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Avoid ldexp call when converting random numbers to double #4159

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Avoid ldexp call when converting random numbers to double #4159

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7 changes: 3 additions & 4 deletions src/random_lcg.cpp
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Original file line number Diff line number Diff line change
@@ -1,7 +1,5 @@
#include "openmc/random_lcg.h"

#include <cmath>

namespace openmc {

// Starting seed
Expand Down Expand Up @@ -78,8 +76,9 @@ double prn(uint64_t* seed)
((*seed >> ((*seed >> 59u) + 5u)) ^ *seed) * 12605985483714917081ull;
uint64_t result = (word >> 43u) ^ word;

// Convert output from unsigned integer to double
return ldexp(result, -64);
// Convert output from unsigned integer to double. Multiplying by an exact
// power of two gives the same result as ldexp but avoids a library call.
return static_cast<double>(result) * 0x1p-64;
}

//==============================================================================
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