@omisego/omg-js-rootchain/src/merkle.js

Module to interact with the Ethereum RootChain

/*
Copyright 2019 OmiseGO Pte Ltd

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. */

// derived from: https://github.com/omisego/plasma-contracts/blob/master/plasma_framework/test/helpers/merkle.js

const web3Utils = require('web3-utils')
const LeafSalt = '0x00'
const NodeSalt = '0x01'
const NullHash = web3Utils.sha3('\0'.repeat(33))

class MerkleNode {
  constructor (data, left = null, right = null) {
    this.data = data
    this.left = left
    this.right = right
  }
}

class MerkleTree {
  constructor (leaves, height = 0) {
    const minHeightForLeaves = leaves.length === 1 ? 1 : parseInt(Math.ceil(Math.log2(leaves.length)), 10)

    if (height === 0) {
      this.height = minHeightForLeaves
    } else if (height > minHeightForLeaves) {
      this.height = height
    } else {
      throw new Error(
        `height should be at least ${minHeightForLeaves} for the list of leaves`
      )
    }

    this.leafCount = 2 ** this.height

    this.leaves = leaves.map(MerkleTree.hashLeaf)

    const fill = Array.from({ length: this.leafCount - this.leaves.length }, () => NullHash)
    this.leaves = this.leaves.concat(fill)
    this.tree = [MerkleTree.createNodes(this.leaves)]
    this.root = this.createTree(this.tree[0])
  }

  static hashLeaf (leaf) {
    return web3Utils.sha3(LeafSalt + leaf.slice(2))
  }

  static createNodes (leaves) {
    return leaves.map(leaf => new MerkleNode(leaf))
  }

  createTree (level) {
    if (level.length === 1) {
      return level[0].data
    }

    const levelSize = level.length
    const nextLevel = []

    let i = 0
    while (i < levelSize) {
      // JS stores hashes as hex-encoded strings
      const combinedData = NodeSalt + level[i].data.slice(2) + level[i + 1].data.slice(2)
      const combined = web3Utils.sha3(combinedData)
      const nextNode = new MerkleNode(combined, level[i], level[i + 1])
      nextLevel.push(nextNode)
      i += 2
    }

    this.tree.push(nextLevel)
    return this.createTree(nextLevel)
  }

  getInclusionProof (leaf) {
    const hashedLeaf = MerkleTree.hashLeaf(leaf)

    let index = this.leaves.indexOf(hashedLeaf)
    if (index === -1) {
      throw new Error('Argument is not a leaf in the tree')
    }

    let proof = '0x'
    for (let i = 0; i < this.height; i++) {
      let siblingIndex
      if (index % 2 === 0) {
        siblingIndex = index + 1
      } else {
        siblingIndex = index - 1
      }
      index = Math.floor(index / 2)

      proof += this.tree[i][siblingIndex].data.slice(2)
    }

    return proof
  }
}

module.exports = MerkleTree