EIP-96: Avoid storing duplicated hashes

EIPS/blockhash_refactoring.md

@@ -7,7 +7,7 @@
     Category: Core
     Status: Draft
     Created: 2017-02-10
-    
+
 ### Summary
 
 Stores blockhashes in the state, reducing the protocol complexity and the need for client implementation complexity in order to process the BLOCKHASH opcode. Also extends the range of how far back blockhash checking can go, with the side effect of creating direct links between blocks with very distant block numbers, facilitating much more efficient initial light client syncing.
@@ -41,44 +41,100 @@ If `block.number >= METROPOLIS_FORK_BLKNUM + 256`, then the BLOCKHASH opcode ins
 
 Also, for blocks where `block.number >= METROPOLIS_FORK_BLKNUM`, the gas cost is increased from 20 to 800 to reflect the higher costs of processing the algorithm in the contract code.
 
+
+### Contract model
+
+The contract, when called by the SYSTEM account, records provided block hashes
+selectively on different levels. The levels differs by intervals (frequency)
+at what the block hashes are stored.
+
+Let `n` be the current block number.
+Let `p = n - 1` be the previous block number.
+Let `h` be the block hash of the previous `p` block.
+Let `B` be the base -- the number of records kept on every level.
+
+The levels are numbered with `k`, starting from 0.
+A given level `k` of stored block hashes has the _interval_ of `B**k` blocks.
+
+The recursive level update formula is:
+
+```python
+def update(k, p):
+    n = p - B**k          # The number of the block hash to be moved.
+    i = (n / B**k) % B    # The index of the storage slot where to move the block hash.
+    if i == 0:
+        update(k + 1, n)  # Update higher level.
+    storage[k][i] = storage[k - 1][0]  # Move the block hash from lower level.
+```
+
+where
+- `storage[k]` is contract storage dedicated to level `k`,
+- `storage[-1][0]` is `h`.
+
+
+### Implementation parameters
+
+- `B` is `256`,
+- `k` is max 2 (3 levels in total).
+
+
 ### BLOCKHASH_CONTRACT_CODE
 
 BLOCKHASH_CONTRACT_CODE is set to:
 
 ```
-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
 ```
 
 The Serpent source code is:
 
 ```python
 # Setting the block hash
 if msg.sender == 2**160 - 2:
-    with prev_block_number = block.number - 1:
-        # Use storage fields 0..255 to store the last 256 hashes
-        ~sstore(prev_block_number % 256, ~calldataload(0))
-        # Use storage fields 256..511 to store the hashes of the last 256
-        # blocks with block.number % 256 == 0
-        if not (prev_block_number % 256):
-            ~sstore(256 + (prev_block_number / 256) % 256, ~calldataload(0))
-        # Use storage fields 512..767 to store the hashes of the last 256
-        # blocks with block.number % 65536 == 0
-        if not (prev_block_number % 65536):
-            ~sstore(512 + (prev_block_number / 65536) % 256, ~calldataload(0))
+
+    # Level 0
+    # Use storage fields 0..255 to store the hashes of the last 256
+    # blocks.
+    n0 = block.number - 1
+    i0 = n0 % 256
+
+    if i0 == 0:
+        # Level 1
+        # Use storage fields 256..511 to store the hashes of 256
+        # blocks with block.number % 256 == 0.
+        n1 = n0 - 256
+        i1 = (n1 / 256) % 256
+
+        if i1 == 0:
+            # Level 2
+            # Use storage fields 512..767 to store the hashes of 256
+            # blocks with block.number % 65536 == 0.
+            n2 = n1 - 256*256
+            i2 = (n2 / (256*256)) % 256
+            # Move to be replaced record from level 1 to level 2.
+            ~sstore(512 + i2, ~sload(256))
+
+        # Move to be replaced record from level 0 to level 1.
+        ~sstore(256 + i1, ~sload(0))
+
+    # Save the provided hash of the previous block.
+    ~sstore(i0, ~calldataload(0))
+
 # Getting the block hash
 else:
-    if ~calldataload(0) >= block.number:
-        return(0)
-    elif block.number - ~calldataload(0) <= 256:
-        return(~sload(~calldataload(0) % 256))
-    elif (not (~calldataload(0) % 256) and block.number - ~calldataload(0) <= 65536):
-        return(~sload(256 + (~calldataload(0) / 256) % 256))
-    elif (not (~calldataload(0) % 65536) and block.number - ~calldataload(0) <= 16777216):
-        return(~sload(512 + (~calldataload(0) / 65536) % 256))
-    else:
-        return(0)
+    number = ~calldataload(0)
+    if block.number > number and number >= 0:
+        distance = block.number - number
+        if distance <= 256:
+            return(~sload(number % 256))
+        if number % 256 == 0:
+            if distance <= 65792:
+                return(~sload(256 + (number / 256) % 256))
+            if (not (number % 65536) and distance <= 16843008):
+                return(~sload(512 + (number / 65536) % 256))
+    return(0)
 ```
 
 ### Rationale
 
-This removes the need for implementaitons to have an explicit way to look into historical block hashes, simplifying the protocol definition and removing a large component of the "implied state" (information that is technically state but is not part of the state tree) and thereby making the protocol more "pure". Additionally, it allows blocks to directly point to blocks far behind them, which enables extremely efficient and secure light client protocols.
+This removes the need for implementations to have an explicit way to look into historical block hashes, simplifying the protocol definition and removing a large component of the "implied state" (information that is technically state but is not part of the state tree) and thereby making the protocol more "pure". Additionally, it allows blocks to directly point to blocks far behind them, which enables extremely efficient and secure light client protocols.

EIPS/eip-96/tests/.gitignore

@@ -0,0 +1,3 @@
+/*.egg-info/
+/.cache/
+/__pycache__/

EIPS/eip-96/tests/README.md

@@ -0,0 +1,5 @@
+### Usage
+
+1. Setup virtualenv
+2. `pip install -r requirements.txt`
+3. `py.test`

EIPS/eip-96/tests/requirements.txt

@@ -0,0 +1,3 @@
+six
+pytest
+ethereum==1.6.1