feat: check if is a banded matrix

src/__tests__/test.js

@@ -37,7 +37,33 @@ describe('Sparse Matrix', () => {
     });
 
     it('transpose', () => {
-        const matrix1 = new SparseMatrix([[1, 2], [3, 4]]);
-        expect(matrix1.transpose().to2DArray()).toEqual([[1, 3], [2, 4]]);
+        const matrix = new SparseMatrix([[1, 2], [3, 4]]);
+        expect(matrix.transpose().to2DArray()).toEqual([[1, 3], [2, 4]]);
+    });
+});
+
+describe('Banded matrices', () => {
+    it('Check band size', () => {
+        const matrix1 = new SparseMatrix([[1, 0], [0, 1]]);
+        const matrix2 = new SparseMatrix([[1, 0, 0], [0, 1, 0]]);
+        const matrix3 = new SparseMatrix([[1, 0, 1], [0, 1, 0]]);
+        const matrix4 = new SparseMatrix([[1, 0, 0], [1, 1, 0]]);
+        const matrix5 = new SparseMatrix([[0, 0, 0], [1, 0, 0], [0, 1, 0]]);
+        expect(matrix1.bandWidth()).toBe(0);
+        expect(matrix2.bandWidth()).toBe(0);
+        expect(matrix3.bandWidth()).toBe(2);
+        expect(matrix4.bandWidth()).toBe(1);
+        expect(matrix5.bandWidth()).toBe(0);
+    });
+
+    it('isBanded', () => {
+        const matrix1 = new SparseMatrix([[1, 0], [0, 1]]);
+        const matrix2 = new SparseMatrix([[1, 0, 0], [0, 1, 0]]);
+        const matrix3 = new SparseMatrix([[1, 0, 1], [0, 1, 0]]);
+        const matrix4 = new SparseMatrix([[1, 0, 0], [1, 1, 0]]);
+        expect(matrix1.isBanded(1)).toBe(true);
+        expect(matrix2.isBanded(1)).toBe(true);
+        expect(matrix3.isBanded(1)).toBe(false);
+        expect(matrix4.isBanded(1)).toBe(true);
     });
 });

src/index.js

@@ -77,6 +77,32 @@ export class SparseMatrix {
         return symmetric;
     }
 
+    /**
+     * Search for the wither band in the main diagonals
+     * @return {number}
+     */
+    bandWidth() {
+        let min = this.columns;
+        let max = -1;
+        this.forEachNonZero((i, j, v) => {
+            let diff = i - j;
+            min = Math.min(min, diff);
+            max = Math.max(max, diff);
+            return v;
+        });
+        return max - min;
+    }
+
+    /**
+     * Test if a matrix is consider banded using a threshold
+     * @param {number} width
+     * @return {boolean}
+     */
+    isBanded(width) {
+        let bandWidth = this.bandWidth();
+        return bandWidth <= width;
+    }
+
     get cardinality() {
         return this.elements.size;
     }