Issue #13219 has been updated by Jabari Zakiya.


After further testing I found the same errors when using **Math.sqrt(n).to_i** with
large number when using ``(n**(1.0/2)).to_i``. This reinforces to me the need to provide 
**sqrt_i** (by whatever name) so users won't fall prey to this undocumented anomaly.

```
2.4.0 :129 > n= 10**35; puts n, a = intsqrt4(n), a*a, b = Math.sqrt(n).to_i, b*b, c = (n**(1.0/2)).to_i, c*c
100000000000000000000000000000000000
316227766016837933
99999999999999999873578871987712489
316227766016837952
100000000000000011890233980627554304
316227766016837952
100000000000000011890233980627554304
 => nil 
2.4.0 :130 >
```

I also realized that the algorithm used to determine the integer squareroot can be easily
generalized to correctly generate integer nth roots for arbitrary size integers.

This capability will now allows Ruby to be used for many Number Theory and Cryptographic problems,
such as generating correct integer values for eliptic curves, real integer roots of polynomials,
public key, et al, encryption, for very large number sets.

Because of this, I am going to add this capability to my **roots** rubygem.

https://rubygems.org/gems/roots

It has two (2) methods **root** and **roots** which generate all the n roots of an nth root for
all **Numeric** types (integers, floats, complex, rational).

In order to not clash with my proposed **class Integer** method name of **sqrt_i** as a Ruby core method,
I will name two more **class Integer** methods **iroot2** and **irootn(n)** to be added to **roots**.
Since finding squareroots is much more frequently done than other roots I give it its own method name.
Below is the code for doing this (which I may refine).

These methods will return the correct nth integer real (not complex) root for positive integers, and
for odd roots of negative integers.

```
class Integer

  def irootn(n)
    return nil if self < 0 && n.even?
    raise "root n is < 2 or not an Integer" unless n.is_a?(Integer) && n > 1
    sign = self < 0 ? -1 : 1
    num  = self.abs
    bits_shift = (num.bit_length)/n + 2
    root, bitn_mask = 0, (1 << bits_shift)
    until (bitn_mask >>= 1) == 0
      root |= bitn_mask
      root ^= bitn_mask if root**n > num
    end
    root*sign
  end

  def iroot2; irootn(2) end

end
```

As with generating integer squareroots, doing ``(num**(1.0/n)).to_i`` produces incorrect answers for
integers past some maximun value, as shown below.

```
2.4.0 :129 > n = 10**37; puts n, a = n.irootn(3), a**3, b = (n**(1.0/3)).to_i, b**3
10000000000000000000000000000000000000
2154434690031
9999999999987694380850132072511299791
2154434690031
9999999999987694380850132072511299791
 => nil 
2.4.0 :130 > n = 10**38; puts n, a = n.irootn(3), a**3, b = (n**(1.0/3)).to_i, b**3
100000000000000000000000000000000000000
4641588833612
99999999999949657815157877549034676928
4641588833612
99999999999949657815157877549034676928
 => nil 
2.4.0 :131 > n = 10**39; puts n, a = n.irootn(3), a**3, b = (n**(1.0/3)).to_i, b**3
1000000000000000000000000000000000000000
10000000000000
1000000000000000000000000000000000000000
9999999999999
999999999999700000000000029999999999999
 => nil 


2.4.0 :134 > n = 10**112; puts n, a = n.irootn(4), a**4, b = (n**(1.0/4)).to_i, b**4
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
10000000000000000000000000000
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
9999999999999999583119736832
9999999999999998332478947328000104273492291412559539763672807300805169073432752214389506884609933472640769458176
 => nil 
2.4.0 :135 >
```
These methods are also preferable because they work correctly for negative integers, while the exponential root
method throws errors.

```
2.4.0 :150 > n = (10**112); puts n, a = n.irootn(3), a**3
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
21544346900318837217592935665193504952
9999999999999999999999999999999999999173635536969138804543654476122863556031297471619838179917400017798906449408
 => nil 
2.4.0 :151 > n = (10**112); puts n, b = (n**(1.0/3)).to_i, b**3
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
21544346900318734919579678222552399872
9999999999999857552405189045342271937486394088257690663380520853991701500619736949767614488813401623055562702848
 => nil 
2.4.0 :152 > n = -(10**112); puts n, a = n.irootn(3), a**3
-10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
-21544346900318837217592935665193504952
-9999999999999999999999999999999999999173635536969138804543654476122863556031297471619838179917400017798906449408
 => nil 
2.4.0 :153 > n = -(10**112); puts n, b = (n**(1.0/3)).to_i, b**3
RangeError: can't convert 1.077217345015937e+37+1.865795172362055e+37i into Integer
        from (irb):153:in `to_i'
        from (irb):153
        from /home/jzakiya/.rvm/rubies/ruby-2.4.0/bin/irb:11:in `<main>'
2.4.0 :154 > 
```

Again, creating C coded core versions of these methods not only will be more efficient but faster.

----------------------------------------
Feature #13219: bug in Math.sqrt(n).to_i, to compute integer squareroot,  new word to accurately fix it
https://bugs.ruby-lang.org/issues/13219#change-63033

* Author: Jabari Zakiya
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
In doing a math application using **Math.sqrt(n).to_i** to compute integer squareroots 
of integers I started noticing errors for numbers > 10**28.


I coded an algorithm that accurately computes the integer squareroot for arbirary sized numbers
but its significantly slower than the math library written in C.

Thus, I request a new method **Math.intsqrt(n)** be created, that is coded in C and part of the
core library, that will compute the integer squareroots of integers accurately and fast.

Here is working highlevel code to accurately compute the integer squareroot.

```
def intsqrt(n)
  bits_shift = (n.to_s(2).size)/2 + 1
  bitn_mask = root = 1 << bits_shift
  while true
    root ^= bitn_mask if (root * root) > n
    bitn_mask >>= 1
    return root if bitn_mask == 0
    root |= bitn_mask
  end
end

def intsqrt1(n)
  return n if n | 1 == 1   # if n is 0 or 1
  bits_shift = (Math.log2(n).ceil)/2 + 1
  bitn_mask = root = 1 << bits_shift
  while true
    root ^= bitn_mask if (root * root) > n
    bitn_mask >>= 1
    return root if bitn_mask == 0
    root |= bitn_mask
  end
end

require "benchmark/ips"

Benchmark.ips do |x|
  n = 10**40
  puts "integer squareroot tests for n = #{n}"
  x.report("intsqrt(n)"       ) { intsqrt(n)  }
  x.report("intsqrt1(n)"      ) { intsqrt1(n) }
  x.report("Math.sqrt(n).to_i") { Math.sqrt(n).to_i }
  x.compare!
end
```
Here's why it needs to be done in C.

```
2.4.0 :178 > load 'intsqrttest.rb'
integer squareroot tests for n = 10000000000000000000000000000000000000000
Warming up --------------------------------------
          intsqrt(n)     5.318k i/100ms
         intsqrt1(n)     5.445k i/100ms
   Math.sqrt(n).to_i   268.281k i/100ms
Calculating -------------------------------------
          intsqrt(n)     54.219k ( 5.5%) i/s -    271.218k in   5.017552s
         intsqrt1(n)     55.872k ( 5.4%) i/s -    283.140k in   5.082953s
   Math.sqrt(n).to_i      5.278M ( 6.1%) i/s -     26.560M in   5.050707s

Comparison:
   Math.sqrt(n).to_i:  5278477.8 i/s
         intsqrt1(n):    55871.7 i/s - 94.47x  slower
          intsqrt(n):    54219.4 i/s - 97.35x  slower

 => true 
2.4.0 :179 > 

```
Here are examples of math errors using **Math.sqrt(n).to_i** run on Ruby-2.4.0.

```
2.4.0 :101 > n = 10**27; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c   
1000000000000000000000000000
31622776601683
999999999999949826038432489
31622776601683
999999999999949826038432489
31622776601683
999999999999949826038432489
 => nil 
2.4.0 :102 > n = 10**28; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c 
10000000000000000000000000000
100000000000000
10000000000000000000000000000
100000000000000
10000000000000000000000000000
100000000000000
10000000000000000000000000000
 => nil 
2.4.0 :103 > n = 10**29; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c 
100000000000000000000000000000
316227766016837
99999999999999409792567484569
316227766016837
99999999999999409792567484569
316227766016837
99999999999999409792567484569
 => nil 
2.4.0 :104 > n = 10**30; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c  
1000000000000000000000000000000
1000000000000000
1000000000000000000000000000000
1000000000000000
1000000000000000000000000000000
1000000000000000
1000000000000000000000000000000
 => nil 
2.4.0 :105 > n = 10**31; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c 
10000000000000000000000000000000
3162277660168379
9999999999999997900254631487641
3162277660168379
9999999999999997900254631487641
3162277660168379
9999999999999997900254631487641
 => nil 
2.4.0 :106 > n = 10**32; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c 
100000000000000000000000000000000
10000000000000000
100000000000000000000000000000000
10000000000000000
100000000000000000000000000000000
10000000000000000
100000000000000000000000000000000
 => nil 
2.4.0 :107 > n = 10**33; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c
1000000000000000000000000000000000
31622776601683793
999999999999999979762122758866849
31622776601683793
999999999999999979762122758866849
31622776601683792
999999999999999916516569555499264
 => nil 
2.4.0 :108 > n = 10**34; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c
10000000000000000000000000000000000
100000000000000000
10000000000000000000000000000000000
100000000000000000
10000000000000000000000000000000000
100000000000000000
10000000000000000000000000000000000
 => nil 
2.4.0 :109 > n = 10**35; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c
100000000000000000000000000000000000
316227766016837933
99999999999999999873578871987712489
316227766016837933
99999999999999999873578871987712489
316227766016837952
100000000000000011890233980627554304
 => nil 
2.4.0 :110 > n = 10**36; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c
1000000000000000000000000000000000000
1000000000000000000
1000000000000000000000000000000000000
1000000000000000000
1000000000000000000000000000000000000
1000000000000000000
1000000000000000000000000000000000000
 => nil 
2.4.0 :111 > n = 10**37; puts n, (a = intsqrt(n)), a*a, (b = intsqrt1(n)), b*b, (c = Math.sqrt(n).to_i), c*c
10000000000000000000000000000000000000
3162277660168379331
9999999999999999993682442519108007561
3162277660168379331
9999999999999999993682442519108007561
3162277660168379392
10000000000000000379480317059650289664
 => nil 
2.4.0 :112 > 
```



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