feature: add mat3Determinant

src/math/mat3/index.ts

@@ -1,3 +1,4 @@
+export { mat3Determinant } from './mat3Determinant';
 export { mat3FromMat4 } from './mat3FromMat4';
 export { mat3FromMat4Transpose } from './mat3FromMat4Transpose';
 export { mat3FromQuaternion } from './mat3FromQuaternion';

src/math/mat3/mat3Determinant.ts

@@ -0,0 +1,13 @@
+import type { RawMatrix3 } from './RawMatrix3';
+
+export function mat3Determinant( m: RawMatrix3 ): number {
+  const
+    n11 = m[ 0 ], n21 = m[ 1 ], n31 = m[ 2 ],
+    n12 = m[ 3 ], n22 = m[ 4 ], n32 = m[ 5 ],
+    n13 = m[ 6 ], n23 = m[ 7 ], n33 = m[ 8 ],
+    t11 = n33 * n22 - n32 * n23,
+    t12 = n32 * n13 - n33 * n12,
+    t13 = n23 * n12 - n22 * n13;
+
+  return n11 * t11 + n21 * t12 + n31 * t13;
+}

src/math/mat3/tests/mat3Determinant.test.ts

@@ -0,0 +1,17 @@
+import '../../../tests/matchers/toBeCloseToArray';
+import { mat3Determinant } from '../mat3Determinant';
+import type { RawMatrix3 } from '../RawMatrix3';
+
+describe( 'mat3Determinant', () => {
+  it( 'returns a determinant of the matrix', () => {
+    const rawMatrixLookAtFrom345: RawMatrix3 = [
+      0.857, 0.000, -0.514,
+      -0.291, 0.825, -0.485,
+      0.424, 0.566, 0.707,
+    ];
+
+    const subject = mat3Determinant( rawMatrixLookAtFrom345 );
+
+    expect( subject ).toBeCloseTo( 1.0 );
+  } );
+} );