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Sources/json-c/json_object_iterator.c

  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
 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
 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
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*
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*
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
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
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 

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