feat(bitwise): Function to check if a number is positive (trekhleb#204)

src/algorithms/math/bits/README.md

@@ -51,6 +51,29 @@ isEven: true
 
 > See [isEven.js](isEven.js) for further details.
 
+#### isPositive
+
+This method determines if the number provided is positive.
+It is based on the fact that all positive numbers have their last
+left bit to be set to 0. However, if the number provided is zero
+or negative zero, it should still return false.
+
+```text
+Number: 1 = 0b0001
+isPositive: true
+
+Number: -1 = -0b0001
+isPositive: false
+
+Number: 0 = 0b0000
+isPositive: false
+
+Number: -0 = 0b0000
+isPositive: false
+```
+
+> See [isPositive.js](isPositive.js) for further details.
+
 #### Multiply By Two
 
 This method shifts original number by one bit to the left.

src/algorithms/math/bits/__test__/isPositive.test.js

@@ -0,0 +1,10 @@
+import isPositive from '../isPositive';
+
+describe('isPositive', () => {
+  it('should detect if a number is positive', () => {
+    expect(isPositive(0)).toBe(false);
+    expect(isPositive(-0)).toBe(false);
+    expect(isPositive(1)).toBe(true);
+    expect(isPositive(-1)).toBe(false);
+  });
+});

src/algorithms/math/bits/isPositive.js

@@ -0,0 +1,13 @@
+/**
+ * @param {number} number
+ * @return {boolean}
+ */
+export default function isPositive(number) {
+  // Zero is neither a positive nor a negative number
+  if (number === 0) {
+    return false;
+  }
+
+  // The most signification bit can be used to determine whether .
+  return ((number >> 31) & 1) === 0;
+}

src/algorithms/math/bits/multiply.js

@@ -1,6 +1,7 @@
 import multiplyByTwo from './multiplyByTwo';
 import divideByTwo from './divideByTwo';
 import isEven from './isEven';
+import isPositive from './isPositive';
 
 /**
  * Multiply two signed numbers using bitwise operations.
@@ -34,7 +35,7 @@ export default function multiply(a, b) {
   const multiplyByOddNegative = () => multiply(multiplyByTwo(a), divideByTwo(b + 1)) - a;
 
   const multiplyByEven = () => multiply(multiplyByTwo(a), divideByTwo(b));
-  const multiplyByOdd = () => (b > 0 ? multiplyByOddPositive() : multiplyByOddNegative());
+  const multiplyByOdd = () => (isPositive(b) ? multiplyByOddPositive() : multiplyByOddNegative());
 
   return isEven(b) ? multiplyByEven() : multiplyByOdd();
 }