Support negative values

Author: chrisullyott

README.md

@@ -60,6 +60,12 @@ $size->add('2G')
 echo $size->asAuto(); // '10.00 GB'
 ```
 
+Negative values are supported. In the case below, 1.2 megabytes are subtracted:
+
+```php
+$size->add('-1.2mb');
+```
+
 You may also use `add()` and `subtract()` with an array of values:
 
 ```php

src/FileSize.php

@@ -107,20 +107,14 @@ private function addSize($size)
     }
 
     /**
-     * Subtract from this filesize, stopping at 0 bytes.
+     * Subtract from this filesize.
      *
      * @param string|int $size Such as '100 MB'
      * @return self
      */
     private function subtractSize($size)
     {
-        $bytesToSubtract = $this->sizeToBytes($size);
-
-        if ($bytesToSubtract <= $this->bytes) {
-            $this->bytes -= $bytesToSubtract;
-        } else {
-            $this->bytes = 0;
-        }
+        $this->bytes -= $this->sizeToBytes($size);
 
         return $this;
     }

src/FileSize/Parser/SizeStringParser.php

@@ -18,7 +18,7 @@ class SizeStringParser
      * - 123 MB
      * - 1 gigabytes
      */
-    const SIZE_STRING_PATTERN = '/^([0-9\.]+)\s*?([A-Za-z]+)?$/';
+    const SIZE_STRING_PATTERN = '/^([0-9\.-]+)\s*?([A-Za-z]+)?$/';
 
     /**
      * Parse a size string into its parts (value, unit).

tests/FileSizeTest.php

@@ -41,6 +41,17 @@ public function testAdd()
         $this->assertSame($size->as('B'), 129128448);
     }
 
+    /**
+     * Test #add with a negative value.
+     */
+    public function testAddNegative()
+    {
+        $size = new FileSize('10MB');
+        $size->add('-20MB');
+
+        $this->assertSame($size->as('MB'), -10.0);
+    }
+
     /**
      * Test #subtract.
      */
@@ -52,6 +63,17 @@ public function testSubtract()
         $this->assertSame($size->as('B'), 128821248);
     }
 
+    /**
+     * Test #subtract with a negative value.
+     */
+    public function testSubtractNegative()
+    {
+        $size = new FileSize('10MB');
+        $size->subtract('-20MB');
+
+        $this->assertSame($size->as('MB'), 30.0);
+    }
+
     /**
      * Test adding an array of items.
      */