update docs

core/src/indexer/README.md

@@ -16,9 +16,13 @@ limited in size, through a mechanism we will defined later on.
 If state is not used then all the processing will happen in parallel until it reaches the chain head. 
 
 ## State
-State refers to intermediate state between blocks (think the state for fold operations). Only the 
-latest data is kept and it is only queryable from the pre-indexer, subgraphs and graphql don't have 
-access to it.
+State refers to intermediate state between blocks (think the state for fold operations). It is only 
+queryable from the pre-indexer, subgraphs and graphql don't have access to it.
+
+In order to support reverts, it is necessary to be able to retrieve a previous state. State should
+store a log of delta operations as well as a snapshot every TBD amount of blocks. Retrieving an old state
+will be possible by getting the latest snapshot and applying the delta operations between that block and 
+the block it is needed at. 
 
 This state is necessary so that users can keep track of things like created contracts on ethereum.
 

core/src/indexer/mod.rs

@@ -20,10 +20,16 @@ mod uniswap;
 
 pub type Item = Box<[u8]>;
 
+enum StateOperation {
+    Set(Key, Item),
+    Unset(Key, Item),
+}
+
 #[derive(Default)]
 pub struct State {
     items: HashMap<Box<[u8]>, Item>,
     tags: HashMap<&'static str, Vec<Box<[u8]>>>,
+    deltas: Vec<StateOperation>,
 }
 
 #[derive(Hash)]
@@ -168,7 +174,7 @@ impl IndexWorker {
                         .transform(EncodedBlock(data), std::mem::take(&mut previous_state));
                     previous_state = state;
                     ctx.store
-                        .set(block_ptr.number, &previous_state, triggers)
+                        .set_with_state(block_ptr.number, &previous_state, triggers)
                         .await?;
 
                     cursor