Merge pull request #45 from codisart/add-arccos-method

[TECH] Add the arccosine function

Author: castarco

src/Decimal.php

@@ -781,7 +781,7 @@ public function sec($scale = null)
     /**
      *	Calculates the arcsine of this with the highest possible accuracy
      *
-     * @param integer $scale [description]
+     * @param integer $scale
      * @return Decimal
      */
     public function arcsin($scale = null)
@@ -810,6 +810,44 @@ public function arcsin($scale = null)
             $scale
         );
     }
+
+    /**
+     *	Calculates the arccosine of this with the highest possible accuracy
+     *
+     * @param integer $scale
+     * @return Decimal
+     */
+    public function arccos($scale = null)
+    {
+        if($this->comp(DecimalConstants::one(), $scale + 2) === 1 || $this->comp(DecimalConstants::negativeOne(), $scale + 2) === -1) {
+            throw new \DomainException(
+                "The arccos of this number is undefined."
+            );
+        }
+
+        $piOverTwo = DecimalConstants::pi()->div(Decimal::fromInteger(2), $scale + 2)->round($scale);
+
+        if ($this->round($scale)->isZero()) {
+            return $piOverTwo;
+        }
+        if ($this->round($scale)->equals(DecimalConstants::one())) {
+            return DecimalConstants::zero();
+        }
+        if ($this->round($scale)->equals(DecimalConstants::negativeOne())) {
+            return DecimalConstants::pi()->round($scale);
+        }
+
+        $scale = ($scale === null) ? 32 : $scale;
+
+        return $piOverTwo->sub(
+            self::powerSerie(
+                $this,
+                DecimalConstants::zero(),
+                $scale
+            )
+        )->round($scale);
+    }
+
     /**
      * Returns exp($this), said in other words: e^$this .
      *

tests/Decimal/DecimalArccosTest.php

@@ -0,0 +1,50 @@
+<?php
+
+use Litipk\BigNumbers\Decimal as Decimal;
+
+/**
+ * @group arcsin
+ */
+class DecimalArccosTest extends PHPUnit_Framework_TestCase
+{
+    public function arccosProvider() {
+        // Some values provided by wolframalpha
+        return [
+            ['0.154', '1.41618102663394', 14],
+            ['1', '0', 17],
+            ['-1', '3.14159265358979324', 17],
+        ];
+    }
+
+    /**
+     * @dataProvider arccosProvider
+     */
+    public function testSimple($nr, $answer, $digits)
+    {
+        $x = Decimal::fromString($nr);
+        $arccosX = $x->arccos($digits);
+
+        $this->assertTrue(
+            Decimal::fromString($answer)->equals($arccosX),
+            "The answer must be " . $answer . ", but was " . $arccosX
+        );
+    }
+
+    /**
+     * @expectedException \DomainException
+     * @expectedExceptionMessage The arccos of this number is undefined.
+     */
+    public function testArcosGreaterThanOne()
+    {
+        Decimal::fromString('25.546')->arccos();
+    }
+
+    /**
+     * @expectedException \DomainException
+     * @expectedExceptionMessage The arccos of this number is undefined.
+     */
+    public function testArccosFewerThanNegativeOne()
+    {
+        Decimal::fromString('-304.75')->arccos();
+    }
+}

tests/Decimal/DecimalArcsinTest.php

@@ -8,7 +8,7 @@
 class DecimalArcsinTest extends PHPUnit_Framework_TestCase
 {
     public function arcsinProvider() {
-        // Some values providede by wolframalpha
+        // Some values provided by wolframalpha
         return [
             ['0.154', '0.15461530016096', 14],
             ['1', '1.57079632679489662', 17],