esp8266/cores/esp8266/sqrt32.cpp

#include <coredecls.h>
#include <stdint.h>

extern "C" {

uint32_t sqrt32 (uint32_t n)
{
    // http://www.codecodex.com/wiki/Calculate_an_integer_square_root#C
    // Another very fast algorithm donated by Tristan.Muntsinger@gmail.com
    // (note: tested across the full 32 bits range, see comment below)

    // 15 iterations (c=1<<15)

    unsigned int c = 0x8000;
    unsigned int g = 0x8000;

    for(;;)
    {
        if (g*g > n)
            g ^= c;
        c >>= 1;
        if (!c)
            return g;
        g |= c;
    }
}

/*
 * tested with:
 *

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

int main (void)
{
    for (uint32_t i = 0; ++i; )
    {
        uint32_t sr = sqrt32(i);
        uint32_t ifsr = sqrt(i);

        if (ifsr != sr)
            printf("%d: i%d f%d\n", i, sr, ifsr);

        if (!(i & 0xffffff))
        {
            printf("%i%% (0x%08x)\r", ((i >> 16) * 100) >> 16, i);
            fflush(stdout);
        }
    }

    printf("\n");
}

 *
 */

};