Clarify that get_block_hash takes a <256 offset, not block index (#28)

evm/processor.cpp

@@ -1103,7 +1103,7 @@ namespace evm
       if (i >= 256)
         ctxt->s.push(0);
       else
-        ctxt->s.push(gs.get_block_hash(i));
+        ctxt->s.push(gs.get_block_hash(i % 256));
     }
 
     void number()

evm/simpleglobalstate.cpp

@@ -44,7 +44,7 @@ namespace evm
     return currentBlock;
   }
 
-  uint256_t SimpleGlobalState::get_block_hash(uint64_t idx)
+  uint256_t SimpleGlobalState::get_block_hash(uint8_t offset)
   {
     return 0u;
   }

evm/simpleglobalstate.h

@@ -33,7 +33,7 @@ namespace evm
     size_t num_accounts() override;
 
     virtual const Block& get_current_block() override;
-    virtual uint256_t get_block_hash(uint64_t idx) override;
+    virtual uint256_t get_block_hash(uint8_t offset) override;
 
     /**
      * For tests which require some initial state, allow manual insertion of

include/globalstate.h

@@ -46,6 +46,6 @@ namespace evm
     virtual size_t num_accounts() = 0;
 
     virtual const Block& get_current_block() = 0;
-    virtual uint256_t get_block_hash(uint64_t idx) = 0;
+    virtual uint256_t get_block_hash(uint8_t offset) = 0;
   };
 } // namespace evm