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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<title>3/wm</title>
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<h1><a href="../webs.html">Sources</a></h1>
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<li><a href="../compiler.html">compiler</a></li>
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<li><a href="../other.html"><b>other tools</b></a></li>
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<li><a href="../units.html">unit test tools</a></li>
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</ul>
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<h2>Other Tools</h2>
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<ul>
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<li><a href="../inblorb/index.html">inblorb</a></li>
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<h2>Foundation</h2>
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<ul>
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<li><a href="../../../inweb/docs/foundation-module/index.html">foundation</a></li>
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</ul>
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<main role="main">
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<!--Weave of '3/bs6' generated by 7-->
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<ul class="crumbs"><li><a href="../webs.html">Source</a></li><li><a href="../other.html">Other Tools</a></li><li><a href="index.html">inblorb</a></li><li><a href="index.html#3">Chapter 3: Other Material</a></li><li><b>Base64</b></li></ul><p class="purpose">To produce base64-encoded story files ready for in-browser play by a Javascript-based interpreter such as Parchment.</p>
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<p class="inwebparagraph"><a id="SP1"></a><b>§1. </b>The Base64 encoding scheme is defined by the Internet standard RFC 1113.
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Broadly, the idea is to take a binary stream of bytes, break it into
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threes, and then convert this into a sequence of four emailable
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characters. To encode 24 bits in four characters, we need six bits per
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character, so we need 2^6 = 64 characters in all. Since 64 = 26 +
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26 + 10 + 2, we can nearly get there with alphanumeric characters alone,
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adding just two others — conventionally, plus and forward-slash.
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That's more or less the whole thing, except that we use an equals sign
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to indicate incompleteness of the final triplet (which might have only
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1 or 2 bytes in it).
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</p>
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<p class="inwebparagraph">RFC 1113 permits white space to be used freely, including in particular
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line breaks, but we don't avail ourselves.
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</p>
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<pre class="display">
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<span class="reserved">char</span><span class="plain"> *</span><span class="identifier">RFC1113_table</span><span class="plain"> = </span><span class="string">"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="</span><span class="plain">;</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="inwebparagraph"><a id="SP2"></a><b>§2. </b>The encoding routine is as follows.
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</p>
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<pre class="display">
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<span class="reserved">void</span><span class="plain"> </span><span class="functiontext">Base64::encode</span><span class="plain">(</span><span class="reserved">filename</span><span class="plain"> *</span><span class="identifier">in_filename</span><span class="plain">, </span><span class="reserved">filename</span><span class="plain"> *</span><span class="identifier">out_filename</span><span class="plain">, </span><span class="reserved">text_stream</span><span class="plain"> *</span><span class="identifier">top</span><span class="plain">, </span><span class="reserved">text_stream</span><span class="plain"> *</span><span class="identifier">tail</span><span class="plain">) {</span>
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<span class="reserved">FILE</span><span class="plain"> *</span><span class="identifier">IN</span><span class="plain"> = </span><span class="functiontext">BinaryFiles::open_for_reading</span><span class="plain">(</span><span class="identifier">in_filename</span><span class="plain">);</span>
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<span class="reserved">text_stream</span><span class="plain"> </span><span class="identifier">TO_struct</span><span class="plain">;</span>
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<span class="reserved">text_stream</span><span class="plain"> *</span><span class="identifier">TO</span><span class="plain"> = &</span><span class="identifier">TO_struct</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">STREAM_OPEN_TO_FILE</span><span class="plain">(</span><span class="identifier">TO</span><span class="plain">, </span><span class="identifier">out_filename</span><span class="plain">, </span><span class="constant">UTF8_ENC</span><span class="plain">) == </span><span class="constant">FALSE</span><span class="plain">)</span>
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<span class="functiontext">BlorbErrors::fatal_fs</span><span class="plain">(</span><span class="string">"can't open base-64 encoded story file for output"</span><span class="plain">, </span><span class="identifier">out_filename</span><span class="plain">);</span>
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<span class="functiontext">Base64::encode_inner</span><span class="plain">(</span><span class="identifier">TO</span><span class="plain">, </span><span class="identifier">IN</span><span class="plain">, </span><span class="identifier">top</span><span class="plain">, </span><span class="identifier">tail</span><span class="plain">);</span>
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<span class="functiontext">BinaryFiles::close</span><span class="plain">(</span><span class="identifier">IN</span><span class="plain">);</span>
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<span class="identifier">STREAM_CLOSE</span><span class="plain">(</span><span class="identifier">TO</span><span class="plain">);</span>
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<span class="plain">}</span>
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<span class="reserved">void</span><span class="plain"> </span><span class="functiontext">Base64::encode_inner</span><span class="plain">(</span><span class="constant">OUTPUT_STREAM</span><span class="plain">, </span><span class="reserved">FILE</span><span class="plain"> *</span><span class="identifier">IN</span><span class="plain">, </span><span class="reserved">text_stream</span><span class="plain"> *</span><span class="identifier">top</span><span class="plain">, </span><span class="reserved">text_stream</span><span class="plain"> *</span><span class="identifier">tail</span><span class="plain">) {</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">top</span><span class="plain">) </span><span class="identifier">WRITE</span><span class="plain">(</span><span class="string">"%S"</span><span class="plain">, </span><span class="identifier">top</span><span class="plain">);</span>
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<span class="reserved">while</span><span class="plain"> (</span><span class="constant">TRUE</span><span class="plain">) {</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">triplet</span><span class="plain">[3], </span><span class="identifier">triplet_size</span><span class="plain"> = 0;</span>
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<<span class="cwebmacro">Read the triplet of binary bytes, storing 0 to 3 in the size read</span> <span class="cwebmacronumber">2.1</span>><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">triplet_size</span><span class="plain"> == 0) </span><span class="reserved">break</span><span class="plain">;</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">quartet</span><span class="plain">[4];</span>
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<<span class="cwebmacro">Convert triplet to a quartet</span> <span class="cwebmacronumber">2.2</span>><span class="plain">;</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">i</span><span class="plain">; </span><span class="reserved">for</span><span class="plain"> (</span><span class="identifier">i</span><span class="plain">=0; </span><span class="identifier">i</span><span class="plain"><4; </span><span class="identifier">i</span><span class="plain">++) </span><span class="identifier">PUT</span><span class="plain">(</span><span class="identifier">RFC1113_table</span><span class="plain">[</span><span class="identifier">quartet</span><span class="plain">[</span><span class="identifier">i</span><span class="plain">]]);</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">triplet_size</span><span class="plain"> < 3) </span><span class="reserved">break</span><span class="plain">;</span>
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<span class="plain">}</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">tail</span><span class="plain">) </span><span class="identifier">WRITE</span><span class="plain">(</span><span class="string">"%S"</span><span class="plain">, </span><span class="identifier">tail</span><span class="plain">);</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">The function Base64::encode is used in 3/rls (<a href="3-rls.html#SP6_5">§6.5</a>).</p>
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<p class="endnote">The function Base64::encode_inner appears nowhere else.</p>
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<p class="inwebparagraph"><a id="SP2_1"></a><b>§2.1. </b>If the file ends in mid-triplet, we pad out with zeros.
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</p>
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<p class="macrodefinition"><code class="display">
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<<span class="cwebmacrodefn">Read the triplet of binary bytes, storing 0 to 3 in the size read</span> <span class="cwebmacronumber">2.1</span>> =
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</code></p>
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<pre class="displaydefn">
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<span class="identifier">triplet</span><span class="plain">[0] = </span><span class="identifier">fgetc</span><span class="plain">(</span><span class="identifier">IN</span><span class="plain">);</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">triplet</span><span class="plain">[0] != </span><span class="identifier">EOF</span><span class="plain">) {</span>
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<span class="identifier">triplet_size</span><span class="plain">++;</span>
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<span class="identifier">triplet</span><span class="plain">[1] = </span><span class="identifier">fgetc</span><span class="plain">(</span><span class="identifier">IN</span><span class="plain">);</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">triplet</span><span class="plain">[1] != </span><span class="identifier">EOF</span><span class="plain">) {</span>
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<span class="identifier">triplet_size</span><span class="plain">++;</span>
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<span class="identifier">triplet</span><span class="plain">[2] = </span><span class="identifier">fgetc</span><span class="plain">(</span><span class="identifier">IN</span><span class="plain">);</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">triplet</span><span class="plain">[2] != </span><span class="identifier">EOF</span><span class="plain">)</span>
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<span class="identifier">triplet_size</span><span class="plain">++;</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">i</span><span class="plain">; </span><span class="reserved">for</span><span class="plain"> (</span><span class="identifier">i</span><span class="plain">=</span><span class="identifier">triplet_size</span><span class="plain">; </span><span class="identifier">i</span><span class="plain"><3; </span><span class="identifier">i</span><span class="plain">++) </span><span class="identifier">triplet</span><span class="plain">[</span><span class="identifier">i</span><span class="plain">] = 0;</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">This code is used in <a href="#SP2">§2</a>.</p>
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<p class="inwebparagraph"><a id="SP2_2"></a><b>§2.2. </b><code class="display">
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<<span class="cwebmacrodefn">Convert triplet to a quartet</span> <span class="cwebmacronumber">2.2</span>> =
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</code></p>
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<pre class="displaydefn">
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">i</span><span class="plain">; </span><span class="reserved">for</span><span class="plain"> (</span><span class="identifier">i</span><span class="plain">=0; </span><span class="identifier">i</span><span class="plain"><4; </span><span class="identifier">i</span><span class="plain">++) </span><span class="identifier">quartet</span><span class="plain">[</span><span class="identifier">i</span><span class="plain">] = 0;</span>
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<span class="identifier">quartet</span><span class="plain">[0] += (</span><span class="identifier">triplet</span><span class="plain">[0] & 0</span><span class="identifier">xFC</span><span class="plain">) >> 2;</span>
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<span class="identifier">quartet</span><span class="plain">[1] += (</span><span class="identifier">triplet</span><span class="plain">[0] & 0</span><span class="identifier">x03</span><span class="plain">) << 4;</span>
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<span class="identifier">quartet</span><span class="plain">[1] += (</span><span class="identifier">triplet</span><span class="plain">[1] & 0</span><span class="identifier">xF0</span><span class="plain">) >> 4;</span>
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<span class="identifier">quartet</span><span class="plain">[2] += (</span><span class="identifier">triplet</span><span class="plain">[1] & 0</span><span class="identifier">x0F</span><span class="plain">) << 2;</span>
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<span class="identifier">quartet</span><span class="plain">[2] += (</span><span class="identifier">triplet</span><span class="plain">[2] & 0</span><span class="identifier">xC0</span><span class="plain">) >> 6;</span>
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<span class="identifier">quartet</span><span class="plain">[3] += (</span><span class="identifier">triplet</span><span class="plain">[2] & 0</span><span class="identifier">x3F</span><span class="plain">) << 0;</span>
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<span class="reserved">switch</span><span class="plain"> (</span><span class="identifier">triplet_size</span><span class="plain">) {</span>
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<span class="reserved">case</span><span class="plain"> 1: </span><span class="identifier">quartet</span><span class="plain">[2] = 64; </span><span class="identifier">quartet</span><span class="plain">[3] = 64; </span><span class="reserved">break</span><span class="plain">;</span>
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<span class="reserved">case</span><span class="plain"> 2: </span><span class="identifier">quartet</span><span class="plain">[3] = 64; </span><span class="reserved">break</span><span class="plain">;</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">This code is used in <a href="#SP2">§2</a>.</p>
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<hr class="tocbar">
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<ul class="toc"><li><a href="3-wm.html">Back to 'Website Maker'</a></li><li><i>(This section ends Chapter 3: Other Material.)</i></li></ul><hr class="tocbar">
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