2.80
/*
* threeforths.c
*/
#include <stdio.h>
#include <assert.h>
#include <limits.h>
/*
* calculate 3/4x, no overflow, round to zero
*
* no overflow means divide 4 first, then multiple 3, diffrent from 2.79 here
*
* rounding to zero is a little complicated.
* every int x, equals f(first 30 bit number) plus l(last 2 bit number)
*
* f = x & ~0x3
* l = x & 0x3
* x = f + l
* threeforths(x) = f/4*3 + l*3/4
*
* f doesn't care about round at all, we just care about rounding from l*3/4
*
* lm3 = (l << 1) + l
*
* when x > 0, rounding to zero is easy
*
* lm3d4 = lm3 >> 2
*
* when x < 0, rounding to zero acts like divide_power2 in 2.78
*
* bias = 0x3 // (1 << 2) - 1
* lm3d4 = (lm3 + bias) >> 2
*/
int threeforths(int x) {
int is_neg = x & INT_MIN;
int f = x & ~0x3;
int l = x & 0x3;
int fd4 = f >> 2;
int fd4m3 = (fd4 << 1) + fd4;
int lm3 = (l << 1) + l;
int bias = (1 << 2) - 1;
(is_neg && (lm3 += bias));
int lm3d4 = lm3 >> 2;
return fd4m3 + lm3d4;
}
int main(int argc, char* argv[]) {
assert(threeforths(8) == 6);
assert(threeforths(9) == 6);
assert(threeforths(10) == 7);
assert(threeforths(11) == 8);
assert(threeforths(12) == 9);
assert(threeforths(-8) == -6);
assert(threeforths(-9) == -6);
assert(threeforths(-10) == -7);
assert(threeforths(-11) == -8);
assert(threeforths(-12) == -9);
return 0;
}