support for handling duplicate keys

This patch adds support for handling duplicate keys. If a duplicate key is found
in an object the last value will be used. If #ordered is used, the first
appearance of the key is considered in the order.

Fixes #84

json/parser.scm

@@ -1,6 +1,6 @@
 ;;; (json parser) --- Guile JSON implementation.
 
-;; Copyright (C) 2013-2020 Aleix Conchillo Flaque <[email protected]>
+;; Copyright (C) 2013-2023 Aleix Conchillo Flaque <[email protected]>
 ;;
 ;; This file is part of guile-json.
 ;;
@@ -212,25 +212,33 @@
 
 (define (json-read-object port null ordered)
   (expect-delimiter port #\{)
-  (let loop ((pairs '()) (added #t))
+  (let loop ((items-hash (make-hash-table))
+             (items-order '())
+             (added #t))
     (skip-whitespaces port)
     (let ((ch (peek-char port)))
       (cond
        ;; End of object.
        ((eqv? ch #\})
         (read-char port)
         (cond
-         (added (if ordered (reverse! pairs) pairs))
+         (added (if ordered
+                    (map (lambda (pair) (hash-ref items-hash (car pair)))
+                         (reverse! items-order))
+                    (hash-map->list (lambda (key value) value) items-hash)))
          (else (json-exception port))))
        ;; Read one pair and continue.
        ((eqv? ch #\")
-        (let ((pair (read-pair port null ordered)))
-          (loop (cons pair pairs) #t)))
+        (let* ((pair (read-pair port null ordered))
+               (key (car pair))
+               (key-exists (hash-ref items-hash key) ))
+          (hash-set! items-hash key pair)
+          (loop items-hash (if key-exists items-order (cons pair items-order)) #t)))
        ;; Skip comma and read more pairs.
        ((eqv? ch #\,)
         (read-char port)
         (cond
-         (added (loop pairs #f))
+         (added (loop items-hash items-order #f))
          (else (json-exception port))))
        ;; Invalid object.
        (else (json-exception port))))))

tests/test-parser.scm

@@ -1,6 +1,6 @@
 ;;; (tests test-parser) --- Guile JSON implementation.
 
-;; Copyright (C) 2018-2022 Aleix Conchillo Flaque <[email protected]>
+;; Copyright (C) 2018-2023 Aleix Conchillo Flaque <[email protected]>
 ;;
 ;; This file is part of guile-json.
 ;;
@@ -86,6 +86,7 @@
 (test-error #t (json-string->scm "[,1]"))
 (test-error #t (json-string->scm "[1,2,,,5]"))
 (test-error #t (json-string->scm "[1,2"))
+(test-error #t (json-string->scm "[1,2,]"))
 
 ;; Objects
 (test-equal '() (json-string->scm "{}"))
@@ -102,6 +103,9 @@
 (test-equal '() (json-string->scm "{}" #:ordered #t))
 (test-equal '(("green" . 1) ("eggs" . 2) ("ham" . 3)) (json-string->scm "{\"green\":1, \"eggs\":2, \"ham\":3}" #:ordered #t))
 
+;; Objects with duplicate keys
+(test-equal '(("foo" . "last") ("bar" . 2) ("baz" . #(1 2 3))) (json-string->scm "{\"foo\": \"first\", \"bar\": 2, \"foo\": \"second\", \"baz\": [1, 2, 3], \"foo\": \"last\"}" #:ordered #t))
+
 ;; Since the following JSON object contains more than one key-value pair, we
 ;; can't use "test-equal" directly since the output could be unordered.
 (define book (json-string->scm "{\"title\":\"A book\",\"author\":\"An author\",\"price\":29.99}"))