isqrt_challenge

#include <stdio.h>
#include <assert.h>
#include <stdint.h>

/*
C function to calculate the truncated square root of an integer <= 65535 without
any loops or if-statements.

Gary Williams 2018-10-31

https://pastebin.com/DRBW7GD2
https://www.quora.com/How-do-you-write-a-C-program-which-calculates-the-square-root-of-a-number-without-using-sqrt-function-neither-for-while-if-or-else-statements-nor-libraries
*/
uint8_t isqrt_16d(uint16_t v) {
    uint8_t result = 0;

    result ^= 1 << 7;
    result ^= ((uint32_t)(v - result * result) >> 31) << 7;
    result ^= 1 << 6;
    result ^= ((uint32_t)(v - result * result) >> 31) << 6;
    result ^= 1 << 5;
    result ^= ((uint32_t)(v - result * result) >> 31) << 5;
    result ^= 1 << 4;
    result ^= ((uint32_t)(v - result * result) >> 31) << 4;
    result ^= 1 << 3;
    result ^= ((uint32_t)(v - result * result) >> 31) << 3;
    result ^= 1 << 2;
    result ^= ((uint32_t)(v - result * result) >> 31) << 2;
    result ^= 1 << 1;
    result ^= ((uint32_t)(v - result * result) >> 31) << 1;
    result ^= 1 << 0;
    result ^= ((uint32_t)(v - result * result) >> 31) << 0;

    return result;
}