Fix issue with encoding large negative numbers

Author: spacesailor24

packages/web3-eth-abi/src/index.js

@@ -268,8 +268,13 @@ ABICoder.prototype.formatParam = function (type, param) {
     if (match) {
         let size = parseInt(match[2] || "256");
         if (size / 8 < param.length) {
-            // pad to correct bit width
-            param = utils.leftPad(param, size);
+            if (param.startsWith('-')) {
+                // pad to correct bit width with - at the beginning
+                param = `-${utils.leftPad(param.substring(1), size)}`;
+            } else {
+                // pad to correct bit width
+                param = utils.leftPad(param, size);
+            }
         }
     }
 

test/eth.abi.encodeParameter.js

@@ -11,6 +11,9 @@ var tests = [{
 },{
     params: ['bytes32[]', ['0xdf3234', '0xfdfd']],
     result: '0x00000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000002df32340000000000000000000000000000000000000000000000000000000000fdfd000000000000000000000000000000000000000000000000000000000000'
+},{
+    params: ['int128', '-170141183460469231731687303715884105727'],
+    result: '0xffffffffffffffffffffffffffffffff80000000000000000000000000000001'
 }];
 
 describe('encodeParameter', function () {