Add new Tx wrapper for btcwire.MsgTx.

Currently, transaction hash caching is provided via Block directly, but a
transaction is not always part of a block and there are several cases
where only the transaction needs to be dealt with without wanting to pass
the entire block and transaction index around to be able to get at the
cached hash.

So, this commit adds a new type named Tx which is a wrapper that provides
easier and more efficient manipulation of raw wire protocol transactions.
It memoizes the hash for the transaction on its first access so subsequent
accesses don't have to repeat the relatively expensive hashing operations.
The idea is the callers can pass around pointers to these Tx types instead
of raw btcwire.MsgTx pointers.

For now, the Block API has not been changed, but the plan is to change it
to provide access to these wrapped transactions rather than having it do
the transaction hash caching directly.

This is only the first part of a series of changes working towards
optimizations noted in conformal/btcd#25.

doc.go

@@ -12,6 +12,13 @@ manipulation of raw wire protocol blocks.  It also memoizes hashes for the
 block and its transactions on their first access so subsequent accesses don't
 have to repeat the relatively expensive hashing operations.
 
+Tx Overview
+
+A Tx defines a bitcoin transaction that provides more efficient manipulation of
+raw wire protocol transactions.  It memoizes the hash for the transaction on its
+first access so subsequent accesses don't have to repeat the relatively
+expensive hashing operations.
+
 Base58 Usage
 
 To decode a base58 string:

tx.go

@@ -0,0 +1,89 @@
+// Copyright (c) 2013 Conformal Systems LLC.
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package btcutil
+
+import (
+	"bytes"
+	"github.com/conformal/btcwire"
+)
+
+// TxIndexUnknown is the value returned for a transaction index that is unknown.
+// This is typically because the transaction has not been inserted into a block
+// yet.
+const TxIndexUnknown = -1
+
+// Tx defines a bitcoin transaction that provides easier and more efficient
+// manipulation of raw transactions.  It also memoizes the hash for the
+// transaction on its first access so subsequent accesses don't have to repeat
+// the relatively expensive hashing operations.
+type Tx struct {
+	msgTx        *btcwire.MsgTx   // Underlying MsgTx
+	serializedTx []byte           // Serialized bytes for the transaction
+	txSha        *btcwire.ShaHash // Cached transaction hash
+	txIndex      int              // Position within a block or TxIndexUnknown
+}
+
+// MsgTx returns the underlying btcwire.MsgTx for the transaction.
+func (t *Tx) MsgTx() *btcwire.MsgTx {
+	// Return the cached transaction.
+	return t.msgTx
+}
+
+// Sha returns the hash of the transaction.  This is equivalent to
+// calling TxSha on the underlying btcwire.MsgTx, however it caches the
+// result so subsequent calls are more efficient.
+func (t *Tx) Sha() *btcwire.ShaHash {
+	// Return the cached hash if it has already been generated.
+	if t.txSha != nil {
+		return t.txSha
+	}
+
+	// Generate the transaction hash.  Ignore the error since TxSha can't
+	// currently fail.
+	sha, _ := t.msgTx.TxSha()
+
+	// Cache the hash and return it.
+	t.txSha = &sha
+	return &sha
+}
+
+// Index returns the saved index of the transaction within a block.  This value
+// will be TxIndexUnknown if it hasn't already explicitly been set.
+func (t *Tx) Index() int {
+	return t.txIndex
+}
+
+// SetIndex sets the index of the transaction in within a block.
+func (t *Tx) SetIndex(index int) {
+	t.txIndex = index
+}
+
+// NewTx returns a new instance of a bitcoin transaction given an underlying
+// btcwire.MsgTx.  See Tx.
+func NewTx(msgTx *btcwire.MsgTx) *Tx {
+	return &Tx{
+		msgTx:   msgTx,
+		txIndex: TxIndexUnknown,
+	}
+}
+
+// NewTxFromBytes returns a new instance of a bitcoin transaction given the
+// serialized bytes.  See Tx.
+func NewTxFromBytes(serializedTx []byte) (*Tx, error) {
+	// Deserialize the bytes into a MsgTx.
+	var msgTx btcwire.MsgTx
+	br := bytes.NewBuffer(serializedTx)
+	err := msgTx.Deserialize(br)
+	if err != nil {
+		return nil, err
+	}
+
+	t := Tx{
+		msgTx:        &msgTx,
+		serializedTx: serializedTx,
+		txIndex:      TxIndexUnknown,
+	}
+	return &t, nil
+}