add mult test from Math::Quaternion, order the code like Wikipedia page

Author: mohawk2

lib/PDL/Graphics/TriD/Quaternion.pm

@@ -48,27 +48,26 @@ sub to_vrmlrot {
 	return [(map {$_/sin($d)} @{$this}[1..3]),2*$d];
 }
 
-# Yuck
 sub multiply {
-	my($this,$with) = @_;
-	return PDL::Graphics::TriD::Quaternion->new(
-		$this->[0] * $with->[0] -
-		$this->[1] * $with->[1] -
-		$this->[2] * $with->[2] -
-		$this->[3] * $with->[3],
-			$this->[2] * $with->[3] -
-			$this->[3] * $with->[2] +
-			$this->[0] * $with->[1] +
-			$this->[1] * $with->[0],
-		$this->[3] * $with->[1] -
-		$this->[1] * $with->[3] +
-		$this->[0] * $with->[2] +
-		$this->[2] * $with->[0],
-			$this->[1] * $with->[2] -
-			$this->[2] * $with->[1] +
-			$this->[0] * $with->[3] +
-			$this->[3] * $with->[0],
-	);
+  my($this,$with) = @_;
+  return PDL::Graphics::TriD::Quaternion->new(
+        $this->[0] * $with->[0]
+      - $this->[1] * $with->[1]
+      - $this->[2] * $with->[2]
+      - $this->[3] * $with->[3],
+            $this->[0] * $with->[1]
+          + $this->[1] * $with->[0]
+          + $this->[2] * $with->[3]
+          - $this->[3] * $with->[2],
+        $this->[0] * $with->[2]
+      - $this->[1] * $with->[3]
+      + $this->[2] * $with->[0]
+      + $this->[3] * $with->[1],
+            $this->[0] * $with->[3]
+          + $this->[1] * $with->[2]
+          - $this->[2] * $with->[1]
+          + $this->[3] * $with->[0],
+  );
 }
 
 sub multiply_scalar {

t/quaternion.t

@@ -18,4 +18,13 @@ is_qua $q, [0,0,0,1];
 
 is_qua +PDL::Graphics::TriD::Quaternion->new(0,0,0,2)->normalise, [0,0,0,1];
 
+my ($q1, $q2) = map PDL::Graphics::TriD::Quaternion->new(@$_), [1,2,3,4], [5,6,7,8];
+
+is_qua $q1->multiply($q2), [
+  5-12-21-32, # $a0*$b0 - $a1*$b1 - $a2*$b2 - $a3*$b3,
+  6+10+24-28, # $a0*$b1 + $b0*$a1 + $a2*$b3 - $a3*$b2,
+  7+15+24-16, # $a0*$b2 + $b0*$a2 + $a3*$b1 - $a1*$b3,
+  8+20+14-18, # $a0*$b3 + $b0*$a3 + $a1*$b2 - $a2*$b1
+];
+
 done_testing;