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106 lines
3.2 KiB

package merkle
import (
"math/bits"
)
// HashFromByteSlices computes a Merkle tree where the leaves are the byte slice,
// in the provided order.
func HashFromByteSlices(items [][]byte) []byte {
switch len(items) {
case 0:
return nil
case 1:
return leafHash(items[0])
default:
k := getSplitPoint(int64(len(items)))
left := HashFromByteSlices(items[:k])
right := HashFromByteSlices(items[k:])
return innerHash(left, right)
}
}
// HashFromByteSliceIterative is an iterative alternative to
// HashFromByteSlice motivated by potential performance improvements.
// (#2611) had suggested that an iterative version of
// HashFromByteSlice would be faster, presumably because
// we can envision some overhead accumulating from stack
// frames and function calls. Additionally, a recursive algorithm risks
// hitting the stack limit and causing a stack overflow should the tree
// be too large.
//
// Provided here is an iterative alternative, a test to assert
// correctness and a benchmark. On the performance side, there appears to
// be no overall difference:
//
// BenchmarkHashAlternatives/recursive-4 20000 77677 ns/op
// BenchmarkHashAlternatives/iterative-4 20000 76802 ns/op
//
// On the surface it might seem that the additional overhead is due to
// the different allocation patterns of the implementations. The recursive
// version uses a single [][]byte slices which it then re-slices at each level of the tree.
// The iterative version reproduces [][]byte once within the function and
// then rewrites sub-slices of that array at each level of the tree.
//
// Experimenting by modifying the code to simply calculate the
// hash and not store the result show little to no difference in performance.
//
// These preliminary results suggest:
//
// 1. The performance of the HashFromByteSlice is pretty good
// 2. Go has low overhead for recursive functions
// 3. The performance of the HashFromByteSlice routine is dominated
// by the actual hashing of data
//
// Although this work is in no way exhaustive, point #3 suggests that
// optimization of this routine would need to take an alternative
// approach to make significant improvements on the current performance.
//
// Finally, considering that the recursive implementation is easier to
// read, it might not be worthwhile to switch to a less intuitive
// implementation for so little benefit.
func HashFromByteSlicesIterative(input [][]byte) []byte {
items := make([][]byte, len(input))
for i, leaf := range input {
items[i] = leafHash(leaf)
}
size := len(items)
for {
switch size {
case 0:
return nil
case 1:
return items[0]
default:
rp := 0 // read position
wp := 0 // write position
for rp < size {
if rp+1 < size {
items[wp] = innerHash(items[rp], items[rp+1])
rp += 2
} else {
items[wp] = items[rp]
rp++
}
wp++
}
size = wp
}
}
}
// getSplitPoint returns the largest power of 2 less than length
func getSplitPoint(length int64) int64 {
if length < 1 {
panic("Trying to split a tree with size < 1")
}
uLength := uint(length)
bitlen := bits.Len(uLength)
k := int64(1 << uint(bitlen-1))
if k == length {
k >>= 1
}
return k
}