Add generateSalt and tests for it

package.json

@@ -36,6 +36,7 @@
     "@types/mocha": "^2.2.41",
     "@types/node": "^7.0.22",
     "awesome-typescript-loader": "^3.1.3",
+    "bignumber.js": "^4.0.2",
     "chai": "^3.5.0",
     "mocha": "^3.4.1",
     "npm-run-all": "^4.0.2",

src/ts/0x.js.ts

@@ -11,6 +11,8 @@ export interface ECSignature {
     s: string;
 }
 
+const MAX_DIGITS_IN_UNSIGNED_256_INT = 78;
+
 export class ZeroEx {
     /**
      * Verifies that the elliptic curve signature `signature` was generated
@@ -34,4 +36,13 @@ export class ZeroEx {
             return false;
         }
     }
+    /** Generates pseudo-random 256 bits salt */
+    public static generatePseudoRandomSalt(): BigNumber.BigNumber {
+        // BigNumber.random returns a pseudo-random number between 0 & 1 with a passed in number of decimal places.
+        // Source: https://mikemcl.github.io/bignumber.js/#random
+        const randomNumber = BigNumber.random(MAX_DIGITS_IN_UNSIGNED_256_INT);
+        const factor = new BigNumber(10).pow(MAX_DIGITS_IN_UNSIGNED_256_INT - 1);
+        const salt = randomNumber.times(factor).round();
+        return salt;
+    }
 }

test/0x.js.ts

@@ -1,6 +1,7 @@
 import {ZeroEx} from '../src/ts/0x.js';
 import {expect} from 'chai';
 import 'mocha';
+import * as BigNumber from 'bignumber.js';
 
 describe('ZeroEx library', () => {
     describe('#isValidSignature', () => {
@@ -73,4 +74,16 @@ describe('ZeroEx library', () => {
             expect(isValid).to.be.true;
         });
     });
+    describe('#generateSalt', () => {
+        it('generates different salts', () => {
+            const equal = ZeroEx.generatePseudoRandomSalt().eq(ZeroEx.generatePseudoRandomSalt());
+            expect(equal).to.be.false;
+        });
+        it('generates salt in range [0..2^256)', () => {
+            const salt = ZeroEx.generatePseudoRandomSalt();
+            expect(salt.greaterThanOrEqualTo(0)).to.be.true;
+            const twoPow256 = new BigNumber(2).pow(256);
+            expect(salt.lessThan(twoPow256)).to.be.true;
+        });
+    });
 });