const Buffer = require('safe-buffer').Buffer
const bcrypto = require('./crypto')
const fastMerkleRoot = require('merkle-lib/fastRoot')
const typeforce = require('typeforce')
const types = require('./types')
const varuint = require('varuint-bitcoin')

const Transaction = require('./transaction')

function Block () {
  this.version = 1
  this.prevHash = null
  this.merkleRoot = null
  this.timestamp = 0
  this.bits = 0
  this.nonce = 0
}

Block.fromBuffer = function (buffer) {
  if (buffer.length < 80) throw new Error('Buffer too small (< 80 bytes)')

  let offset = 0
  function readSlice (n) {
    offset += n
    return buffer.slice(offset - n, offset)
  }

  function readUInt32 () {
    const i = buffer.readUInt32LE(offset)
    offset += 4
    return i
  }

  function readInt32 () {
    const i = buffer.readInt32LE(offset)
    offset += 4
    return i
  }

  const block = new Block()
  block.version = readInt32()
  block.prevHash = readSlice(32)
  block.merkleRoot = readSlice(32)
  block.timestamp = readUInt32()
  block.bits = readUInt32()
  block.nonce = readUInt32()

  if (buffer.length === 80) return block

  function readVarInt () {
    const vi = varuint.decode(buffer, offset)
    offset += varuint.decode.bytes
    return vi
  }

  function readTransaction () {
    const tx = Transaction.fromBuffer(buffer.slice(offset), true)
    offset += tx.byteLength()
    return tx
  }

  const nTransactions = readVarInt()
  block.transactions = []

  for (var i = 0; i < nTransactions; ++i) {
    const tx = readTransaction()
    block.transactions.push(tx)
  }

  return block
}

Block.prototype.byteLength = function (headersOnly) {
  if (headersOnly || !this.transactions) return 80

  return 80 + varuint.encodingLength(this.transactions.length) + this.transactions.reduce(function (a, x) {
    return a + x.byteLength()
  }, 0)
}

Block.fromHex = function (hex) {
  return Block.fromBuffer(Buffer.from(hex, 'hex'))
}

Block.prototype.getHash = function () {
  return bcrypto.hash256(this.toBuffer(true))
}

Block.prototype.getId = function () {
  return this.getHash().reverse().toString('hex')
}

Block.prototype.getUTCDate = function () {
  const date = new Date(0) // epoch
  date.setUTCSeconds(this.timestamp)

  return date
}

// TODO: buffer, offset compatibility
Block.prototype.toBuffer = function (headersOnly) {
  const buffer = Buffer.allocUnsafe(this.byteLength(headersOnly))

  let offset = 0
  function writeSlice (slice) {
    slice.copy(buffer, offset)
    offset += slice.length
  }

  function writeInt32 (i) {
    buffer.writeInt32LE(i, offset)
    offset += 4
  }
  function writeUInt32 (i) {
    buffer.writeUInt32LE(i, offset)
    offset += 4
  }

  writeInt32(this.version)
  writeSlice(this.prevHash)
  writeSlice(this.merkleRoot)
  writeUInt32(this.timestamp)
  writeUInt32(this.bits)
  writeUInt32(this.nonce)

  if (headersOnly || !this.transactions) return buffer

  varuint.encode(this.transactions.length, buffer, offset)
  offset += varuint.encode.bytes

  this.transactions.forEach(function (tx) {
    const txSize = tx.byteLength() // TODO: extract from toBuffer?
    tx.toBuffer(buffer, offset)
    offset += txSize
  })

  return buffer
}

Block.prototype.toHex = function (headersOnly) {
  return this.toBuffer(headersOnly).toString('hex')
}

Block.calculateTarget = function (bits) {
  const exponent = ((bits & 0xff000000) >> 24) - 3
  const mantissa = bits & 0x007fffff
  const target = Buffer.alloc(32, 0)
  target.writeUInt32BE(mantissa, 28 - exponent)
  return target
}

Block.calculateMerkleRoot = function (transactions) {
  typeforce([{ getHash: types.Function }], transactions)
  if (transactions.length === 0) throw TypeError('Cannot compute merkle root for zero transactions')

  const hashes = transactions.map(function (transaction) {
    return transaction.getHash()
  })

  return fastMerkleRoot(hashes, bcrypto.hash256)
}

Block.prototype.checkMerkleRoot = function () {
  if (!this.transactions) return false

  const actualMerkleRoot = Block.calculateMerkleRoot(this.transactions)
  return this.merkleRoot.compare(actualMerkleRoot) === 0
}

Block.prototype.checkProofOfWork = function () {
  const hash = this.getHash().reverse()
  const target = Block.calculateTarget(this.bits)

  return hash.compare(target) <= 0
}

module.exports = Block