# 3 - More Mini-XML Programming Techniques

This chapter shows additional ways to use the Mini-XML library in your programs.

## Load Callbacks

Chapter 2 introduced the `mxmlLoadFile()` and `mxmlLoadString()` functions. The last argument to these functions is a callback function which is used to determine the value type of each data node in an XML document.

Mini-XML defines several standard callbacks for simple XML data files:

- `MXML_INTEGER_CALLBACK` - All data nodes contain whitespace-separated integers.
- `MXML_OPAQUE_CALLBACK` - All data nodes contain opaque strings ("CDATA").
- `MXML_REAL_CALLBACK` - All data nodes contain whitespace-separated floating-point numbers.
- `MXML_TEXT_CALLBACK` - All data nodes contain whitespace-separated strings.

You can provide your own callback functions for more complex XML documents. Your callback function will receive a pointer to the current element node and must return the value type of the immediate children for that element node: `MXML_INTEGER`, `MXML_OPAQUE`, `MXML_REAL`, or `MXML_TEXT`. The function is called after the element and its attributes have been read, so you can look at the element name, attributes, and attribute values to determine the proper value type to return.

The following callback function looks for an attribute named "type" or the element name to determine the value type for its child nodes:

        /*
         * 'type_cb()' - XML data type callback for mxmlLoadFile()...
         */

        mxml_type_t                             /* O - Data type */
        type_cb(mxml_node_t *node)              /* I - Element node */
        {
          const char    *type;                  /* Type string */

         /*
          * You can lookup attributes and/or use the element name, hierarchy, etc...
          */

          if ((type = mxmlElementGetAttr(node, "type")) == NULL)
    	type = node->value.element.name;

          if (!strcmp(type, "integer"))
    	return (MXML_INTEGER);
          else if (!strcmp(type, "opaque"))
    	return (MXML_OPAQUE);
          else if (!strcmp(type, "real"))
    	return (MXML_REAL);
          else
    	return (MXML_TEXT);
        }

To use this callback function, simply use the name when you call any of the load functions:

        FILE *fp;
        mxml_node_t *tree;

        fp = fopen("filename.xml", "r");
        tree = mxmlLoadFile(NULL, fp, type_cb);
        fclose(fp);

## Save Callbacks

Chapter 2 also introduced the `mxmlSaveFile()`, `mxmlSaveString()`, and `mxmlSaveAllocString()` functions. The last argument to these functions is a callback function which is used to automatically insert whitespace in an XML document.

Your callback function will be called up to four times for each element node with a pointer to the node and a "where" value of `MXML_WS_BEFORE_OPEN`, `MXML_WS_AFTER_OPEN`, `MXML_WS_BEFORE_CLOSE`, or `MXML_WS_AFTER_CLOSE`. The callback function should return `NULL` if no whitespace should be added and the string to insert (spaces, tabs, carriage returns, and newlines) otherwise. The following whitespace callback can be used to add whitespace to XHTML output to make it more readable in a standard text editor:

        /*
         * 'whitespace_cb()' - Let the mxmlSaveFile() function know when to insert
         *                     newlines and tabs...
         */

        const char *                            /* O - Whitespace string or NULL */
        whitespace_cb(mxml_node_t *node,        /* I - Element node */
            	  int         where)        /* I - Open or close tag? */
        {
          const char *name;                     /* Name of element */

         /*
          * We can conditionally break to a new line before or after any element.
          * These are just common HTML elements...
          */

          name = node->value.element.name;

          if (!strcmp(name, "html") || !strcmp(name, "head") || !strcmp(name, "body") ||
    	  !strcmp(name, "pre") || !strcmp(name, "p") ||
    	  !strcmp(name, "h1") || !strcmp(name, "h2") || !strcmp(name, "h3") ||
    	  !strcmp(name, "h4") || !strcmp(name, "h5") || !strcmp(name, "h6"))
          {
           /*
    	* Newlines before open and after close...
    	*/

    	if (where == MXML_WS_BEFORE_OPEN || where == MXML_WS_AFTER_CLOSE)
    	  return ("\n");
          }
          else if (!strcmp(name, "dl") || !strcmp(name, "ol") || !strcmp(name, "ul"))
          {
           /*
    	* Put a newline before and after list elements...
    	*/

    	return ("\n");
          }
          else if (!strcmp(name, "dd") || !strcmp(name, "dt") || !strcmp(name, "li"))
          {
           /*
    	* Put a tab before <li>'s, <dd>'s, and <dt>'s, and a newline after them...
    	*/

    	if (where == MXML_WS_BEFORE_OPEN)
    	  return ("\t");
    	else if (where == MXML_WS_AFTER_CLOSE)
    	  return ("\n");
          }

         /*
          * Return NULL for no added whitespace...
          */

          return (NULL);
        }

To use this callback function, simply use the name when you call any of the save functions:

        FILE *fp;
        mxml_node_t *tree;

        fp = fopen("filename.xml", "w");
        mxmlSaveFile(tree, fp, whitespace_cb);
        fclose(fp);

## Custom Data Types

Mini-XML supports custom data types via global load and save callbacks. Only a single set of callbacks can be active at any time, however your callbacks can store additional information in order to support multiple custom data types as needed. The `MXML_CUSTOM` node type identifies custom data nodes.

The load callback receives a pointer to the current data node and a string of opaque character data from the XML source with character entities converted to the corresponding UTF-8 characters. For example, if we wanted to support a custom date/time type whose value is encoded as "yyyy-mm-ddThh:mm:ssZ" (ISO format), the load callback would look like the following:

        typedef struct
        {
          unsigned      year,                   /* Year */
                        month,                  /* Month */
                        day,                    /* Day */
                        hour,                   /* Hour */
                        minute,                 /* Minute */
                        second;                 /* Second */
          time_t        unix;                   /* UNIX time value */
        } iso_date_time_t;

        int                                     /* I - 0 on success, -1 on error */
        load_custom(mxml_node_t *node,          /* I - Node */
                    const char  *data)          /* I - Value */
        {
          iso_date_time_t *dt;                  /* Date/time value */
          struct tm       tmdata;               /* UNIX time data */

         /*
          * Allocate data structure...
          */

          dt = calloc(1, sizeof(iso_date_time_t));

         /*
          * Try reading 6 unsigned integers from the data string...
          */

          if (sscanf(data, "%u-%u-%uT%u:%u:%uZ",
                     &(dt->year), &(dt->month), &(dt->day),
                     &(dt->hour), &(dt->minute), &(dt->second)) != 6)
          {
           /*
            * Unable to read numbers, free the data structure and return an
            * error...
            */

            free(dt);

            return (-1);
          }

         /*
          * Range check values...
          */

          if (dt->month < 1 || dt->month > 12 ||
              dt->day < 1 || dt->day > 31 ||
              dt->hour < 0 || dt->hour > 23 ||
              dt->minute < 0 || dt->minute > 59 ||
              dt->second < 0 || dt->second > 59)
          {
           /*
            * Date information is out of range...
            */

            free(dt);

            return (-1);
          }

         /*
          * Convert ISO time to UNIX time in seconds...
          */

          tmdata.tm_year = dt->year - 1900;
          tmdata.tm_mon  = dt->month - 1;
          tmdata.tm_day  = dt->day;
          tmdata.tm_hour = dt->hour;
          tmdata.tm_min  = dt->minute;
          tmdata.tm_sec  = dt->second;

          dt->unix = gmtime(&tmdata);

         /*
          * Assign custom node data and destroy function pointers...
          */

          node->value.custom.data    = dt;
          node->value.custom.destroy = free;

         /*
          * Return with no errors...
          */

          return (0);
        }

The function itself can return 0 on success or -1 if it is unable to decode the custom data or the data contains an error. Custom data nodes contain a `void` pointer to the allocated custom data for the node and a pointer to a destructor function which will free the custom data when the node is deleted.

The save callback receives the node pointer and returns an allocated string containing the custom data value. The following save callback could be used for our ISO date/time type:

        char *                                  /* I - Allocated string */
        save_custom(mxml_node_t *node)          /* I - Node */
        {
          char            data[255];            /* Data string */
          iso_date_time_t *dt;                  /* ISO date/time pointer */

          dt = (iso_date_time_t *)node->custom.data;

          snprintf(data, sizeof(data), "%04u-%02u-%02uT%02u:%02u:%02uZ",
                   dt->year, dt->month, dt->day, dt->hour,
                   dt->minute, dt->second);

          return (strdup(data));
        }

You register the callback functions using the `mxmlSetCustomHandlers()` function:

        mxmlSetCustomHandlers(load_custom, save_custom);

## Changing Node Values

All of the examples so far have concentrated on creating and loading new XML data nodes. Many applications, however, need to manipulate or change the nodes during their operation, so Mini-XML provides functions to change node values safely and without leaking memory.

Existing nodes can be changed using the `mxmlSetElement()`, `mxmlSetInteger()`, `mxmlSetOpaque()`, `mxmlSetReal()`, and `mxmlSetText()` functions. For example, use the following function call to change a text node to contain the text "new" with leading whitespace:

        mxml_node_t *node;

        mxmlSetText(node, 1, "new");

## Formatted Text

The `mxmlNewTextf()` and `mxmlSetTextf()` functions create and change text nodes, respectively, using `printf`-style format strings and arguments. For example, use the following function call to create a new text node:

        mxml_node_t *node;

        node = mxmlNewTextf(node, 1, "%s/%s",
                            path, filename);

## Indexing

Mini-XML provides functions for managing indices of nodes. The current implementation provides the same functionality as the `mxmlFindElement()`. The advantage of using an index is that searching and enumeration of elements is significantly faster. The only disadvantage is that each index is a static snapshot of the XML document, so indices are not well suited to XML data that is updated more often than it is searched. The overhead of creating an index is approximately equal to walking the XML document tree. Nodes in the index are sorted by element name and attribute value.

Indices are stored in `mxml_index_t` structures. The `mxmlIndexNew()` function creates a new index:

        mxml_node_t *tree;
        mxml_index_t *ind;

        ind = mxmlIndexNew(tree, "element", "attribute");

The first argument is the XML node tree to index. Normally this will be a pointer to the `?xml` element.

The second argument contains the element to index; passing `NULL` indexes all element nodes alphabetically.

The third argument contains the attribute to index; passing `NULL` causes only the element name to be indexed.

Once the index is created, the `mxmlIndexEnum()`, `mxmlIndexFind()`, and `mxmlIndexReset()` functions are used to access the nodes in the index. The `mxmlIndexReset()` function resets the "current" node pointer in the index, allowing you to do new searches and enumerations on the same index. Typically you will call this function prior to your calls to `mxmlIndexEnum()` and `mxmlIndexFind()`.

The `mxmlIndexEnum()` function enumerates each of the nodes in the index and can be used in a loop as follows:

        mxml_node_t *node;
        mxml_index_t *ind;

        mxmlIndexReset(ind);

        while ((node = mxmlIndexEnum(ind)) != NULL)
        {
          // do something with node
        }

The `mxmlIndexFind()` function locates the next occurrence of the named element and attribute value in the index. It can be used to find all matching elements in an index, as follows:

        mxml_node_t *node;
        mxml_index_t *ind;

        mxmlIndexReset(ind);

        while ((node = mxmlIndexFind(ind, "element", "attr-value")) != NULL)
        {
          // do something with node
        }

The second and third arguments represent the element name and attribute value, respectively. A `NULL` pointer is used to return all elements or attributes in the index. Passing `NULL` for both the element name and attribute value is equivalent to calling `mxmlIndexEnum`.

When you are done using the index, delete it using the `mxmlIndexDelete()` function:

        mxml_index_t *ind;

        mxmlIndexDelete(ind);