Add more bit manipulation functions.

trekhleb · trekhleb · commit 933848b2155a · 2018-06-27T18:05:18.000+03:00
diff --git a/README.md b/README.md
@@ -49,7 +49,7 @@ a set of rules that precisely define a sequence of operations.
 ### Algorithms by Topic
 
 * **Math**
-  * `B` [Bit Manipulation](src/algorithms/math/bits) - set/get/update/clear bits, multiplication/division by two etc.
+  * `B` [Bit Manipulation](src/algorithms/math/bits) - set/get/update/clear bits, multiplication/division by two, make negative etc.
   * `B` [Factorial](src/algorithms/math/factorial) 
   * `B` [Fibonacci Number](src/algorithms/math/fibonacci)
   * `B` [Primality Test](src/algorithms/math/primality-test) (trial division method)
diff --git a/src/algorithms/math/bits/README.md b/src/algorithms/math/bits/README.md
@@ -53,6 +53,8 @@ Number: 0b1010 = 10
 Powers of two: 2^3 + 0 + 2^1 + 0 
 ```
 
+> See `multiplyByTwo` function for further details.
+
 #### Divide By Two
 
 This method shifts original number by one bit to the right.
@@ -69,3 +71,29 @@ After the shift
 Number: 0b0010 = 2
 Powers of two: 0 + 0 + 2^1 + 0 
 ```
+
+> See `divideByTwo` function for further details.
+
+#### Switch Sign
+
+This method make positive numbers to be negative and backwards.
+To do so it uses "Twos Complement" approach which does it by
+inverting all of the bits of the number and adding 1 to it.
+
+```
+1101 -3
+1110 -2
+1111 -1
+0000  0
+0001  1
+0010  2
+0011  3
+``` 
+
+> See `switchSign` function for further details.
+
+## References
+
+- [Bit Manipulation on YouTube](https://www.youtube.com/watch?v=NLKQEOgBAnw&t=0s&index=28&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8)
+- [Negative Numbers in binary on YouTube](https://www.youtube.com/watch?v=4qH4unVtJkE&t=0s&index=30&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8)
+- [Bit Hacks on stanford.edu](https://graphics.stanford.edu/~seander/bithacks.html)
diff --git a/src/algorithms/math/bits/__test__/switchSign.test.js b/src/algorithms/math/bits/__test__/switchSign.test.js
@@ -0,0 +1,13 @@
+import switchSign from '../switchSign';
+
+describe('switchSign', () => {
+  it('should switch the sign of the number using twos complement approach', () => {
+    expect(switchSign(0)).toBe(0);
+    expect(switchSign(1)).toBe(-1);
+    expect(switchSign(-1)).toBe(1);
+    expect(switchSign(32)).toBe(-32);
+    expect(switchSign(-32)).toBe(32);
+    expect(switchSign(23)).toBe(-23);
+    expect(switchSign(-23)).toBe(23);
+  });
+});
diff --git a/src/algorithms/math/bits/switchSign.js b/src/algorithms/math/bits/switchSign.js
@@ -0,0 +1,8 @@
+/**
+ * Switch the sign of the number using "Twos Complement" approach.
+ * @param {number} number
+ * @return {number}
+ */
+export default function switchSign(number) {
+  return ~number + 1;
+}
