Add numCheckpoints

contracts/governance/utils/Votes.sol

@@ -7,6 +7,7 @@ import "../../utils/Nonces.sol";
 import "../../utils/Checkpoints.sol";
 import "../../utils/cryptography/EIP712.sol";
 import "./IVotes.sol";
+import "../../utils/math/SafeCast.sol";
 
 /**
  * @dev This is a base abstract contract that tracks voting units, which are a measure of voting power that can be
@@ -167,6 +168,20 @@ abstract contract Votes is IVotes, Context, EIP712, Nonces {
         }
     }
 
+    /**
+     * @dev Get number of checkpoints for `account`.
+     */
+    function _numCheckpoints(address account) internal view virtual returns (uint32) {
+        return SafeCast.toUint32(_delegateCheckpoints[account].length());
+    }
+
+    /**
+     * @dev Get the `pos`-th checkpoint for `account`.
+     */
+    function _checkpoints(address account, uint32 pos) internal view virtual returns (Checkpoints.Checkpoint memory) {
+        return _delegateCheckpoints[account].getAtPosition(pos);
+    }
+
     function _add(uint256 a, uint256 b) private pure returns (uint256) {
         return a + b;
     }

contracts/token/ERC20/extensions/ERC20Votes.sol

@@ -44,6 +44,20 @@ abstract contract ERC20Votes is ERC20, Votes {
         super._afterTokenTransfer(from, to, amount);
     }
 
+    /**
+     * @dev Get number of checkpoints for `account`.
+     */
+    function numCheckpoints(address account) public view virtual returns (uint32) {
+        return super._numCheckpoints(account);
+    }
+
+    /**
+     * @dev Get the `pos`-th checkpoint for `account`.
+     */
+    function checkpoints(address account, uint32 pos) public view virtual returns (Checkpoints.Checkpoint memory) {
+        return super._checkpoints(account, pos);
+    }
+
     /**
      * @dev Returns the balance of `account`.
      */

contracts/utils/Checkpoints.sol

@@ -68,6 +68,14 @@ library Checkpoints {
         return pos == 0 ? 0 : _unsafeAccess(self._checkpoints, pos - 1)._value;
     }
 
+    /**
+     * @dev Returns checkpoint at given position.
+     */
+    function getAtPosition(History storage self, uint32 pos) internal view returns (Checkpoint memory) {
+        Checkpoint storage last = _unsafeAccess(self._checkpoints, pos);
+        return last;
+    }
+
     /**
      * @dev Pushes a value onto a History so that it is stored as the checkpoint for the current block.
      *

test/token/ERC20/extensions/ERC20Votes.test.js

@@ -341,6 +341,55 @@ contract('ERC20Votes', function (accounts) {
       await this.token.mint(holder, supply);
     });
 
+    describe('numCheckpoints', function () {
+      it('returns the number of checkpoints for a delegate', async function () {
+        await this.token.transfer(recipient, '100', { from: holder }); //give an account a few tokens for readability
+        expect(await this.token.numCheckpoints(other1)).to.be.bignumber.equal('0');
+
+        const t1 = await this.token.delegate(other1, { from: recipient });
+        expect(await this.token.numCheckpoints(other1)).to.be.bignumber.equal('1');
+
+        const t2 = await this.token.transfer(other2, 10, { from: recipient });
+        expect(await this.token.numCheckpoints(other1)).to.be.bignumber.equal('2');
+
+        const t3 = await this.token.transfer(other2, 10, { from: recipient });
+        expect(await this.token.numCheckpoints(other1)).to.be.bignumber.equal('3');
+
+        const t4 = await this.token.transfer(recipient, 20, { from: holder });
+        expect(await this.token.numCheckpoints(other1)).to.be.bignumber.equal('4');
+
+        expect(await this.token.checkpoints(other1, 0)).to.be.deep.equal([ t1.receipt.blockNumber.toString(), '100' ]);
+        expect(await this.token.checkpoints(other1, 1)).to.be.deep.equal([ t2.receipt.blockNumber.toString(), '90' ]);
+        expect(await this.token.checkpoints(other1, 2)).to.be.deep.equal([ t3.receipt.blockNumber.toString(), '80' ]);
+        expect(await this.token.checkpoints(other1, 3)).to.be.deep.equal([ t4.receipt.blockNumber.toString(), '100' ]);
+
+        await time.advanceBlock();
+        expect(await this.token.getPastVotes(other1, t1.receipt.blockNumber)).to.be.bignumber.equal('100');
+        expect(await this.token.getPastVotes(other1, t2.receipt.blockNumber)).to.be.bignumber.equal('90');
+        expect(await this.token.getPastVotes(other1, t3.receipt.blockNumber)).to.be.bignumber.equal('80');
+        expect(await this.token.getPastVotes(other1, t4.receipt.blockNumber)).to.be.bignumber.equal('100');
+      });
+
+      it('does not add more than one checkpoint in a block', async function () {
+        await this.token.transfer(recipient, '100', { from: holder });
+        expect(await this.token.numCheckpoints(other1)).to.be.bignumber.equal('0');
+
+        const [ t1, t2, t3 ] = await batchInBlock([
+          () => this.token.delegate(other1, { from: recipient, gas: 100000 }),
+          () => this.token.transfer(other2, 10, { from: recipient, gas: 100000 }),
+          () => this.token.transfer(other2, 10, { from: recipient, gas: 100000 }),
+        ]);
+        expect(await this.token.numCheckpoints(other1)).to.be.bignumber.equal('1');
+        expect(await this.token.checkpoints(other1, 0)).to.be.deep.equal([ t1.receipt.blockNumber.toString(), '80' ]);
+        // expectReve(await this.token.checkpoints(other1, 1)).to.be.deep.equal([ '0', '0' ]); // Reverts due to array overflow check
+        // expect(await this.token.checkpoints(other1, 2)).to.be.deep.equal([ '0', '0' ]); // Reverts due to array overflow check
+
+        const t4 = await this.token.transfer(recipient, 20, { from: holder });
+        expect(await this.token.numCheckpoints(other1)).to.be.bignumber.equal('2');
+        expect(await this.token.checkpoints(other1, 1)).to.be.deep.equal([ t4.receipt.blockNumber.toString(), '100' ]);
+      });
+    });
+
     describe('balanceOf', function () {
       it('grants to initial account', async function () {
         expect(await this.token.balanceOf(holder)).to.be.bignumber.equal('10000000000000000000000000');