From d23f38b4f3eea39f02b967ad77106ac8bf580517 Mon Sep 17 00:00:00 2001
From: Ethan Buchman <ethan@coinculture.info>
Date: Sat, 12 Mar 2016 13:01:08 -0500
Subject: [PATCH] txs.Hash() to avoid extra allocs

---
 types/block.go |  8 ++------
 types/tx.go    | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/types/block.go b/types/block.go
index 739a61b1f..9ce45bce0 100644
--- a/types/block.go
+++ b/types/block.go
@@ -329,7 +329,7 @@ type Data struct {
 	// Txs that will be applied by state @ block.Height+1.
 	// NOTE: not all txs here are valid.  We're just agreeing on the order first.
 	// This means that block.AppHash does not include these txs.
-	Txs []Tx `json:"txs"`
+	Txs Txs `json:"txs"`
 
 	// Volatile
 	hash []byte
@@ -342,11 +342,7 @@ func (data *Data) Hash() []byte {
 		return data.hash
 	}
 	if data.hash == nil {
-		txs := make([]interface{}, len(data.Txs))
-		for i, tx := range data.Txs {
-			txs[i] = tx
-		}
-		data.hash = merkle.SimpleHashFromBinaries(txs) // NOTE: leaves are TxIDs.
+		data.hash = data.Txs.Hash() // NOTE: leaves of merkle tree are TxIDs
 	}
 	return data.hash
 }
diff --git a/types/tx.go b/types/tx.go
index a3cb9fc04..63a7afad4 100644
--- a/types/tx.go
+++ b/types/tx.go
@@ -1,3 +1,24 @@
 package types
 
+import (
+	"github.com/tendermint/go-merkle"
+)
+
 type Tx []byte
+
+type Txs []Tx
+
+func (txs Txs) Hash() []byte {
+	// Recursive impl.
+	// Copied from go-merkle to avoid allocations
+	switch len(txs) {
+	case 0:
+		return nil
+	case 1:
+		return txs[0]
+	default:
+		left := Txs(txs[:(len(txs)+1)/2]).Hash()
+		right := Txs(txs[(len(txs)+1)/2:]).Hash()
+		return merkle.SimpleHashFromTwoHashes(left, right)
+	}
+}