Use __$...$__ for placeholder.

Author: chriseth

linker.js

@@ -4,6 +4,10 @@ function keccak256 (input) {
   return keccak('keccak256').update(input).digest();
 }
 
+function libraryHashPlaceholder (input) {
+  return '$' + keccak256(input).toString('hex').slice(0, 34) + '$';
+}
+
 var linkBytecode = function (bytecode, libraries) {
   // NOTE: for backwards compatibility support old compiler which didn't use file names
   var librariesComplete = {};
@@ -45,7 +49,7 @@ var linkBytecode = function (bytecode, libraries) {
     };
 
     replace(libraryName);
-    replace(keccak256(libraryName).toString('hex'));
+    replace(libraryHashPlaceholder(libraryName));
   }
 
   return bytecode;

test/linker.js

@@ -141,7 +141,7 @@ tape('Linking', function (t) {
   });
 
   t.test('hashed placeholder', function (st) {
-    var bytecode = '6060604052341561000__cb901161e812ceb78cfe30ca65050c433771__66606060606060';
+    var bytecode = '6060604052341561000__$cb901161e812ceb78cfe30ca65050c4337$__66606060606060';
     bytecode = linker.linkBytecode(bytecode, { 'lib2.sol:L': '0x123456' });
     st.equal(bytecode, '6060604052341561000000000000000000000000000000000000012345666606060606060');
     st.end();