#!/usr/bin/perl
use strict;
use warnings;

sub middle ($$) {
  my ( $a, $b ) = ( shift, shift );
  return ($a + $b) / 2
}

sub square ($) {
  my $x = shift;
  return $x**2;
}

sub abs ($) {
  my $n = shift;
  return $n < 0 ? -$n : $n;
}

sub sqrt_search{
  my ( $x, $base, $top, $guess, $counter ) = ( shift, shift, shift, shift, shift );
  print "sqrt-search( $x, $base, $top, $guess )\n";
  return $guess if( abs($x - square $guess) < 0.0001);
  return undef if $counter == 0;
  return sqrt_search( $x, $guess, $top, middle( $guess, $top ), --$counter)
    if( ($x > 0) and (square $guess < $x) );
  return sqrt_search( $x, $base, $guess, middle( $base, $guess ), --$counter)
    if( ($x > 0) and (square $guess > $x) );
  return undef;
}

sub sqrt ($) {
  my $x = shift;
  return sqrt_search( $x, 0, $x, middle(0, $x), 50);
}

# Tests
print "SQRT 16:\n",  &sqrt( 16 ), $/, $/;
print "SQRT 4:\n",   &sqrt( 4  ), $/, $/;
print "SQRT 2:\n",   &sqrt( 2  ), $/, $/;
print "SQRT 0.5:\n", &sqrt( .5 ), $/, $/;
print "SQRT -1:\n",  &sqrt( -1 ), $/, $/;
print "SQRT 0:\n",   &sqrt( 0  ), $/, $/;