cups/api-array.shtml

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  Array API introduction for the Common UNIX Printing System (CUPS).

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<h2 class='title'><a name='OVERVIEW'>Overview</a></h2>

<p>The CUPS array API provides a high-performance generic array container.
The contents of the array container can be sorted and the container itself is
designed for optimal speed and memory usage under a wide variety of conditions.
Sorted arrays use a binary search algorithm from the last found or inserted
element to quickly find matching elements in the array. Arrays created with the
optional hash function can often find elements with a single lookup. The
<a href='#cups_array_t'><code>cups_array_t</code></a> type is used when
referring to a CUPS array.</p>

<p>The CUPS scheduler (<tt>cupsd</tt>) and many of the CUPS API
functions use the array API to efficiently manage large lists of
data.</p>

<h3><a name='MANAGING_ARRAYS'>Managing Arrays</a></h3>

<p>Arrays are created using either the
<a href='#cupsArrayNew'><code>cupsArrayNew</code></a> or
<a href='#cupsArrayNew2'><code>cupsArrayNew2</code></a> functions. The
first function creates a new array with the specified callback function
and user data pointer:</p>

<pre class='example'>
#include &lt;cups/array.h&gt;

static int compare_func(void *first, void *second, void *user_data);

void *user_data;
<a href='#cups_array_t'>cups_array_t</a> *array = <a href='#cupsArrayNew'>cupsArrayNew</a>(compare_func, user_data);
</pre>

<p>The comparison function (type
<a href="#cups_arrayfunc_t"><code>cups_arrayfunc_t</code></a>) is called
whenever an element is added to the array and can be <code>NULL</code> to
create an unsorted array. The function returns -1 if the first element should
come before the second, 0 if the first and second elements should have the same
ordering, and 1 if the first element should come after the second.</p>

<p>The "user_data" pointer is passed to your comparison function. Pass
<code>NULL</code> if you do not need to associate the elements in your array
with additional information.</p>

<p>The <a href='#cupsArrayNew2'><code>cupsArrayNew2</code></a> function adds
two more arguments to support hashed lookups, which can potentially provide
instantaneous ("O(1)") lookups in your array:</p>

<pre class='example'>
#include &lt;cups/array.h&gt;

#define HASH_SIZE 512 /* Size of hash table */

static int compare_func(void *first, void *second, void *user_data);
static int hash_func(void *element, void *user_data);

void *user_data;
<a href='#cups_array_t'>cups_array_t</a> *array = <a href='#cupsArrayNew2'>cupsArrayNew2</a>(compare_func, user_data, hash_func, HASH_SIZE);
</pre>

<p>The hash function (type
<a href="#cups_ahash_func_t"><code>cups_ahash_func_t</code></a>) returns a
number from 0 to (hash_size-1) that (hopefully) uniquely identifies the
element and is called whenever you look up an element in the array with
<a href='#cupsArrayFind'><code>cupsArrayFind</code></a>. The hash size is
only limited by available memory, but generally should not be larger than
16384 to realize any performance improvement.</p>

<p>Once you have created the array, you add elements using the
<a href='#cupsArrayAdd'><code>cupsArrayAdd</code></a>
<a href='#cupsArrayInsert'><code>cupsArrayInsert</code></a> functions.
The first function adds an element to the array, adding the new element
after any elements that have the same order, while the second inserts the
element before others with the same order. For unsorted arrays,
<a href='#cupsArrayAdd'><code>cupsArrayAdd</code></a> appends the elemnt to
the end of the array while
<a href='#cupsArrayInsert'><code>cupsArrayInsert</code></a> inserts the
element at the beginning of the array. For example, the following code
creates a sorted array of character strings:</p>

<pre class='example'>
#include &lt;cups/array.h&gt;

/* Use strcmp() to compare strings - it will ignore the user_data pointer */
<a href='#cups_array_t'>cups_array_t</a> *array = <a href='#cupsArrayNew'>cupsArrayNew</a>((<a href='#cups_array_func_t'>cups_array_func_t</a>)strcmp, NULL);

/* Add four strings to the array */
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "One Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Two Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Red Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Blue Fish");
</pre>

<p>Elements are removed using the
<a href='#cupsArrayRemove'><code>cupsArrayRemove</code></a> function, for
example:</p>

<pre class='example'>
#include &lt;cups/array.h&gt;

/* Use strcmp() to compare strings - it will ignore the user_data pointer */
<a href='#cups_array_t'>cups_array_t</a> *array = <a href='#cupsArrayNew'>cupsArrayNew</a>((<a href='#cups_array_func_t'>cups_array_func_t</a>)strcmp, NULL);

/* Add four strings to the array */
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "One Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Two Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Red Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Blue Fish");

/* Remove "Red Fish" */
<a href='#cupsArrayRemove'>cupsArrayRemove</a>(array, "Red Fish");
</pre>

<p>Finally, you free the memory used by the array using the
<a href='#cupsArrayDelete'><code>cupsArrayDelete</code></a> function. All
of the memory for the array and hash table (if any) is freed, however <em>CUPS
does not free the elements</em> - if necessary, you must allocate and free the
elements yourself.</p>

<h3><a name='FINDING_AND_ENUMERATING'>Finding and Enumerating Elements</a></h3>

<p>CUPS provides several functions to find and enumerate elements in an
array. Each one sets or updates a "current index" into the array, such that
future lookups will start where the last one left off:</p>

<dl>
	<dt><a href='#cupsArrayFind'><code>cupsArrayFind</code></a></dt>
	<dd>Returns the first matching element .</dd>
	<dt><a href='#cupsArrayFirst'><code>cupsArrayFirst</code></a></dt>
	<dd>Returns the first element in the array.</dd>
	<dt><a href='#cupsArrayIndex'><code>cupsArrayIndex</code></a></dt>
	<dd>Returns the Nth element in the array.</dd>
	<dt><a href='#cupsArrayLast'><code>cupsArrayLast</code></a></dt>
	<dd>Returns the last element in the array.</dd>
	<dt><a href='#cupsArrayNext'><code>cupsArrayNext</code></a></dt>
	<dd>Returns the next element in the array.</dd>
	<dt><a href='#cupsArrayPrev'><code>cupsArrayPrev</code></a></dt>
	<dd>Returns the previous element in the array.</dd>
</dl>

<p>Each of these functions returns <code>NULL</code> when there is no
corresponding element.  For example, a simple <code>for</code> loop using the
<a href='#cupsArrayFirst'><code>cupsArrayFirst</code></a> and
<a href='#cupsArrayNext'><code>cupsArrayNext</code></a> functions will
enumerate all of the strings in our previous example:</p> 

<pre class='example'>
#include &lt;cups/array.h&gt;

/* Use strcmp() to compare strings - it will ignore the user_data pointer */
<a href='#cups_array_t'>cups_array_t</a> *array = <a href='#cupsArrayNew'>cupsArrayNew</a>((<a href='#cups_array_func_t'>cups_array_func_t</a>)strcmp, NULL);

/* Add four strings to the array */
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "One Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Two Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Red Fish");
<a href='#cupsArrayAdd'>cupsArrayAdd</a>(array, "Blue Fish");

/* Show all of the strings in the array */
char *s;
for (s = (char *)<a href='#cupsArrayFirst'>cupsArrayFirst</a>(array); s != NULL; s = (char *)<a href='#cupsArrayNext'>cupsArrayNext</a>(array))
  puts(s);
</pre>