Math.average

contracts/math/Math.sol

@@ -13,4 +13,12 @@ library Math {
   function min(uint256 _a, uint256 _b) internal pure returns (uint256) {
     return _a < _b ? _a : _b;
   }
+
+  function average(uint256 _a, uint256 _b) internal pure returns (uint256) {
+    // (_a + _b) / 2 can overflow, so we distribute
+    uint256 c = (_a / 2) + (_b / 2) + ((_a % 2 + _b % 2) / 2);
+    assert((_a <= c && c <= _b) || (_a >= c && c >= _b));
+
+    return c;
+  }
 }

contracts/mocks/MathMock.sol

@@ -12,4 +12,8 @@ contract MathMock {
   function min(uint256 _a, uint256 _b) public pure returns (uint256) {
     return Math.min(_a, _b);
   }
+
+  function average(uint256 _a, uint256 _b) public pure returns (uint256) {
+    return Math.average(_a, _b);
+  }
 }

test/library/Math.test.js

@@ -1,5 +1,11 @@
 const MathMock = artifacts.require('MathMock');
 
+const BigNumber = web3.BigNumber;
+
+require('chai')
+  .use(require('chai-bignumber')(BigNumber))
+  .should();
+
 contract('Math', function () {
   const min = 1234;
   const max = 5678;
@@ -31,4 +37,28 @@ contract('Math', function () {
       assert.equal(result, min);
     });
   });
+
+  describe('average', function () {
+    function bnAverage (a, b) {
+      return a.plus(b).div(2).truncated();
+    }
+
+    it('is correctly calculated with two odd numbers', async function () {
+      const a = new BigNumber(57417);
+      const b = new BigNumber(95431);
+      (await this.math.average(a, b)).should.be.bignumber.equal(bnAverage(a, b));
+    });
+
+    it('is correctly calculated with two even numbers', async function () {
+      const a = new BigNumber(42304);
+      const b = new BigNumber(84346);
+      (await this.math.average(a, b)).should.be.bignumber.equal(bnAverage(a, b));
+    });
+
+    it('is correctly calculated with one even and one odd number', async function () {
+      const a = new BigNumber(57417);
+      const b = new BigNumber(84346);
+      (await this.math.average(a, b)).should.be.bignumber.equal(bnAverage(a, b));
+    });
+  });
 });