alistair23-linux/lib/math/int_sqrt.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2013 Davidlohr Bueso <davidlohr.bueso@hp.com>
 *
 *  Based on the shift-and-subtract algorithm for computing integer
 *  square root from Guy L. Steele.
 */

#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/bitops.h>

/**
 * int_sqrt - computes the integer square root
 * @x: integer of which to calculate the sqrt
 *
 * Computes: floor(sqrt(x))
 */
unsigned long int_sqrt(unsigned long x)
{
	unsigned long b, m, y = 0;

	if (x <= 1)
		return x;

	m = 1UL << (__fls(x) & ~1UL);
	while (m != 0) {
		b = y + m;
		y >>= 1;

		if (x >= b) {
			x -= b;
			y += m;
		}
		m >>= 2;
	}

	return y;
}
EXPORT_SYMBOL(int_sqrt);

#if BITS_PER_LONG < 64
/**
 * int_sqrt64 - strongly typed int_sqrt function when minimum 64 bit input
 * is expected.
 * @x: 64bit integer of which to calculate the sqrt
 */
u32 int_sqrt64(u64 x)
{
	u64 b, m, y = 0;

	if (x <= ULONG_MAX)
		return int_sqrt((unsigned long) x);

	m = 1ULL << ((fls64(x) - 1) & ~1ULL);
	while (m != 0) {
		b = y + m;
		y >>= 1;

		if (x >= b) {
			x -= b;
			y += m;
		}
		m >>= 2;
	}

	return y;
}
EXPORT_SYMBOL(int_sqrt64);
#endif