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    <p>A string is an array, so <code>"hello"[ 1 ] == 'e'</code>. Furthermore an array decays to a pointer to its first element, and the subscripting operation is defined as <code>array[ index ] == * ( array + index )</code>. The addition produces a pointer after the first element.</p> <p>The expression in the <code>#define</code> is confusing because it relies on precedence rules rather than parentheses, and does this in a funny order besides. This is really not an example of good code.</p> <p>It's unnecessary to force the compiler to evaluate something at compile time. All compile-time constant expressions will be optimized as such when optimization is enabled, and otherwise you might want to step through the evaluation in the debugger! So here is a reformatted version:</p> <pre><code>if ( ( op % 8 ) &amp; // low 3 bits select conditions for execution ( s[ op / 8 ] &gt; 'Z'? // lowercase letters go through complicated mapping ( " (),-089&lt;&gt;?BCFGHJLSVWZ[^hlmnxy|}"[ s[ op / 8 ] - 'Z' - 4 ] ) + 130 : s[ op / 8 ] - " (("[ s [ op / 8 ] / 39 ] ) ) { // uppercase is simpler code; } </code></pre> <p>This is still quite obfuscated and likely inefficient. I can't tell why the author is doing things this way (didn't watch the YouTube) but since <code>op8 = op % 8</code> generates the low three bits, i.e. a number in the range 0..7, it seems pointless to do something such as add 130 before extracting a subset of the low three bits again (this time with the <code>&amp;</code> operator).</p> <p>Anyway…</p> <p>This calculates an index into the macro's argument string:</p> <pre><code>[ s[ op / 8 ] - 'Z' - 4 ] </code></pre> <p>This maps uppercase letters in the macro argument string to other letters… don't ask me why. Looks like evidence of flawed design; at this level, the string should be in a format without a need for translation.</p> <pre><code>( " (),-089&lt;&gt;?BCFGHJLSVWZ[^hlmnxy|}"[ s[ op / 8 ] - 'Z' - 4 ] ) </code></pre> <p>Finally, the number 130 is added. Since <code>130 % 8 == 2</code>, I would think 2 would also work. Beats me. Also, performing addition here has the side effect of changing the expression's type to <code>int</code>, but it doesn't matter whether 130 is inside the range of <code>char</code>.</p> <pre><code>( " (),-089&lt;&gt;?BCFGHJLSVWZ[^hlmnxy|}"[ s[ op / 8 ] - 'Z' - 4 ] ) + 130 </code></pre> <hr> <p>Putting binary data into string literals is a favorite strategy for obfuscation and "Code Golf", the sport of making a program as short as possible. It is a way to make a program unreadable and fairly compact in memory, <em>not</em> a way to make it faster, and not the best way to optimize for memory consumption either. If you want a table of numbers, specify them as numbers in hex, or decimal.</p>
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