defmodule SApp.PageStore do
  @moduledoc """
  A module that implements a content-adressable storage (pages,
  identified by the hash of their contents).

  This is not a shard, it is a side process that a shard may use to store its data.

  Uses an ETS table of:
  
      { page_id, why_have_it }         # waiting for data
      { page_id, why_have_it, data }   # once we have the data

      why_have_it := :root
                   | {:req_by, some_other_page_id}
                   | {:cached, expiry_date}

  TODO: at the moment we are trying to pull all missing pages at once from our peers.
  This can work for metadata that isn't too big but won't work with bigger objects.
  Have a smart strategy where we limit the number of requests currently in-flight but
  still make sure everything gets pulled in. This will also pave the way to selectively
  pulling in pages, for instance if we have a function to give them a priority score and
  a maximum stored page count.

  A `SApp.PageStore` can be used as a `SData.PageStore` in the following way:

      %SApp.PageStore{pid: store_pid}

  or:

      %SApp.PageStore{pid: store_pid, prefer_ask: [connection_pid, ...]}

  In the second case, missing pages will be requested first to the specified peers.
  """

  use GenServer

  @enforce_keys [:pid]
  defstruct [:pid, :prefer_ask]

  @cache_ttl 600          # Ten minutes
  @clean_cache_every 60   # One minute
  @max_failures 4         # Maximum of peers that reply not_found before we abandon

  defmodule State do
    @moduledoc"""
    Internal state struct of pagestore process.
    """
    defstruct [:shard_id, :path, :netgroup, :store, :reqs, :retries, :store_path]
  end


  def start_link(shard_id, path, netgroup) do
    GenServer.start_link(__MODULE__, [shard_id, path, netgroup])
  end

  def init([shard_id, path, netgroup]) do
    Shard.Manager.dispatch_to(shard_id, path, self())

    store_path = [Application.get_env(:shard, :data_path), "#{shard_id|>Base.encode16}.#{path}"] |> Path.join
    {:ok, store} = :dets.open_file(String.to_atom(store_path), [type: :set])

    Process.send_after(self(), :clean_cache, 1000)

    {:ok, %State{shard_id: shard_id, path: path, netgroup: netgroup, store: store, reqs: %{}, retries: %{}, store_path: store_path}}
  end


  def handle_call(:delete_store, _from, state) do
    :dets.close state.store
    File.rm state.store_path
    {:stop, :normal, :ok, state}
  end

  def handle_call({:get, key, prefer_ask}, from, state) do
    case :dets.lookup state.store, key do
      [{_, _, bin}] ->
        {:reply, bin, state}
      [{_, _}] ->
        state = add_request(state, key, from)
        {:noreply, state}
      [] ->
        why = {:cached, System.os_time(:seconds) + @cache_ttl}
        init_rec_pull(state, key, why, prefer_ask)
        state = add_request(state, key, from)
        {:noreply, state}
    end
  end

  def handle_call({:put, bin}, _from, state) do
    hash = SData.bin_hash bin
    store_put(state, hash, bin)
    {:reply, hash, state}
  end

  def handle_call({:have_rec, root}, _from, state) do
    {:reply, have_rec(state, root), state}
  end

  defp add_request(state, key, from) do
    reqs_key = case state.reqs[key] do
      nil ->
        MapSet.put(MapSet.new(), from)
      ms ->
        MapSet.put(ms, from)
    end
    put_in(state.reqs[key], reqs_key)
  end

  defp store_put(state, hash, bin) do
    case :dets.lookup state.store, hash do
      [] ->
        :dets.insert state.store, {hash, {:cached, System.os_time(:seconds) + @cache_ttl}, bin}
        :dets.sync state.store
        nil
      [{_, why}] ->
        :dets.insert state.store, {hash, why, bin}
        :dets.sync state.store
        why
      [{_, _, _}] ->
        nil
    end
  end

  defp have_rec(state, root) do
    case :dets.lookup state.store, root do
      [{_, _, bin}] ->
        pg = SData.term_unbin bin
        pg |> SData.Page.refs |> Enum.map(&(have_rec(state, &1))) |> Enum.all?
      _ ->
        false
    end
  end

  defp init_rec_pull(state, key, why, prefer_ask) do
    case prefer_ask do
      [_|_] ->
        for peer <- prefer_ask do
          SNet.Manager.send_pid(peer, {state.shard_id, state.path, {:get, key}})
        end
      _ ->
        ask_random_peers(state, key)
    end
    :dets.insert state.store, {key, why}
  end

  def handle_cast({:rec_pull, hash, ask_to}, state) do
    if :dets.lookup state.store, hash == [] do
      why = {:cached, System.os_time(:seconds) + @cache_ttl}
      init_rec_pull(state, hash, why, ask_to)
    end
    {:noreply, state}
  end

  def handle_cast({:msg, conn_pid, auth, _shard_id, _path, msg}, state) do
    if not SNet.Group.in_group?(state.netgroup, conn_pid, auth) do
      {:noreply, state}
    else
      state = case msg do
        {:get, key} ->
          case :dets.lookup state.store, key do
            [{_, _, bin}] ->
              SNet.Manager.send_pid(conn_pid, {state.shard_id, state.path, {:info, key, bin}})
            _ ->
              SNet.Manager.send_pid(conn_pid, {state.shard_id, state.path, {:not_found, key}})
          end
          state
        {:info, hash, bin} ->
          already_have_it = case :dets.lookup state.store, hash do
            [{_, _, _}] -> true
            _ -> false
          end
          if SData.bin_hash(bin) == hash and not already_have_it do
            reqs = case state.reqs[hash] do
              nil -> state.reqs
              pids ->
                for pid <- pids do
                  GenServer.reply(pid, bin)
                end
                Map.delete(state.reqs, hash)
            end
            state = %{state | reqs: reqs, retries: Map.delete(state.retries, hash)}
            rec_why = store_put(state, hash, bin)
            if rec_why != nil do
              sub_why = case rec_why do
                {:cached, ttl} -> {:cached, ttl}
                _ -> {:req_by, hash}
              end
              value = SData.term_unbin bin
              for dep <- SData.Page.refs value do
                if :dets.lookup state.store, dep == [] do
                  init_rec_pull(state, dep, sub_why, [conn_pid])
                end
              end
            end
            state
          else
            state
          end
        {:not_found, key} ->
          if state.reqs[key] != nil do
            nretry = case state.retries[key] do
              nil -> 1
              n -> n+1
            end
            if nretry < @max_failures do
              ask_random_peers(state, key)
              %{state | retries: Map.put(state.retries, key, nretry)}
            else
              for pid <- state.reqs[key] do
                GenServer.reply(pid, nil)
              end
              state = %{state | reqs: Map.delete(state.reqs, key)}
              state = %{state | retries: Map.delete(state.retries, key)}
              state
            end
          else
            state
          end
      end
      {:noreply, state}
    end
  end
  
  def handle_cast({:set_roots, roots}, state) do
    cached_why = {:cached, System.os_time(:seconds) + @cache_ttl}

    # Set old roots and their deps as cached
    for ent <- :dets.select state.store, [{ {:"$1", :root, :"$2"},         [], [:"$$"] },
                                         { {:"$1", :root},                [], [:"$$"] },
                                         { {:"$1", {:req_by, :_}, :"$2"}, [], [:"$$"] },
                                         { {:"$1", {:req_by, :_}},        [], [:"$$"] }]
    do
      case ent do
        [id, bin] ->
          :dets.insert state.store, {id, cached_why, bin}
        [id] ->
          :dets.insert state.store, {id, cached_why}
      end
    end

    # Set new roots as roots
    for root <- roots do
      case :dets.lookup state.store, root do
        [{^root, _, bin}] ->
          :dets.insert state.store, {root, :root, bin}
          rec_set_dep(state, root, SData.term_unbin bin)
        [{^root, _}] ->
          :dets.insert state.store, {root, :root}
        [] ->
          init_rec_pull state, root, :root, []
      end
    end
    {:noreply, state}
  end

  defp rec_set_dep(state, hash, val0) do
    for dep <- SData.Page.refs val0 do
      case :dets.lookup state.store, dep do
        [{^dep, _, bin}] ->
          :dets.insert state.store, {dep, {:req_by, hash}, bin}
          rec_set_dep(state, dep, SData.term_unbin bin)
        [{^dep, _}] ->
          :dets.insert state.store, {dep, {:req_by, hash}}
        [] ->
          init_rec_pull state, dep, {:req_by, hash}, []
      end
    end
  end

  def handle_info(:clean_cache, state) do
    currtime = System.os_time :seconds

    cache_cleanup = [ {{:_, {:cached, :'$1'}, :_}, [{:<, :'$1', currtime}], [true]},
                      {{:_, {:cached, :'$1'}},     [{:<, :'$1', currtime}], [true]} ]
    :dets.select_delete(state.store, cache_cleanup)

    Process.send_after(self(), :clean_cache, @clean_cache_every * 1000)
    {:noreply, state}
  end

  defp ask_random_peers(state, key) do
    SNet.Group.broadcast(state.netgroup, {state.shard_id, state.path, {:get, key}}, nmax: 3)
  end

  defimpl SData.PageStore do
    def put(store, page) do
      bin = SData.term_bin page
      hash = GenServer.call(store.pid, {:put, bin})
      { hash, store }
    end

    def get(store, hash) do
      try do
        case GenServer.call(store.pid, {:get, hash, store.prefer_ask}) do
          nil -> nil
          bin -> SData.term_unbin bin
        end
      catch
        :exit, {:timeout, _} -> nil
      end
    end

    def copy(store, other_store, hash) do
      GenServer.cast(store.pid, {:rec_pull, hash, other_store.prefer_ask})
      store
    end

    def free(store, _hash) do
      store ## DO SOMETHING???
    end
  end

  # ====================
  # PAGE STORE INTERFACE
  # ====================

  @doc"""
  Returns `true` if the page store currently stores the specified root page
  and all its dependencies, recursively.
  """
  def have_rec?(pid, root) do
    GenServer.call(pid, {:have_rec, root})
  end

  @doc"""
  Define the set of root pages we are interested in. This will start pulling in
  the defined pages and all their dependencies recursively if we don't have them.
  """
  def set_roots(pid, roots) do
    GenServer.cast(pid, {:set_roots, roots})
  end

  @doc"""
  Delete the page store. The process is stopped and the data file is deleted from disk.
  """
  def delete_store(pid) do
    GenServer.call(pid, :delete_store)
  end
end