Refactor snapshot arrays into a struct.

contracts/drafts/ERC20Snapshot.sol

@@ -18,14 +18,15 @@ contract ERC20Snapshot is ERC20 {
     // Snapshots store a value at the time a snapshot is taken (and a new snapshot id created), and the corresponding
     // snapshot id. Each account has individual snapshots taken on demand, as does the token's total supply.
 
-    // These two fields (value and id) belong together, but are not part of a struct so that functions that work on
-    // arrays can be called on them.
-
-    mapping (address => uint256[]) private _accountSnapshotIds;
-    mapping (address => uint256[]) private _accountSnapshotValues;
+    // Snapshoted values have arrays of ids and the value corresponding to that id. These could be an array of a
+    // Snapshot struct, but that would impede usage of functions that work on an array.
+    struct Snapshots {
+        uint256[] ids;
+        uint256[] values;
+    }
 
-    uint256[] private _totalSupplySnapshotIds;
-    uint256[] private _totalSupplySnapshotValues;
+    mapping (address => Snapshots) private _accountBalanceSnapshots;
+    Snapshots private _totalSupplySnaphots;
 
     // Snapshot ids increase monotonically, with the first value being 1. An id of 0 is invalid.
     Counters.Counter private _currentSnapshotId;
@@ -44,14 +45,13 @@ contract ERC20Snapshot is ERC20 {
     }
 
     function balanceOfAt(address account, uint256 snapshotId) public view returns (uint256) {
-        (bool snapshotted, uint256 value) =
-            _valueAt(snapshotId, _accountSnapshotValues[account], _accountSnapshotIds[account]);
+        (bool snapshotted, uint256 value) = _valueAt(snapshotId, _accountBalanceSnapshots[account]);
 
         return snapshotted ? value : balanceOf(account);
     }
 
     function totalSupplyAt(uint256 snapshotId) public view returns(uint256) {
-        (bool snapshotted, uint256 value) = _valueAt(snapshotId, _totalSupplySnapshotValues, _totalSupplySnapshotIds);
+        (bool snapshotted, uint256 value) = _valueAt(snapshotId, _totalSupplySnaphots);
 
         return snapshotted ? value : totalSupply();
     }
@@ -93,34 +93,34 @@ contract ERC20Snapshot is ERC20 {
     // In summary, we need to find an element in an array, returning the index of the smallest value that is larger if
     // it is not found, unless said value doesn't exist (e.g. when all values are smaller). Arrays.findUpperBound does
     // exactly this.
-    function _valueAt(uint256 snapshotId, uint256[] storage values, uint256[] storage ids)
+    function _valueAt(uint256 snapshotId, Snapshots storage snapshots)
         private view returns (bool, uint256)
     {
         require(snapshotId > 0);
         require(snapshotId <= _currentSnapshotId.current());
 
-        uint256 index = ids.findUpperBound(snapshotId);
+        uint256 index = snapshots.ids.findUpperBound(snapshotId);
 
-        if (index == ids.length) {
+        if (index == snapshots.ids.length) {
             return (false, 0);
         } else {
-            return (true, values[index]);
+            return (true, snapshots.values[index]);
         }
     }
 
     function _updateAccountSnapshot(address account) private {
-        _updateSnapshot(_accountSnapshotValues[account], _accountSnapshotIds[account], balanceOf(account));
+        _updateSnapshot(_accountBalanceSnapshots[account], balanceOf(account));
     }
 
     function _updateTotalSupplySnapshot() private {
-        _updateSnapshot(_totalSupplySnapshotValues, _totalSupplySnapshotIds, totalSupply());
+        _updateSnapshot(_totalSupplySnaphots, totalSupply());
     }
 
-    function _updateSnapshot(uint256[] storage values, uint256[] storage ids, uint256 currentValue) private {
+    function _updateSnapshot(Snapshots storage snapshots, uint256 currentValue) private {
         uint256 currentId = _currentSnapshotId.current();
-        if (_lastSnapshotId(ids) < currentId) {
-            ids.push(currentId);
-            values.push(currentValue);
+        if (_lastSnapshotId(snapshots.ids) < currentId) {
+            snapshots.ids.push(currentId);
+            snapshots.values.push(currentValue);
         }
     }