Added sqrt function and tests for it. Also added two() function to return a BigInteger of value 2.

src/BigDecimal.php

@@ -112,6 +112,22 @@ final class BigDecimal extends BigNumber implements \Serializable
         return $one;
     }
 
+    /**
+     * Returns a BigDecimal representing two, with a scale of zero.
+     *
+     * @return BigDecimal
+     */
+    public static function two()
+    {
+        static $two = null;
+
+        if ($two === null) {
+            $two = new BigDecimal('2');
+        }
+
+        return $two;
+    }
+
     /**
      * Returns a BigDecimal representing ten, with a scale of zero.
      *
@@ -413,6 +429,32 @@ final class BigDecimal extends BigNumber implements \Serializable
 
         return [$quotient, $remainder];
     }
+    /**
+     * Returns the square root of this number.
+     *
+     * The quotient has a scale of `0`, and the remainder has a scale of `0`.
+     *
+     * @return BigDecimal With the square root of this number.
+     *
+     * @throws IsNegativeException If this number is negative.
+     */
+    public function sqrt()
+    {
+        if ($this->isNegative()) {
+            throw IsNegativeException::isNegative($this);
+        }
+        $two = BigDecimal::two();
+        $guess = $this->quotient($two);
+        while (true) {
+            $last = $guess;
+            $guess = $this->quotient($guess)->add($guess)->quotient($two);
+            // (($n / $guess) + $guess) / 2;
+            if($last->compareTo($guess) == 0) {
+                break;
+            }
+        }
+        return $guess;
+    }
 
     /**
      * Returns a BigDecimal with the current value and the specified scale.

src/Exception/IsNegativeException.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace Brick\Math\Exception;
+
+use Brick\Math\BigInteger;
+
+/**
+ * Exception thrown when a is negative, but should be positive..
+ */
+class IsNegativeException extends \RuntimeException
+{
+    /**
+     * @param BigInteger $value
+     *
+     * @return IsNegativeException
+     */
+    public static function isNegative(BigInteger $value)
+    {
+        $message = '%s is negative, but should be positive.';
+
+        return new self(sprintf($message, (string) $value));
+    }
+}

tests/BigDecimalTest.php

@@ -1360,6 +1360,8 @@ class BigDecimalTest extends AbstractTestCase
         BigDecimal::of(1.2)->quotient(0);
     }
     
+    
+
     /**
      * @expectedException \Brick\Math\Exception\DivisionByZeroException
      */
@@ -1368,12 +1370,9 @@ class BigDecimalTest extends AbstractTestCase
         BigDecimal::of(1.2)->remainder(0);
     }
 
-    /**
-     * @expectedException \Brick\Math\Exception\DivisionByZeroException
-     */
-    public function testQuotientAndRemainderOfZeroThrowsException()
+    public function testsqrt()
     {
-        BigDecimal::of(1.2)->quotientAndRemainder(0);
+        $this->assertSame(BigDecimal::of(625)->sqrt(), BigDecimal::of(25));
     }
 
     /**