lib/int_sqrt.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Copyright (C) 2013 Davidlohr Bueso <davidlohr.bueso@hp.com>
   4  *
   5  *  Based on the shift-and-subtract algorithm for computing integer
   6  *  square root from Guy L. Steele.
   7  */
   8
   9 #include <linux/kernel.h>
  10 #include <linux/export.h>
  11 #include <linux/bitops.h>
  12
  13 /**
  14  * int_sqrt - computes the integer square root
  15  * @x: integer of which to calculate the sqrt
  16  *
  17  * Computes: floor(sqrt(x))
  18  */
  19 unsigned long int_sqrt(unsigned long x)
  20 {
  21         unsigned long b, m, y = 0;
  22
  23         if (x <= 1)
  24                 return x;
  25
  26         m = 1UL << (__fls(x) & ~1UL);
  27         while (m != 0) {
  28                 b = y + m;
  29                 y >>= 1;
  30
  31                 if (x >= b) {
  32                         x -= b;
  33                         y += m;
  34                 }
  35                 m >>= 2;
  36         }
  37
  38         return y;
  39 }
  40 EXPORT_SYMBOL(int_sqrt);
  41
  42 #if BITS_PER_LONG < 64
  43 /**
  44  * int_sqrt64 - strongly typed int_sqrt function when minimum 64 bit input
  45  * is expected.
  46  * @x: 64bit integer of which to calculate the sqrt
  47  */
  48 u32 int_sqrt64(u64 x)
  49 {
  50         u64 b, m, y = 0;
  51
  52         if (x <= ULONG_MAX)
  53                 return int_sqrt((unsigned long) x);
  54
  55         m = 1ULL << ((fls64(x) - 1) & ~1ULL);
  56         while (m != 0) {
  57                 b = y + m;
  58                 y >>= 1;
  59
  60                 if (x >= b) {
  61                         x -= b;
  62                         y += m;
  63                 }
  64                 m >>= 2;
  65         }
  66
  67         return y;
  68 }
  69 EXPORT_SYMBOL(int_sqrt64);
  70 #endif