Changed prev_hash to block_hash, state transition now uses vm env

* PREVHASH => BLOCKHASH( N )
* State transition object uses VMEnv as it's query interface
* Updated vm.Enviroment has GetHash( n ) for BLOCKHASH instruction
* Added GetHash to xeth, core, utils & test environments

vm/environment.go

@@ -14,11 +14,10 @@ type Environment interface {
 
 	Origin() []byte
 	BlockNumber() *big.Int
-	PrevHash() []byte
+	GetHash(n uint64) []byte
 	Coinbase() []byte
 	Time() int64
 	Difficulty() *big.Int
-	BlockHash() []byte
 	GasLimit() *big.Int
 	Transfer(from, to Account, amount *big.Int) error
 	AddLog(state.Log)

vm/types.go

@@ -59,7 +59,7 @@ const (
 const (
 
 	// 0x40 range - block operations
-	PREVHASH OpCode = 0x40 + iota
+	BLOCKHASH OpCode = 0x40 + iota
 	COINBASE
 	TIMESTAMP
 	NUMBER
@@ -216,7 +216,7 @@ var opCodeToString = map[OpCode]string{
 	GASPRICE:     "TXGASPRICE",
 
 	// 0x40 range - block operations
-	PREVHASH:    "PREVHASH",
+	BLOCKHASH:   "BLOCKHASH",
 	COINBASE:    "COINBASE",
 	TIMESTAMP:   "TIMESTAMP",
 	NUMBER:      "NUMBER",

vm/vm_debug.go

@@ -42,9 +42,9 @@ func (self *DebugVm) Run(me, caller ContextRef, code []byte, value, gas, price *
 
 	msg := self.env.State().Manifest().AddMessage(&state.Message{
 		To: me.Address(), From: caller.Address(),
-		Input:  callData,
-		Origin: self.env.Origin(),
-		Block:  self.env.BlockHash(), Timestamp: self.env.Time(), Coinbase: self.env.Coinbase(), Number: self.env.BlockNumber(),
+		Input:     callData,
+		Origin:    self.env.Origin(),
+		Timestamp: self.env.Time(), Coinbase: self.env.Coinbase(), Number: self.env.BlockNumber(),
 		Value: value,
 	})
 	context := NewContext(msg, caller, me, code, gas, price)
@@ -516,12 +516,15 @@ func (self *DebugVm) Run(me, caller ContextRef, code []byte, value, gas, price *
 			self.Printf(" => %v", context.Price)
 
 			// 0x40 range
-		case PREVHASH:
-			prevHash := self.env.PrevHash()
+		case BLOCKHASH:
+			num := stack.Pop()
+			if num.Cmp(new(big.Int).Sub(self.env.BlockNumber(), ethutil.Big256)) < 0 {
+				stack.Push(ethutil.Big0)
+			} else {
+				stack.Push(ethutil.BigD(self.env.GetHash(num.Uint64())))
+			}
 
-			stack.Push(ethutil.BigD(prevHash))
-
-			self.Printf(" => 0x%x", prevHash)
+			self.Printf(" => 0x%x", stack.Peek().Bytes())
 		case COINBASE:
 			coinbase := self.env.Coinbase()