defmodule GenFST do
  @moduledoc """
  GenFST implements a generic finite state transducer with
  customizable rules expressed in a DSL.

  A finite-state transducer (FST) is a finite-state machine 
  with two memory tapes, following the terminology for Turing 
  machines: an input tape and an output tape.

  A FST will read a set of strings on the input tape and 
  generates a set of relations on the output tape. An FST 
  can be thought of as a translator or relater between strings in a set.

  In morphological parsing, an example would be inputting a string of letters 
  into the FST, the FST would then output a string of 
  [morphemes](https://en.wikipedia.org/wiki/Morphemes).

  ## Example

  Here we implement a simple morphological parser for English language. This
  morphological parser recognize different inflectional morphology of the verbs.

  ```elixir
  fst = GenFST.new
  |> GenFST.rule(["play", {"s", "^s"}])
  |> GenFST.rule(["act", {"s", "^s"}])
  |> GenFST.rule(["act", {"ed", "^ed"}])
  |> GenFST.rule(["act", {"ing", ""}])

  assert "play^s" == fst |> GenFST.parse("plays")
  ```

  For example if we pass the third-person singluar tense of the verb _act_,
  `GenFST.parse(fst, "acts")`, the morphological parser will output
  `"act^s"`. The semantic of rule definition is given at `rule/2`.

  """

  @opaque fst :: %Graph{}

  @doc """
  Create a new finite state transducer.

  See example usage in [Module Example](#module-example)
  """
  @spec new :: fst
  def new do
    Graph.new
  end

  @doc """
  Define a transducing rule, adding it to the fst

  A transducing rule is a `List` of `String.t | {String.t, String.t}`.

  For example: `rule fst, ["play", {"s", "^s"}]` means outputing `"play"` verbatimly,
  and transform `"s"` into `"^s"`. If a finite state transducer built with
  this rule is fed with string `"plays"`, then the output will be `"play^s"`

  See example usage in [Module Example](#module-example)
  """
  @type fst_rule :: String.t | {String.t, String.t}
  @spec rule(fst, [fst_rule]) :: fst
  def rule(fst, r) do
    process_rule(fst, r)
  end

  @doc """
  Parse the input by transducing it with the given fst.

  See example usage in [Module Example](#module-example)
  """
  @spec parse(fst, String.t) :: String.t
  def parse(fst, input) do
    input_cps = String.codepoints(input)
    transduced = transduce(fst, input_cps, {:root, :initial}, "")
    transduced_len = Enum.count(transduced)

    cond do
      transduced_len == 0 ->
        {:error, "not possible"}
      transduced_len == 1 ->
        {:ok, List.first(transduced)}
      transduced_len > 1 ->
        {:ambigious, transduced}
    end

  end

  defp transduce(_fst_graph, [] = _input_cps, state, transduced) do
    if elem(state, 1) == :terminal do
      [transduced]
    else
      nil
    end
  end

  defp transduce(fst_graph, [x | xs] = _input_cps, state, transduced) do
    edges = Enum.filter(Graph.out_edges(fst_graph, state), fn(edge) -> 
      {e_from, _e_to} = edge.label
      e_from == x
    end)
    
    possible_transduced = for edge <- edges do
      transduce(fst_graph, xs, edge.v2, transduced <> elem(edge.label, 1))
    end
    List.flatten(Enum.filter(possible_transduced, fn(x) -> x end))
  end

  @doc false
  defp process_rule(fst_graph, rule) do
    rule_length = Enum.count(rule)
    {fst_graph, _, _} =  Enum.reduce(Enum.with_index(rule), {fst_graph, {:root, :initial}, ""}, fn({rule_item, i}, {fst_graph, vertex, prefix}) ->
      process_rule_item(fst_graph, vertex, prefix, rule_item, i == rule_length - 1)
    end)
    fst_graph
  end

  @doc false
  defp process_rule_item(fst_graph, vertex, prefix, rule_item, is_terminal) do
    if is_binary rule_item do
      Enum.reduce(String.codepoints(rule_item), {fst_graph, vertex, prefix}, fn(char, {fst_graph, vertex, prefix}) -> 
        process_rule_item_char(fst_graph, vertex, prefix, {char, char}, is_terminal)
      end)
    else
      {from, to} = rule_item
      from_len = String.length from
      to_len = String.length to
      cond do
        from_len > to_len ->
          h_from = String.slice(from, 0, to_len)
          h_to = String.slice(from, 0, to_len)
          h_cps_pairs = Enum.zip(String.codepoints(h_from), String.codepoints(h_to))

          {fst_graph, vertex, prefix} = Enum.reduce(h_cps_pairs,
                      {fst_graph, vertex, prefix}, 
                      fn({from_cp, to_cp}, {fst_graph, vertex, prefix}) -> 
            process_rule_item_char(fst_graph, vertex, prefix, {from_cp, to_cp}, false)
          end)

          t_from_cps = String.codepoints(String.slice(from, to_len, from_len))
          t_from_cps_len = Enum.count(t_from_cps)
          {fst_graph, vertex, prefix} = Enum.reduce(Enum.with_index(t_from_cps),
                      {fst_graph, vertex, prefix}, 
                      fn({t_from_cp, i}, {fst_graph, vertex, prefix}) -> 
            process_rule_item_char(fst_graph, vertex, prefix, {t_from_cp, ""}, t_from_cps_len - 1 == i)
          end)

        from_len < to_len -> 
          h_from = String.slice(from, 0, from_len - 1)
          h_to = String.slice(to, 0, from_len - 1)
          h_cps_pairs = Enum.zip(String.codepoints(h_from), String.codepoints(h_to))
          {fst_graph, vertex, prefix} = Enum.reduce(h_cps_pairs,
                      {fst_graph, vertex, prefix}, 
                      fn({from_cp, to_cp}, {fst_graph, vertex, prefix}) -> 
            process_rule_item_char(fst_graph, vertex, prefix, {from_cp, to_cp}, false)
          end)
          process_rule_item_char(fst_graph, vertex, prefix, {
            String.slice(from, from_len - 1, from_len),
            String.slice(to, from_len - 1, to_len)
          }, true)

        from_len == to_len ->
          cps_pairs = Enum.zip(String.codepoints(from), String.codepoints(to))
          cps_pairs_len = Enum.count(cps_pairs)
          Enum.reduce(Enum.with_index(cps_pairs),
                      {fst_graph, vertex, prefix}, 
                      fn({{from_cp, to_cp}, i}, {fst_graph, vertex, prefix}) -> 
            process_rule_item_char(fst_graph, vertex, prefix, {from_cp, to_cp}, i == cps_pairs_len - 1)
          end)

      end
    end
  end

  @doc false
  defp process_rule_item_char(fst_graph, vertex, prefix, {from, to}, is_terminal) do
    new_prefix = prefix <> from
    target_v = if is_terminal do
      {new_prefix <> ":" <> to, :terminal}
    else
      {new_prefix <> ":" <> to, :transitional}
    end
    edge = Graph.Edge.new(vertex, target_v, label: {from, to})
    {Graph.add_edge(fst_graph, edge), target_v, new_prefix}
  end
end