From f106344504fcb6b2448b572b83613cf4b7e48f54 Mon Sep 17 00:00:00 2001
From: Jon Janzen <jonjanzen@me.com>
Date: Mon, 29 Mar 2021 19:18:21 -0600
Subject: [PATCH] identity matrix

---
 matrix/src/lib.rs | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/matrix/src/lib.rs b/matrix/src/lib.rs
index a559e63..ff99554 100644
--- a/matrix/src/lib.rs
+++ b/matrix/src/lib.rs
@@ -46,6 +46,15 @@ impl Matrix {
     pub fn new_empty_4() -> Matrix {
         Matrix::new_empty(4, 4)
     }
+
+    pub fn identity() -> Matrix {
+        Matrix::new(vec![
+            vec![1.0, 0.0, 0.0, 0.0],
+            vec![0.0, 1.0, 0.0, 0.0],
+            vec![0.0, 0.0, 1.0, 0.0],
+            vec![0.0, 0.0, 0.0, 1.0],
+        ])
+    }
 }
 
 impl Index<usize> for Matrix {
@@ -277,4 +286,31 @@ mod tests {
 
         assert_eq!(matrix * tuple, expected);
     }
+
+    #[test]
+    fn matrix_by_identity() {
+        let matrix = Matrix::from_array([
+            [0.0, 1.0, 2.0, 4.0,],
+            [1.0, 2.0, 4.0, 8.0,],
+            [2.0, 4.0, 8.0, 16.0],
+            [4.0, 8.0, 16.0, 32.0,]
+        ]);
+
+        let expected = Matrix::from_array([
+            [0.0, 1.0, 2.0, 4.0,],
+            [1.0, 2.0, 4.0, 8.0,],
+            [2.0, 4.0, 8.0, 16.0],
+            [4.0, 8.0, 16.0, 32.0,]
+        ]);
+
+        assert_eq!(matrix * Matrix::identity(), expected);
+    }
+
+    #[test]
+    fn tuple_by_identity() {
+        let t = Tuple::new(1.0, 2.0, 3.0, 4.0);
+        let expected = Tuple::new(1.0, 2.0, 3.0, 4.0);
+
+        assert_eq!(Matrix::identity() * t, expected);
+    }
 }