Pos localization

packages/contracts-bedrock/semver-lock.json

@@ -100,8 +100,8 @@
     "sourceCodeHash": "0xa995b54dce03ddf5c9c47451bd7181996b91398ad66b54ab0b8cbf582863a33e"
   },
   "src/dispute/OutputBisectionGame.sol": {
-    "initCodeHash": "0x4db1ded83f8584c983bc6f5d7dfcde9281ba895c793d2fd9f30571c17d93a5fd",
-    "sourceCodeHash": "0x6bd528d5824fe6d093141c07f5590a3c434dbb67f56ab8fa96d7b0a23dac38af"
+    "initCodeHash": "0x6efe83410be6fd58eb07a5297492c6a45598d1f0f84d4ec286d93beade28f40f",
+    "sourceCodeHash": "0x1cc70ebc403581213a4853e1ef1579abeb63496d0625424fa5b9ac8351b2eeca"
   },
   "src/legacy/DeployerWhitelist.sol": {
     "initCodeHash": "0x8de80fb23b26dd9d849f6328e56ea7c173cd9e9ce1f05c9beea559d1720deb3d",

packages/contracts-bedrock/src/dispute/OutputBisectionGame.sol

@@ -147,9 +147,11 @@ contract OutputBisectionGame is IOutputBisectionGame, Clone, ISemver {
             // prestate.
             // If the step is an attack at a trace index > 0, the prestate exists elsewhere in
             // the game state.
-            // WARN: This will not work, except for in the first execution trace bisection game!
-            //       We need to replace `0` with the correct starting index for the given sub-game.
-            preStateClaim = stepPos.indexAtDepth() == 0
+            // NOTE: We localize the `indexAtDepth` for the current execution trace subgame by finding
+            //       the remainder of the index at depth divided by 2 ** (MAX_GAME_DEPTH - SPLIT_DEPTH),
+            //       which is the number of leaves in each execution trace subgame. This is so that we can
+            //       determine whether or not the step position is represents the `ABSOLUTE_PRESTATE`.
+            preStateClaim = (stepPos.indexAtDepth() % (2 ** (MAX_GAME_DEPTH - SPLIT_DEPTH))) == 0
                 ? ABSOLUTE_PRESTATE
                 : findTraceAncestor(Position.wrap(Position.unwrap(parentPos) - 1), parent.parentIndex, false).claim;
             // For all attacks, the poststate is the parent claim.