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improved documentation

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Krishna Vedala 2020-05-26 09:05:44 -04:00
parent 0ad756f860
commit 139964d325
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1 changed files with 23 additions and 17 deletions
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math/fast_power.cpp
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@ -1,22 +1,29 @@
#include <iostream>
#include <cstdlib>
#include <cstdint>
#include <cassert>
#include <ctime>
#include <cmath>
/*
Program that computes a^b in O(logN) time.
/**
* @file
Program that computes \f$a^b\f$ in \f$O(logN)\f$ time.
It is based on formula that:
case1) if b is even: a^b = a^(b/2) * a^(b/2) = (a^(b/2))ˆ2
case2) if b is odd: a^b = a^((b-1)/2) * a^((b-1)/2) * a = (a^((b-1)/2))^2 * a
We can compute a^b recursively using above algorithm.
1. if \f$b\f$ is even: \f$a^b = a^\frac{b}{2} \cdot a^\frac{b}{2} =
{a^\frac{b}{2}}^2\f$
2. if \f$b\f$ is odd: \f$a^b = a^\frac{b-1}{2} \cdot
a^\frac{b-1}{2} \cdot a = {a^\frac{b-1}{2}}^2 \cdot a\f$
We can compute \f$a^b\f$
recursively using above algorithm.
*/
#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdlib>
#include <ctime>
#include <iostream>
/**
* algorithm implementation for \f$a^b\f$
*/
double fast_power_recursive(int64_t a, int64_t b) {
// negative power. a^b = 1 / (a^-b)
if (b < 0)
return 1.0 / fast_power_recursive(a, -b);
if (b < 0) return 1.0 / fast_power_recursive(a, -b);
if (b == 0) return 1;
int64_t bottom = fast_power_recursive(a, b >> 1);
@ -31,14 +38,13 @@ double fast_power_recursive(int64_t a, int64_t b) {
return result;
}
/*
/**
Same algorithm with little different formula.
It still calculates in O(logN)
*/
double fast_power_linear(int64_t a, int64_t b) {
// negative power. a^b = 1 / (a^-b)
if (b < 0)
return 1.0 / fast_power_linear(a, -b);
if (b < 0) return 1.0 / fast_power_linear(a, -b);
double result = 1;
while (b) {