defmodule UltraDark.Validator do
  alias UltraDark.Blockchain.Block
  alias UltraDark.Utilities
  alias UltraDark.KeyPair

  @moduledoc """
    Responsible for implementing the consensus rules to all blocks and transactions
  """

  @doc """
    A block is considered valid if the index is greater than the index of the previous block,
    the previous_hash is equal to the hash of the previous block, and the hash of the block,
    when recalculated, is the same as what the listed block hash is
  """
  @spec is_block_valid?(Block, list, number) :: :ok | {:error, String.t}
  def is_block_valid?(block, chain, difficulty) do
    last_block = List.first(chain)

    with :ok <- valid_index(block.index, last_block.index),
       :ok <- valid_prev_hash(block.previous_hash, last_block.hash),
       :ok <- valid_hash(block),
       :ok <- valid_coinbase?(block),
  	   :ok <- valid_transactions?(block),
  	   :ok <- valid_difficulty?(block, difficulty)
    do :ok
    else err -> err
    end
  end

  defp valid_index(index, prev_index) when index > prev_index, do: :ok
  defp valid_index(index, prev_index) when index <= prev_index, do: {:error, "Block has invalid index"}

  defp valid_prev_hash(prev_hash, last_block_hash) when prev_hash == last_block_hash, do: :ok
  defp valid_prev_hash(prev_hash, last_block_hash) when prev_hash != last_block_hash, do: {:error, "Blocks prev_hash is not equal to the last block's hash"}

  defp valid_hash(%{index: index, previous_hash: previous_hash, timestamp: timestamp, nonce: nonce, hash: hash, merkle_root: merkle_root, difficulty: difficulty}) do
    with :ok <- compare_hash({index, previous_hash, timestamp, nonce, merkle_root}, hash),
       :ok <- fn -> if Block.hash_beat_target?(%{hash: hash, difficulty: difficulty}), do: :ok, else: {:error, "Hash did not beat target"} end
    do :ok
	  else err -> err
	  end
  end

  defp compare_hash({index, previous_hash, timestamp, nonce, merkle_root}, hash) do
    if Utilities.sha3_base16([Integer.to_string(index), previous_hash, timestamp, Integer.to_string(nonce), merkle_root]) == hash, do: :ok, else: {:error, "Computed hash doesnt match privided hash"}
  end

  @spec valid_coinbase?(Block) :: :ok | {:error, String.t}
  def valid_coinbase?(%{transactions: transactions, index: block_index}) do
    coinbase = List.first(transactions)

    with :ok <- (&(if &1 != nil, do: :ok, else: {:error, "Block has no coinbase"})).(coinbase),
       :ok <- is_coinbase?(coinbase),
       :ok <- appropriate_coinbase_output?(transactions, block_index)
    do :ok
    else err -> err
    end
  end

  @spec valid_transaction?(Transaction) :: boolean
  def valid_transaction?(%{inputs: inputs}) do
    inputs
    |> Enum.map(fn input ->
      case {Base.decode16(input.addr), Base.decode16(input.signature)} do
        {{:ok, pub}, {:ok, sig}} -> KeyPair.verify_signature(pub, sig, input.txoid)
        _ -> false
      end
    end)
    |> Enum.all?(&(&1 == true))
  end

  @spec valid_transactions?(Block) :: :ok | {:error, String.t}
  def valid_transactions?(%{transactions: transactions}) do
    if Enum.all?(transactions, &(valid_transaction? &1)), do: :ok, else: {:error, "Transaction contains invalid input(s)"}
  end

  defp is_coinbase?(tx) do
    if tx.txtype == "COINBASE", do: :ok, else: {:error, "Transaction 1 of block is not of txtype COINBASE"}
  end

  defp appropriate_coinbase_output?([coinbase | transactions], block_index) do
    if Block.total_block_fees(transactions) + Block.calculate_block_reward(block_index) == List.first(coinbase.outputs).amount, do: :ok, else: {:error, "Coinbase output is invalid"}
  end

  @spec valid_difficulty?(Block, number) :: :ok | {:error, String.t}
  def valid_difficulty?(%{difficulty: difficulty}, diff) do
	  if difficulty == diff, do: :ok, else: {:error, "Invalid difficulty. Got #{difficulty}, want #{diff}"}
  end
end