1 /**
2 *******************************************************************************
3 * @file json_object_iterator.c
4 *
5 * Copyright (c) 2009-2012 Hewlett-Packard Development Company, L.P.
6 *
7 * This library is free software; you can redistribute it and/or modify
8 * it under the terms of the MIT license. See COPYING for details.
9 *
10 * @brief  json-c forces clients to use its private data
11 *         structures for JSON Object iteration.  This API
12 *         implementation corrects that by abstracting the
13 *         private json-c details.
14 *
15 *******************************************************************************
16 */
17 
18 #include <stddef.h>
19 
20 #include "json.h"
21 #include "json_object_private.h"
22 
23 #include "json_object_iterator.h"
24 
25 /**
26  * How It Works
27  *
28  * For each JSON Object, json-c maintains a linked list of zero
29  * or more lh_entry (link-hash entry) structures inside the
30  * Object's link-hash table (lh_table).
31  *
32  * Each lh_entry structure on the JSON Object's linked list
33  * represents a single name/value pair.  The "next" field of the
34  * last lh_entry in the list is set to NULL, which terminates
35  * the list.
36  *
37  * We represent a valid iterator that refers to an actual
38  * name/value pair via a pointer to the pair's lh_entry
39  * structure set as the iterator's opaque_ field.
40  *
41  * We follow json-c's current pair list representation by
42  * representing a valid "end" iterator (one that refers past the
43  * last pair) with a NULL value in the iterator's opaque_ field.
44  *
45  * A JSON Object without any pairs in it will have the "head"
46  * field of its lh_table structure set to NULL.  For such an
47  * object, json_object_iter_begin will return an iterator with
48  * the opaque_ field set to NULL, which is equivalent to the
49  * "end" iterator.
50  *
51  * When iterating, we simply update the iterator's opaque_ field
52  * to point to the next lh_entry structure in the linked list.
53  * opaque_ will become NULL once we iterate past the last pair
54  * in the list, which makes the iterator equivalent to the "end"
55  * iterator.
56  */
57 
58 /// Our current representation of the "end" iterator;
59 ///
60 /// @note May not always be NULL
61 static const void* kObjectEndIterValue = NULL;
62 
63 /**
64  * ****************************************************************************
65  */
66 struct json_object_iterator
json_object_iter_begin(struct json_object * obj)67 json_object_iter_begin(struct json_object* obj)
68 {
69     struct json_object_iterator iter;
70     struct lh_table* pTable;
71 
72     /// @note json_object_get_object will return NULL if passed NULL
73     ///       or a non-json_type_object instance
74     pTable = json_object_get_object(obj);
75     JASSERT(NULL != pTable);
76 
77     /// @note For a pair-less Object, head is NULL, which matches our
78     ///       definition of the "end" iterator
79     iter.opaque_ = pTable->head;
80     return iter;
81 }
82 
83 /**
84  * ****************************************************************************
85  */
86 struct json_object_iterator
json_object_iter_end(const struct json_object * obj)87 json_object_iter_end(const struct json_object* obj)
88 {
89     struct json_object_iterator iter;
90 
91     JASSERT(NULL != obj);
92     JASSERT(json_object_is_type(obj, json_type_object));
93 
94     iter.opaque_ = kObjectEndIterValue;
95 
96     return iter;
97 }
98 
99 /**
100  * ****************************************************************************
101  */
102 void
json_object_iter_next(struct json_object_iterator * iter)103 json_object_iter_next(struct json_object_iterator* iter)
104 {
105     JASSERT(NULL != iter);
106     JASSERT(kObjectEndIterValue != iter->opaque_);
107 
108     iter->opaque_ = ((struct lh_entry *)iter->opaque_)->next;
109 }
110 
111 
112 /**
113  * ****************************************************************************
114  */
115 const char*
json_object_iter_peek_name(const struct json_object_iterator * iter)116 json_object_iter_peek_name(const struct json_object_iterator* iter)
117 {
118     JASSERT(NULL != iter);
119     JASSERT(kObjectEndIterValue != iter->opaque_);
120 
121     return (const char*)(((struct lh_entry *)iter->opaque_)->k);
122 }
123 
124 
125 /**
126  * ****************************************************************************
127  */
128 struct json_object*
json_object_iter_peek_value(const struct json_object_iterator * iter)129 json_object_iter_peek_value(const struct json_object_iterator* iter)
130 {
131     JASSERT(NULL != iter);
132     JASSERT(kObjectEndIterValue != iter->opaque_);
133 
134     return (struct json_object*)(((struct lh_entry *)iter->opaque_)->v);
135 }
136 
137 
138 /**
139  * ****************************************************************************
140  */
141 json_bool
json_object_iter_equal(const struct json_object_iterator * iter1,const struct json_object_iterator * iter2)142 json_object_iter_equal(const struct json_object_iterator* iter1,
143                        const struct json_object_iterator* iter2)
144 {
145     JASSERT(NULL != iter1);
146     JASSERT(NULL != iter2);
147 
148     return (iter1->opaque_ == iter2->opaque_);
149 }
150 
151 
152 /**
153  * ****************************************************************************
154  */
155 struct json_object_iterator
json_object_iter_init_default(void)156 json_object_iter_init_default(void)
157 {
158     struct json_object_iterator iter;
159 
160     /**
161      * @note Make this a negative, invalid value, such that
162      *       accidental access to it would likely be trapped by the
163      *       hardware as an invalid address.
164      */
165     iter.opaque_ = NULL;
166 
167     return iter;
168 }
169