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<html>
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<head>
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<title>Simple Preform Cache</title>
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<link href="../docs-assets/Breadcrumbs.css" rel="stylesheet" rev="stylesheet" type="text/css">
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<meta name="viewport" content="width=device-width initial-scale=1">
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<script>
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function togglePopup(material_id) {
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var popup = document.getElementById(material_id);
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popup.classList.toggle("show");
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}
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</script>
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<body class="commentary-font">
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<nav role="navigation">
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<h1><a href="../index.html">
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<img src="../docs-assets/Inform.png" height=72">
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</a></h1>
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<ul><li><a href="../index.html">home</a></li>
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</ul><h2>Compiler</h2><ul>
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<li><a href="../structure.html">structure</a></li>
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<li><a href="../services.html">services</a></li>
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</ul><h2>Resources</h2><ul>
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<li><a href="../extensions.html">extensions</a></li>
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<li><a href="../kits.html">kits</a></li>
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</ul><h2>Repository</h2><ul>
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<li><a href="https://github.com/ganelson/inform"><img src="../docs-assets/github.png" height=18> github</a></li>
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</ul><h2>Related Projects</h2><ul>
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<li><a href="../../../inweb/index.html">inweb</a></li>
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<li><a href="../../../intest/index.html">intest</a></li>
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</ul>
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</nav>
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<main role="main">
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<!--Weave of 'Simple Preform Cache' generated by Inweb-->
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<div class="breadcrumbs">
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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../services.html">Services</a></li><li><a href="index.html">syntax</a></li><li><a href="index.html#2">Chapter 2: The Parse Tree</a></li><li><b>Simple Preform Cache</b></li></ul></div>
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<p class="purpose">A simple way to speed up repeated Preform parses of the same text.</p>
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. </b>Inform runs substantially faster if <a href="../values-module/4-ets.html" class="internal">Chapter 4: The S-Parser (in values)</a> can cache
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its findings: so, for instance, if Inform parses the text in words 507 to 511
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once, it need not do so again in the same context.
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</p>
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<p class="commentary">We provide a cache, then, for Preform nonterminals whose return type is
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<span class="extract"><span class="extract-syntax">parse_node</span></span>. The cache takes the form of a modest ring buffer for each
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of the contexts:
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</p>
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<pre class="definitions code-font"><span class="definition-keyword">define</span> <span class="constant-syntax">MAXIMUM_CACHE_SIZE</span><span class="plain-syntax"> </span><span class="constant-syntax">20</span><span class="plain-syntax"> </span><span class="comment-syntax"> a Goldilocks value: too high slows us down, too low doesn't cache enough</span>
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<span class="definition-keyword">define</span> <span class="constant-syntax">NUMBER_OF_CACHED_NONTERMINALS</span><span class="plain-syntax"> </span><span class="constant-syntax">5</span>
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</pre>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="reserved-syntax">typedef</span><span class="plain-syntax"> </span><span class="reserved-syntax">struct</span><span class="plain-syntax"> </span><span class="reserved-syntax">expression_cache</span><span class="plain-syntax"> {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">struct</span><span class="plain-syntax"> </span><span class="reserved-syntax">expression_cache_entry</span><span class="plain-syntax"> </span><span class="identifier-syntax">pe_cache</span><span class="plain-syntax">[</span><span class="constant-syntax">MAXIMUM_CACHE_SIZE</span><span class="plain-syntax">];</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">pe_cache_size</span><span class="plain-syntax">; </span><span class="comment-syntax"> number of entries used, 0 to </span><span class="extract"><span class="extract-syntax">MAXIMUM_CACHE_SIZE</span></span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">pe_cache_posn</span><span class="plain-syntax">; </span><span class="comment-syntax"> next write position, 0 to </span><span class="extract"><span class="extract-syntax">pe_cache_size</span></span><span class="comment-syntax"> minus 1</span>
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<span class="plain-syntax">} </span><span class="reserved-syntax">expression_cache</span><span class="plain-syntax">;</span>
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<span class="reserved-syntax">typedef</span><span class="plain-syntax"> </span><span class="reserved-syntax">struct</span><span class="plain-syntax"> </span><span class="reserved-syntax">expression_cache_entry</span><span class="plain-syntax"> {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">struct</span><span class="plain-syntax"> </span><span class="identifier-syntax">wording</span><span class="plain-syntax"> </span><span class="identifier-syntax">cached_query</span><span class="plain-syntax">; </span><span class="comment-syntax"> the word range whose parsing this is</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">struct</span><span class="plain-syntax"> </span><span class="reserved-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">cached_result</span><span class="plain-syntax">; </span><span class="comment-syntax"> and the result (quite possibly </span><span class="extract"><span class="extract-syntax">UNKNOWN_NT</span></span><span class="comment-syntax">)</span>
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<span class="plain-syntax">} </span><span class="reserved-syntax">expression_cache_entry</span><span class="plain-syntax">;</span>
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<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">expression_cache_has_been_used</span><span class="plain-syntax"> = </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="reserved-syntax">expression_cache</span><span class="plain-syntax"> </span><span class="identifier-syntax">contextual_cache</span><span class="plain-syntax">[</span><span class="constant-syntax">NUMBER_OF_CACHED_NONTERMINALS</span><span class="plain-syntax">];</span>
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</pre>
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<ul class="endnotetexts"><li>The structure expression_cache is private to this section.</li><li>The structure expression_cache_entry is private to this section.</li></ul>
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<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>§2. </b></p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="reserved-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="function-syntax">PreformCache::parse</span><span class="plain-syntax">(</span><span class="identifier-syntax">wording</span><span class="plain-syntax"> </span><span class="identifier-syntax">W</span><span class="plain-syntax">, </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">context</span><span class="plain-syntax">, </span><span class="identifier-syntax">nonterminal</span><span class="plain-syntax"> *</span><span class="identifier-syntax">nt</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Wordings::empty</span><span class="plain-syntax">(</span><span class="identifier-syntax">W</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><a href="2-spc.html#SP3" class="function-link"><span class="function-syntax">PreformCache::not_found</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">W</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">context</span><span class="plain-syntax"> < </span><span class="constant-syntax">0</span><span class="plain-syntax">) || (</span><span class="identifier-syntax">context</span><span class="plain-syntax"> >= </span><span class="constant-syntax">NUMBER_OF_CACHED_NONTERMINALS</span><span class="plain-syntax">))</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">internal_error</span><span class="plain-syntax"> (</span><span class="string-syntax">"bad expression parsing context"</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="named-paragraph-container code-font"><a href="2-spc.html#SP2_1" class="named-paragraph-link"><span class="named-paragraph">Check the expression cache to see if we already know the answer</span><span class="named-paragraph-number">2.1</span></a></span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">unwanted</span><span class="plain-syntax"> = </span><span class="constant-syntax">0</span><span class="plain-syntax">; </span><span class="reserved-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="identifier-syntax">spec</span><span class="plain-syntax"> = </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">plm</span><span class="plain-syntax"> = </span><span class="identifier-syntax">preform_lookahead_mode</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">preform_lookahead_mode</span><span class="plain-syntax"> = </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Preform::parse_nt_against_word_range</span><span class="plain-syntax">(</span><span class="identifier-syntax">nt</span><span class="plain-syntax">, </span><span class="identifier-syntax">W</span><span class="plain-syntax">, &</span><span class="identifier-syntax">unwanted</span><span class="plain-syntax">, (</span><span class="reserved-syntax">void</span><span class="plain-syntax"> **) &</span><span class="identifier-syntax">spec</span><span class="plain-syntax">)) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Wordings::empty</span><span class="plain-syntax">(</span><a href="2-pn.html#SP4" class="function-link"><span class="function-syntax">Node::get_text</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">spec</span><span class="plain-syntax">))) </span><a href="2-pn.html#SP4" class="function-link"><span class="function-syntax">Node::set_text</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">spec</span><span class="plain-syntax">, </span><span class="identifier-syntax">W</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> } </span><span class="reserved-syntax">else</span><span class="plain-syntax"> </span><span class="identifier-syntax">spec</span><span class="plain-syntax"> = </span><a href="2-spc.html#SP3" class="function-link"><span class="function-syntax">PreformCache::not_found</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">W</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">preform_lookahead_mode</span><span class="plain-syntax"> = </span><span class="identifier-syntax">plm</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="named-paragraph-container code-font"><a href="2-spc.html#SP2_2" class="named-paragraph-link"><span class="named-paragraph">Write the newly discovered specification to the cache for future use</span><span class="named-paragraph-number">2.2</span></a></span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><a href="2-tv.html#SP3" class="function-link"><span class="function-syntax">VerifyTree::verify_structure_from</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">spec</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">spec</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">}</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP2_1" class="paragraph-anchor"></a><b>§2.1. </b>The following seeks a previously cached answer:
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</p>
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<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Check the expression cache to see if we already know the answer</span><span class="named-paragraph-number">2.1</span></span><span class="comment-syntax"> =</span>
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="reserved-syntax">expression_cache</span><span class="plain-syntax"> *</span><span class="identifier-syntax">ec</span><span class="plain-syntax"> = &(</span><span class="identifier-syntax">contextual_cache</span><span class="plain-syntax">[</span><span class="identifier-syntax">context</span><span class="plain-syntax">]);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">expression_cache_has_been_used</span><span class="plain-syntax"> == </span><span class="identifier-syntax">FALSE</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><a href="2-spc.html#SP4" class="function-link"><span class="function-syntax">PreformCache::warn_of_changes</span></a><span class="plain-syntax">(); </span><span class="comment-syntax"> this empties all the caches</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">expression_cache_has_been_used</span><span class="plain-syntax"> = </span><span class="identifier-syntax">TRUE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">for</span><span class="plain-syntax"> (</span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">i</span><span class="plain-syntax">=0; </span><span class="identifier-syntax">i</span><span class="function-syntax"><ec-></span><span class="element-syntax">pe_cache_size</span><span class="plain-syntax">; </span><span class="identifier-syntax">i</span><span class="plain-syntax">++)</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">Wordings::eq</span><span class="plain-syntax">(</span><span class="identifier-syntax">W</span><span class="plain-syntax">, </span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">].</span><span class="element-syntax">cached_query</span><span class="plain-syntax">))</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">].</span><span class="element-syntax">cached_result</span><span class="plain-syntax">;</span>
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</pre>
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<ul class="endnotetexts"><li>This code is used in <a href="2-spc.html#SP2">§2</a>.</li></ul>
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<p class="commentary firstcommentary"><a id="SP2_2" class="paragraph-anchor"></a><b>§2.2. </b>The cache expands until it reaches <span class="extract"><span class="extract-syntax">MAXIMUM_CACHE_SIZE</span></span>; after that,
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entries are written in a position cycling through the ring. In either case
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it takes <span class="extract"><span class="extract-syntax">MAXIMUM_CACHE_SIZE</span></span> further parses (not found in the cache) to
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overwrite the one we put down now.
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</p>
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<p class="commentary"><span class="named-paragraph-container code-font"><span class="named-paragraph-defn">Write the newly discovered specification to the cache for future use</span><span class="named-paragraph-number">2.2</span></span><span class="comment-syntax"> =</span>
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="reserved-syntax">expression_cache</span><span class="plain-syntax"> *</span><span class="identifier-syntax">ec</span><span class="plain-syntax"> = &(</span><span class="identifier-syntax">contextual_cache</span><span class="plain-syntax">[</span><span class="identifier-syntax">context</span><span class="plain-syntax">]);</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache</span><span class="plain-syntax">[</span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache_posn</span><span class="plain-syntax">].</span><span class="element-syntax">cached_query</span><span class="plain-syntax"> = </span><span class="identifier-syntax">W</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache</span><span class="plain-syntax">[</span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache_posn</span><span class="plain-syntax">].</span><span class="element-syntax">cached_result</span><span class="plain-syntax"> = </span><span class="identifier-syntax">spec</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache_posn</span><span class="plain-syntax">++;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache_size</span><span class="plain-syntax"> < </span><span class="constant-syntax">MAXIMUM_CACHE_SIZE</span><span class="plain-syntax">) </span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache_size</span><span class="plain-syntax">++;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache_posn</span><span class="plain-syntax"> == </span><span class="constant-syntax">MAXIMUM_CACHE_SIZE</span><span class="plain-syntax">) </span><span class="identifier-syntax">ec</span><span class="plain-syntax">-></span><span class="element-syntax">pe_cache_posn</span><span class="plain-syntax"> = </span><span class="constant-syntax">0</span><span class="plain-syntax">;</span>
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</pre>
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<ul class="endnotetexts"><li>This code is used in <a href="2-spc.html#SP2">§2</a>.</li></ul>
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<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>§3. </b>In Inform, this returns an UNKNOWN specification.
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="reserved-syntax">parse_node</span><span class="plain-syntax"> *</span><span class="function-syntax">PreformCache::not_found</span><button class="popup" onclick="togglePopup('usagePopup1')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup1">Usage of <span class="code-font"><span class="function-syntax">PreformCache::not_found</span></span>:<br/><a href="2-spc.html#SP2">§2</a></span></button><span class="plain-syntax">(</span><span class="identifier-syntax">wording</span><span class="plain-syntax"> </span><span class="identifier-syntax">W</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> #</span><span class="identifier-syntax">ifdef</span><span class="plain-syntax"> </span><span class="identifier-syntax">UNKNOWN_PREFORM_RESULT_SYNTAX_CALLBACK</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">UNKNOWN_PREFORM_RESULT_SYNTAX_CALLBACK</span><span class="plain-syntax">(</span><span class="identifier-syntax">W</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> #</span><span class="identifier-syntax">endif</span>
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<span class="plain-syntax"> #</span><span class="identifier-syntax">ifndef</span><span class="plain-syntax"> </span><span class="identifier-syntax">UNKNOWN_PREFORM_RESULT_SYNTAX_CALLBACK</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="identifier-syntax">NULL</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> #</span><span class="identifier-syntax">endif</span>
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<span class="plain-syntax">}</span>
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</pre>
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<p class="commentary firstcommentary"><a id="SP4" class="paragraph-anchor"></a><b>§4. </b>As with all caches, we have to be careful that the information does not fall
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out of date. There are two things which can go wrong: the S-node in the cache
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might be altered, perhaps as a result of the type-checker trying to force a
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round peg into a square hole; or the stock of Inform's defined names might
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change, so that the same text now has to be read differently.
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</p>
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<p class="commentary">The first problem can't be fixed here. It's tempting to try something like
|
|
flagging S-nodes which have been altered, and then ensuring that the
|
|
cache never serves up an altered result. But that fails for timing reasons —
|
|
by the time the S-node might be altered, pointers to it may exist
|
|
in multiple data structures already, because the cache might have served
|
|
it more than once by that time. (Not just a theoretical possibility — tests
|
|
show that this does, albeit rarely, happen.) The brute force solution is to
|
|
serve a copy of the cache entry, and thus never send out the same pointer
|
|
twice. But this more than doubles the memory required to store S-nodes,
|
|
which is unacceptable, and also slows Inform down, because allocating memory
|
|
for all those copies is laborious. We therefore just have to be very careful
|
|
about modifying S-nodes which have arisen from parsing.
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|
</p>
|
|
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<p class="commentary">The second problem is easier. We require other parts of Inform which make
|
|
or unmake name definitions to warn us, by calling this routine. Definitions
|
|
are made and unmade relatively rarely, so the performance hit is small.
|
|
</p>
|
|
|
|
<pre class="displayed-code all-displayed-code code-font">
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|
<span class="reserved-syntax">void</span><span class="plain-syntax"> </span><span class="function-syntax">PreformCache::warn_of_changes</span><button class="popup" onclick="togglePopup('usagePopup2')"><span class="comment-syntax">?</span><span class="popuptext" id="usagePopup2">Usage of <span class="code-font"><span class="function-syntax">PreformCache::warn_of_changes</span></span>:<br/><a href="2-spc.html#SP2_1">§2.1</a></span></button><span class="plain-syntax">(</span><span class="reserved-syntax">void</span><span class="plain-syntax">) {</span>
|
|
<span class="plain-syntax"> </span><span class="reserved-syntax">for</span><span class="plain-syntax"> (</span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">i</span><span class="plain-syntax">=0; </span><span class="identifier-syntax">i</span><span class="plain-syntax"><</span><span class="constant-syntax">NUMBER_OF_CACHED_NONTERMINALS</span><span class="plain-syntax">; </span><span class="identifier-syntax">i</span><span class="plain-syntax">++) {</span>
|
|
<span class="plain-syntax"> </span><span class="identifier-syntax">contextual_cache</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">].</span><span class="element-syntax">pe_cache_size</span><span class="plain-syntax"> = </span><span class="constant-syntax">0</span><span class="plain-syntax">;</span>
|
|
<span class="plain-syntax"> </span><span class="identifier-syntax">contextual_cache</span><span class="plain-syntax">[</span><span class="identifier-syntax">i</span><span class="plain-syntax">].</span><span class="element-syntax">pe_cache_posn</span><span class="plain-syntax"> = </span><span class="constant-syntax">0</span><span class="plain-syntax">;</span>
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|
<span class="plain-syntax"> }</span>
|
|
<span class="plain-syntax">}</span>
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|
</pre>
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