From 5c28af3a53cf6006bba2ba40151f5ff41447fcbe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Crozet?= <sebastien@crozet.re>
Date: Fri, 2 Feb 2018 12:26:31 +0100
Subject: [PATCH] Add 3d cross-product matrix construction from a vector.

---
 CHANGELOG.md       |  3 +++
 src/core/matrix.rs | 15 +++++++++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 37a276ee..0c8fb210 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   * `UnitQuaternion::scaled_rotation_between_axis` and
     `UnitQuaternion::rotation_between_axis` that take Unit vectors instead of
     Vector as arguments.
+  * `.cross_matrix()` returns the cross-product matrix of a given vector, i.e.,
+    the matrix `M` such that for all vector `v` we have `M * v ==
+    self.cross(&v)`.
 
 
 
diff --git a/src/core/matrix.rs b/src/core/matrix.rs
index 34f6b2db..55ed6d06 100644
--- a/src/core/matrix.rs
+++ b/src/core/matrix.rs
@@ -16,8 +16,8 @@ use abomonation::Abomonation;
 
 use alga::general::{Ring, Real};
 
-use core::{Scalar, DefaultAllocator, Unit, VectorN, MatrixMN};
-use core::dimension::{Dim, DimAdd, DimSum, U1, U2};
+use core::{Scalar, DefaultAllocator, Unit, VectorN, MatrixN, MatrixMN};
+use core::dimension::{Dim, DimAdd, DimSum, U1, U2, U3};
 use core::constraint::{ShapeConstraint, SameNumberOfRows, SameNumberOfColumns, DimEq};
 use core::iter::{MatrixIter, MatrixIterMut};
 use core::allocator::{Allocator, SameShapeAllocator, SameShapeR, SameShapeC};
@@ -895,6 +895,17 @@ impl<N: Scalar + Ring, R: Dim, C: Dim, S: Storage<N, R, C>> Matrix<N, R, C, S> {
     }
 }
 
+impl<N: Real, S: Storage<N, U3>> Vector<N, U3, S>
+    where DefaultAllocator: Allocator<N, U3> {
+    /// Computes the matrix `M` such that for all vector `v` we have `M * v == self.cross(&v)`.
+    #[inline]
+    pub fn cross_matrix(&self) -> MatrixN<N, U3> {
+        MatrixN::<N, U3>::new(N::zero(), -self[2],   self[1],
+                              self[2],    N::zero(), -self[0],
+                              -self[1],   self[0],    N::zero())
+    }
+}
+
 impl<N: Real, R: Dim, C: Dim, S: Storage<N, R, C>> Matrix<N, R, C, S> {
     /// The smallest angle between two vectors.
     #[inline]