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<title>Comparison - avram - a virtual machine code interpreter</title>
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<h4 class="subsection">3.1.5 Comparison</h4>

<p>The file <samp><span class="file">compare.h</span></samp> contains a few function declarations
pertaining to the computation of the comparison predicate described in
<a href="Compare.html#Compare">Compare</a>. Some of the work is done by static functions in
<samp><span class="file">compare.c</span></samp> that are not recommended entry points to the library.

<div class="defun">
&mdash; Function: void <b>avm_initialize_compare</b> ()<var><a name="index-avm_005finitialize_005fcompare-476"></a></var><br>
<blockquote><p><a name="index-g_t_0040code_007bcompare_007d-477"></a>This function should be called once before the first call to
<code>avm_comparison</code>, as it initializes some necessary internal data
structures. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: void <b>avm_count_compare</b> ()<var><a name="index-avm_005fcount_005fcompare-478"></a></var><br>
<blockquote><p>This function can be used to check for memory leaks, by detecting
unreclaimed storage at the end of a run. The data structures relevant to
comparison that could be reported as unreclaimed are known as &ldquo;decision&rdquo;
nodes, but these should always be handled properly by the library
without intervention. If <code>avm_count_lists</code> is also being used, the
call to this function must precede it. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: list <b>avm_comparison</b> (<var>list operand, int *fault</var>)<var><a name="index-avm_005fcomparison-479"></a></var><br>
<blockquote><p>This function takes a list operand representing a pair of trees and
returns a list representing the logical value of their equality. If the
operand is <code>NULL</code>, a message of invalid comparison is returned and
the <code>*</code><var>fault</var> is set to a non-zero value. If the <code>head</code> of the
operand is unequal to the <code>tail</code>, a <code>NULL</code> value is
returned. If they are equal, a list is returned whose <code>head</code> and
<code>tail</code> are both <code>NULL</code>. The equality in question is structural
<a name="index-pointer-equality-480"></a>rather than pointer equality.

        <p>The list operand to this function may be modified by this function, but
not in a way that should make any difference to a client program. If two
lists are found to be equal, or if even two sublists are found to be
equal in the course of the comparison, one of them is deallocated and
made to point to the other. This action saves memory and may make
subsequent comparisons faster. However, it could disrupt client programs
<a name="index-pointers-481"></a>that happen to be holding stale list pointers.

        <p><a name="index-discontiguous-field-482"></a>As of <code>avram</code> version 0.6.0, a logical field called
<code>discontiguous</code> has been added to the <code>node</code> record type
declared in <code>lists.h</code>, which is checked by the comparison
function. If a list node has its <code>discontiguous</code> field set to a
non-zero value, and if it also has a non-null <code>value</code> field, then
it won't be deallocated in the course of comparison even if it is
found to be equal to something else. This feature can be used by
client modules to create lists in which value fields refer to data
structures that are meant to exist independently of them. See
<samp><span class="file">mpfr.c</span></samp> for an example.

        <p>This function is likely to have better performance and memory usage than
a naive implementation of comparison, for the above reasons and also
because of optimizations pertaining to comparison of lists representing
characters. Moreover, it is not subject to stack overflow exceptions
<a name="index-recursion-483"></a>because it is not written in a recursive style. 
</p></blockquote></div>

<div class="defun">
&mdash; Function: list <b>avm_binary_comparison</b> (<var>list left_operand, list right_operand, int *fault</var>)<var>;<a name="index-avm_005fbinary_005fcomparison-484"></a></var><br>
<blockquote><p>This function is the same as <code>avm_comparison</code> except that it
allows the left and right operands to be passed as separate lists
rather than taking them from the <code>head</code> and the <code>tail</code> of a
single list. 
</p></blockquote></div>

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