Add merkle tree size measure

This change creates a merkletree package which will be used in the future
for an implementation of file system API.

PiperOrigin-RevId: 315952451

1 files changed, 71 insertions, 0 deletions

diff --git a/pkg/merkletree/merkletree.go b/pkg/merkletree/merkletree.go
new file mode 100644
index 000000000..965f3670b
--- /dev/null
+++ b/pkg/merkletree/merkletree.go
@@ -0,0 +1,71 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package merkletree implements Merkle tree generating and verification.
+package merkletree
+
+import (
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+const (
+	// sha256DigestSize specifies the digest size of a SHA256 hash.
+	sha256DigestSize = 32
+)
+
+// Size defines the scale of a Merkle tree.
+type Size struct {
+	// blockSize is the size of a data block to be hashed.
+	blockSize int64
+	// digestSize is the size of a generated hash.
+	digestSize int64
+	// hashesPerBlock is the number of hashes in a block. For example, if
+	// blockSize is 4096 bytes, and digestSize is 32 bytes, there will be 128
+	// hashesPerBlock. Therefore 128 hashes in a lower level will be put into a
+	// block and generate a single hash in an upper level.
+	hashesPerBlock int64
+	// levelStart is the start block index of each level. The number of levels in
+	// the tree is the length of the slice. The leafs (level 0) are hashes of
+	// blocks in the input data. The levels above are hashes of lower level
+	// hashes.  The highest level is the root hash.
+	levelStart []int64
+}
+
+// MakeSize initializes and returns a new Size object describing the structure
+// of a tree. dataSize specifies the number of the file system size in bytes.
+func MakeSize(dataSize int64) Size {
+	size := Size{
+		blockSize: usermem.PageSize,
+		// TODO(b/156980949): Allow config other hash methods (SHA384/SHA512).
+		digestSize:     sha256DigestSize,
+		hashesPerBlock: usermem.PageSize / sha256DigestSize,
+	}
+	numBlocks := (dataSize + size.blockSize - 1) / size.blockSize
+	level := int64(0)
+	offset := int64(0)
+
+	// Calcuate the number of levels in the Merkle tree and the beginning offset
+	// of each level. Level 0 is the level directly above the data blocks, while
+	// level NumLevels - 1 is the root.
+	for numBlocks > 1 {
+		size.levelStart = append(size.levelStart, offset)
+		// Round numBlocks up to fill up a block.
+		numBlocks += (size.hashesPerBlock - numBlocks%size.hashesPerBlock) % size.hashesPerBlock
+		offset += numBlocks / size.hashesPerBlock
+		numBlocks = numBlocks / size.hashesPerBlock
+		level++
+	}
+	size.levelStart = append(size.levelStart, offset)
+	return size
+}