6211eef7ee
This commit adds a new function which is similar to the DoubleSha256
function except it returns a ShaHash copy instead of a byte slice. It
also adds a new benchmark for it.
This can be a slight optimization in certain cases where the caller
ultimately wants a ShaHash since it can avoid a heap allocation and
additional copy to convert the result to a ShaHash (the function simply
performs a type cast against the returned array which is not possible
against a []byte).
existing: DoubleSha256 500000 3081 ns/op 32 B/op 1 allocs/op
new: DoubleSha256SH 500000 2939 ns/op 0 B/op 0 allocs/op
The hashing functions for blocks and transactions have also been updated
to make use of the new function since they directly return the ShaHash.
The transaction change in particular is quite useful since transactions
are frequently hashed and this change allows all of those hashes to avoid
an additional heap allocation.
106 lines
3.7 KiB
Go
106 lines
3.7 KiB
Go
// Copyright (c) 2013-2014 Conformal Systems LLC.
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// Use of this source code is governed by an ISC
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// license that can be found in the LICENSE file.
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package blockchain
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import (
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"math"
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"github.com/btcsuite/btcd/wire"
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"github.com/btcsuite/btcutil"
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)
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// nextPowerOfTwo returns the next highest power of two from a given number if
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// it is not already a power of two. This is a helper function used during the
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// calculation of a merkle tree.
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func nextPowerOfTwo(n int) int {
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// Return the number if it's already a power of 2.
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if n&(n-1) == 0 {
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return n
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}
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// Figure out and return the next power of two.
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exponent := uint(math.Log2(float64(n))) + 1
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return 1 << exponent // 2^exponent
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}
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// HashMerkleBranches takes two hashes, treated as the left and right tree
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// nodes, and returns the hash of their concatenation. This is a helper
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// function used to aid in the generation of a merkle tree.
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func HashMerkleBranches(left *wire.ShaHash, right *wire.ShaHash) *wire.ShaHash {
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// Concatenate the left and right nodes.
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var sha [wire.HashSize * 2]byte
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copy(sha[:wire.HashSize], left[:])
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copy(sha[wire.HashSize:], right[:])
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newSha := wire.DoubleSha256SH(sha[:])
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return &newSha
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}
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// BuildMerkleTreeStore creates a merkle tree from a slice of transactions,
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// stores it using a linear array, and returns a slice of the backing array. A
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// linear array was chosen as opposed to an actual tree structure since it uses
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// about half as much memory. The following describes a merkle tree and how it
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// is stored in a linear array.
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//
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// A merkle tree is a tree in which every non-leaf node is the hash of its
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// children nodes. A diagram depicting how this works for bitcoin transactions
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// where h(x) is a double sha256 follows:
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//
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// root = h1234 = h(h12 + h34)
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// / \
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// h12 = h(h1 + h2) h34 = h(h3 + h4)
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// / \ / \
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// h1 = h(tx1) h2 = h(tx2) h3 = h(tx3) h4 = h(tx4)
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//
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// The above stored as a linear array is as follows:
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//
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// [h1 h2 h3 h4 h12 h34 root]
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//
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// As the above shows, the merkle root is always the last element in the array.
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//
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// The number of inputs is not always a power of two which results in a
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// balanced tree structure as above. In that case, parent nodes with no
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// children are also zero and parent nodes with only a single left node
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// are calculated by concatenating the left node with itself before hashing.
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// Since this function uses nodes that are pointers to the hashes, empty nodes
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// will be nil.
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func BuildMerkleTreeStore(transactions []*btcutil.Tx) []*wire.ShaHash {
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// Calculate how many entries are required to hold the binary merkle
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// tree as a linear array and create an array of that size.
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nextPoT := nextPowerOfTwo(len(transactions))
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arraySize := nextPoT*2 - 1
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merkles := make([]*wire.ShaHash, arraySize)
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// Create the base transaction shas and populate the array with them.
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for i, tx := range transactions {
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merkles[i] = tx.Sha()
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}
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// Start the array offset after the last transaction and adjusted to the
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// next power of two.
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offset := nextPoT
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for i := 0; i < arraySize-1; i += 2 {
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switch {
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// When there is no left child node, the parent is nil too.
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case merkles[i] == nil:
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merkles[offset] = nil
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// When there is no right child, the parent is generated by
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// hashing the concatenation of the left child with itself.
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case merkles[i+1] == nil:
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newSha := HashMerkleBranches(merkles[i], merkles[i])
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merkles[offset] = newSha
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// The normal case sets the parent node to the double sha256
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// of the concatentation of the left and right children.
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default:
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newSha := HashMerkleBranches(merkles[i], merkles[i+1])
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merkles[offset] = newSha
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}
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offset++
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}
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return merkles
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}
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