feat: inverse()

src/index.js

@@ -166,6 +166,44 @@ class Matrix extends Map {
   determinant () {
     return (this.get('a') * this.get('d')) - (this.get('b') * this.get('c'))
   }
+
+  /**
+   * @return {boolean} true if matrix is invertible
+   */
+  isInvertible () {
+    return this.determinant() !== 0
+  }
+
+  /**
+   * @return {Matrix} inverse of the current matrix.
+   * @throws Will throw an error if the matrix is not invertable
+   */
+  inverse () {
+    if (this.isIdentity()) {
+      return new Matrix()
+    }
+
+    if (!this.isInvertible()) {
+      throw new Error('Matrix is not invertible.')
+    }
+
+    const dt = this.determinant()
+    const a = this.get('a')
+    const b = this.get('b')
+    const c = this.get('c')
+    const d = this.get('d')
+    const e = this.get('e')
+    const f = this.get('f')
+
+    return this.withMutations(matrix => matrix
+      .set('a', d / dt)
+      .set('b', -b / dt)
+      .set('c', -c / dt)
+      .set('d', a / dt)
+      .set('e', ((c * f) - (d * e)) / dt)
+      .set('f', -((a * f) - (b * e)) / dt)
+    )
+  }
 }
 
 export default Matrix

src/index.spec.js

@@ -121,3 +121,19 @@ describe('determinant', () => {
   expect(new Matrix().determinant()).toBe(1)
   expect(new Matrix(1, 1, 1, 1, 0, 0).determinant()).toBe(0)
 })
+
+describe('isInvertable', () => {
+  expect(mockMatrix.isInvertible()).toBe(true)
+  expect(new Matrix().isInvertible()).toBe(true)
+  expect(new Matrix(1, 1, 1, 1, 0, 0).isInvertible()).toBe(false)
+})
+
+describe('inverse', () => {
+  const mockMatrixInverted = new Matrix(-2, 1, 1.5, -0.5, 1, -2)
+
+  expect(mockMatrix.inverse().equals(mockMatrixInverted)).toBe(true)
+  expect(new Matrix().inverse().equals(new Matrix())).toBe(true)
+  expect(() => {
+    new Matrix(1, 1, 1, 1, 0, 0).inverse()
+  }).toThrow()
+})