-parse-string))