turi::fast_integer_power Class Reference

#include <core/util/fast_integer_power.hpp>

Public Member Functions
	fast_integer_power (double a=1.0)
void	set_base (double a)
double	pow (size_t b) const GL_HOT_INLINE_FLATTEN

Detailed Description

A class that gives highly optimized version of the power function for when a^b needs to be computed for many different values of b. b must be a non-negative integer (type size_t).

Usage:

fast_integer_power fip(a); // Slower than std::pow by a factor of 10 or so.

   fip.pow(b);   // Very fast, returns std::pow(a, b);

Internals:

 Let the size_t value x be laid out in 4 bit blocks on disk.

 | b0 b1 b2 b3 | b4 b5 b6 b7 | ... |

 +-------------+-------------+ ...
   block 1        block 2

 First note that a^b can be computed for by looking at all the bits of
 b that are 1, e.g. if b0, b4, and b5, are 1 (so b = 2^0 + 2^4 + 2^5),
 then a^b = a^(2^0) * a^(2^4) * a^(2^5).

 Thus caching the values for each of these powers of 2 gives us the
 final value of a.

 The method used internally employs this technique, but treats each
 block as a unit.  Thus we actually compute

 a^b = a^(lookup1[block1 bits]) * a^(lookup2[block2 bits]) * ...

 with each lookup having 8 bits and thus 256 possible values.

Definition at line 53 of file fast_integer_power.hpp.

Constructor & Destructor Documentation

fast_integer_power()

turi::fast_integer_power::fast_integer_power ( double  a = 1.0 )

inline

Constructs lookup tables to return std::pow(a, b).

Definition at line 58 of file fast_integer_power.hpp.

Member Function Documentation

pow()

double turi::fast_integer_power::pow ( size_t  n ) const

inline

Returns a^b, where a is given in the constructor.

Calculate out a^n.

Definition at line 93 of file fast_integer_power.hpp.

set_base()

void turi::fast_integer_power::set_base ( double  a )

inline

Sets the base of the power function.

Definition at line 64 of file fast_integer_power.hpp.

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The documentation for this class was generated from the following file:

• core/util/fast_integer_power.hpp