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POC "double to num" conversion code: why is it written like this?
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I don't understand how the following C conversion functions work (and why they're written this way); I'm fairly certain that the original author knew what he was doing: <pre><code>typedef union TValue { uint64_t u64; double n; struct { uint32_t lo; /* Lower 32 bits of number. */ uint32_t hi; /* Upper 32 bits of number. */ } u32; [...] } TValue; static int32_t num2bit(double n) { TValue o; o.n = n + 6755399441055744.0; /* 2^52 + 2^51 */ return (int32_t)o.u32.lo; } static uint64_t num2u64(double n) { #ifdef _MSC_VER if (n >= 9223372036854775808.0) /* They think it's a feature. */ return (uint64_t)(int64_t)(n - 18446744073709551616.0); else #endif return (uint64_t)n; } </code></pre> <ul> <li>Does num2bit actually just cast a <code>double</code> into <code>int32_t</code>? Why the addition? Why write it like this?</li> <li>What is this "feature" that is alluded to in num2u64? (I believe _MSC_VER means it's the code-path for Microsofts C compiler).</li> </ul> Note that those functions are not always used (depending on CPU architecture), this is for little-endian (I resolved some preprocessor macros to simplify). Links to online browseable mirror (the code is from the <a href="http://luajit.org" rel="nofollow">LuaJIT</a> project): Surrounding <a href="http://repo.or.cz/w/luajit-2.0.git/blob_plain/HEAD:/src/lj_obj.h" rel="nofollow">Header file</a> (or <a href="http://repo.or.cz/w/luajit-2.0.git/" rel="nofollow">whole project</a>). Every hint is appreciated.
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<c><double><type-conversion><low-level>
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C "double to num" conversion code: why is it written like this?
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