defmodule Fsdb do
  @moduledoc """
    Fsdb module.

    ```elixir
    #run with: mix sample_direct

    db = Path.expand("_build/dev/.fsdb")

    :ok = Fsdb.init(db)

    #drop to start over
    :ok = Fsdb.drop(db, "table1")

    :ok = Fsdb.create(db, "table1")

    #insert generates and returns an autoincrementing id
    {:ok, 1} = Fsdb.insert(db, "table1", "row1")
    {:ok, "row1"} = Fsdb.fetch(db, "table1", 1)

    #operation on nonexisting ids return not found
    :nf = Fsdb.delete(db, "table1", 5)
    :nf = Fsdb.update(db, "table1", 5, "row5+")

    #save overrides and updates id generator
    :ok = Fsdb.save(db, "table1", 5, "row5")
    :ok = Fsdb.save(db, "table1", 3, "row3")
    #continue +1 of largest id used/generated before
    {:ok, 6} = Fsdb.insert(db, "table1", "row6")

    #operations that get back previous value
    {:ok, "row5"} = Fsdb.update(db, "table1", 5, "row5+")
    {:ok, "row5+"} = Fsdb.save(db, "table1", 5, "row5++")
    {:ok, "row5++"} = Fsdb.delete(db, "table1", 5)

    #tuples may not be id ordered
    [{1, "row1"}, {3, "row3"}, {6, "row6"}] = Fsdb.list(db, "table1")
    ```
  """
  @rowre ~r/row\.[0-9]+/
  @rowid "row.id"

  def init(path) do
    File.mkdir_p!(path)
  end

  def create(path, table) do
    folder = Path.join(path, table)
    if !File.exists?(folder) do
      File.mkdir_p!(folder)
      file = Path.join(folder, @rowid)
      :ok = dump(file, 0)
    end
    :ok
  end

  def drop(path, table) do
    folder = Path.join(path, table)
    File.rm_rf!(folder)
    :ok #rm returns list of deleted files
  end

  def list(path, table) do
    folder = Path.join(path, table)
    for file <- File.ls!(folder),
      Regex.match?(@rowre, file) do
        parse(Path.join(folder, file))
    end
  end

  defp parse(file) do
    txt = File.read!(file) |> :erlang.binary_to_list
    {:ok, tokens, _} = :erl_scan.string(txt)
    {:ok, row} = :erl_parse.parse_term(tokens)
    row
  end

  def insert(path, table, row) do
    folder = Path.join(path, table)
    id = nextid(folder)
    file = Path.join(folder, rowid(id))
    :ok = dump(file, {id, row})
    {:ok, id}
  end

  def save(path, table, id, row) do
    folder = Path.join(path, table)
    file = Path.join(folder, rowid(id))
    if File.exists?(file) do
      {^id, crow} = parse(file)
      :ok = dump(file, {id, row})
      maxid(folder, id)
      {:ok, crow}
    else
      :ok = dump(file, {id, row})
      maxid(folder, id)
      :ok
    end
  end

  defp nextid(folder) do
    file = Path.join(folder, @rowid)
    id = parse(file) + 1
    :ok = dump(file, id)
    id
  end

  defp maxid(folder, id) do
    file = Path.join(folder, @rowid)
    cid = parse(file)
    mid = if id > cid, do: id, else: cid
    :ok = dump(file, mid)
    mid
  end

  defp dump(file, row) do
    File.write! file, :io_lib.format("~p.", [row])
  end

  def fetch(path, table, id) do
    folder = Path.join(path, table)
    file = Path.join(folder, rowid(id))
    if File.exists?(file) do
      {^id, row} = parse(file)
      {:ok, row}
    else
      :nf
    end
  end

  defp rowid(id) do
    fid = Integer.to_string(id) |> String.pad_leading(6, "0")
    "row.#{fid}"
  end

  def update(path, table, id, row) do
    folder = Path.join(path, table)
    file = Path.join(folder, rowid(id))
    if File.exists?(file) do
      {^id, crow} = parse(file)
      :ok = dump(file, {id, row})
      {:ok, crow}
    else
      :nf
    end
  end

  def delete(path, table, id) do
    folder = Path.join(path, table)
    file = Path.join(folder, rowid(id))
    if File.exists?(file) do
      {^id, row} = parse(file)
      File.rm!(file)
      {:ok, row}
    else
      :nf
    end
  end

end