Add negative function.

.gitignore

@@ -14,3 +14,6 @@
 # Node.js
 /node_modules/
 /package-lock.json
+
+# PHPUnit
+/.phpunit.result.cache

src/Number.php

@@ -195,6 +195,16 @@ public function is_zero() {
 		return 0 === $calculator->compare( $this, new self( '0' ) );
 	}
 
+	/**
+	 * Negative.
+	 *
+	 * @link https://github.com/pronamic/wp-number/issues/1
+	 * @return Number
+	 */
+	public function negative() {
+		return Number::from_int( 0 )->subtract( $this );
+	}
+
 	/**
 	 * JSON serialize.
 	 *

tests/src/NumberTest.php

@@ -417,6 +417,20 @@ public function test_to_string() {
 		$this->assertSame( '123456.78', \strval( $number ) );
 	}
 
+	/**
+	 * Test negative.
+	 * 
+	 * @link https://github.com/pronamic/wp-number/issues/1
+	 */
+	public function test_negative() {
+		$number = new Number( 29.95 );
+
+		$negative = $number->negative();
+
+		$this->assertSame( '29.95', \strval( $number ) );
+		$this->assertSame( '-29.95', \strval( $negative ) );
+	}
+
 	/**
 	 * Test calculator exception.
 	 */
@@ -448,7 +462,6 @@ public function test_calculator() {
 		$calculators_property->setValue( $calculators );
 	}
 
-
 	/**
 	 * Test format i18n.
 	 *