bmt: Binary Merkle Tree Hash (#14334)

bmt is a new package that provides hashers for binary merkle tree hashes on
size-limited chunks. the main motivation is that using BMT hash as the chunk
hash of the swarm hash offers logsize inclusion proofs for arbitrary files on a
32-byte resolution completely viable to use in challenges on the blockchain.

bmt/bmt_r.go

@@ -0,0 +1,85 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// simple nonconcurrent reference implementation for hashsize segment based
+// Binary Merkle tree hash on arbitrary but fixed maximum chunksize
+//
+// This implementation does not take advantage of any paralellisms and uses
+// far more memory than necessary, but it is easy to see that it is correct.
+// It can be used for generating test cases for optimized implementations.
+// see testBMTHasherCorrectness function in bmt_test.go
+package bmt
+
+import (
+	"hash"
+)
+
+// RefHasher is the non-optimized easy to read reference implementation of BMT
+type RefHasher struct {
+	span    int
+	section int
+	cap     int
+	h       hash.Hash
+}
+
+// NewRefHasher returns a new RefHasher
+func NewRefHasher(hasher BaseHasher, count int) *RefHasher {
+	h := hasher()
+	hashsize := h.Size()
+	maxsize := hashsize * count
+	c := 2
+	for ; c < count; c *= 2 {
+	}
+	if c > 2 {
+		c /= 2
+	}
+	return &RefHasher{
+		section: 2 * hashsize,
+		span:    c * hashsize,
+		cap:     maxsize,
+		h:       h,
+	}
+}
+
+// Hash returns the BMT hash of the byte slice
+// implements the SwarmHash interface
+func (rh *RefHasher) Hash(d []byte) []byte {
+	if len(d) > rh.cap {
+		d = d[:rh.cap]
+	}
+
+	return rh.hash(d, rh.span)
+}
+
+func (rh *RefHasher) hash(d []byte, s int) []byte {
+	l := len(d)
+	left := d
+	var right []byte
+	if l > rh.section {
+		for ; s >= l; s /= 2 {
+		}
+		left = rh.hash(d[:s], s)
+		right = d[s:]
+		if l-s > rh.section/2 {
+			right = rh.hash(right, s)
+		}
+	}
+	defer rh.h.Reset()
+	rh.h.Write(left)
+	rh.h.Write(right)
+	h := rh.h.Sum(nil)
+	return h
+}