defmodule RDF.Query.BGP.Simple do
  @moduledoc false

  @behaviour RDF.Query.BGP.Matcher

  alias RDF.Query.BGP
  alias RDF.Query.BGP.{QueryPlanner, BlankNodeHandler}
  alias RDF.{Graph, Description}

  @impl RDF.Query.BGP.Matcher
  def execute(bgp, graph, opts \\ [])

  # https://www.w3.org/TR/sparql11-query/#emptyGroupPattern
  def execute(%BGP{triple_patterns: []}, _, _), do: [%{}]

  def execute(%BGP{triple_patterns: triple_patterns}, %Graph{} = graph, opts) do
    {bnode_state, preprocessed_triple_patterns} = BlankNodeHandler.preprocess(triple_patterns)

    preprocessed_triple_patterns
    |> QueryPlanner.query_plan()
    |> do_execute(graph)
    |> BlankNodeHandler.postprocess(triple_patterns, bnode_state, opts)
  end

  @impl RDF.Query.BGP.Matcher
  def stream(bgp, graph, opts \\ []) do
    execute(bgp, graph, opts)
    |> Stream.into([])
  end

  defp do_execute([triple_pattern | remaining], graph) do
    do_execute(remaining, graph, match(graph, triple_pattern))
  end

  defp do_execute(triple_patterns, graph, solutions)

  defp do_execute(_, _, []), do: []

  defp do_execute([], _, solutions), do: solutions

  defp do_execute([triple_pattern | remaining], graph, solutions) do
    do_execute(remaining, graph, match_with_solutions(graph, triple_pattern, solutions))
  end

  defp match_with_solutions(graph, {s, p, o} = triple_pattern, existing_solutions)
       when is_tuple(s) or is_tuple(p) or is_tuple(o) do
    triple_pattern
    |> apply_solutions(existing_solutions)
    |> Enum.flat_map(&merging_match(&1, graph))
  end

  defp match_with_solutions(graph, triple_pattern, existing_solutions) do
    graph
    |> match(triple_pattern)
    |> Enum.flat_map(fn solution ->
      Enum.map(existing_solutions, &Map.merge(solution, &1))
    end)
  end

  defp apply_solutions(triple_pattern, solutions) do
    apply_solution =
      case triple_pattern do
        {{s}, {p}, {o}} -> fn solution -> {solution, {solution[s], solution[p], solution[o]}} end
        {{s}, {p}, o} -> fn solution -> {solution, {solution[s], solution[p], o}} end
        {{s}, p, {o}} -> fn solution -> {solution, {solution[s], p, solution[o]}} end
        {{s}, p, o} -> fn solution -> {solution, {solution[s], p, o}} end
        {s, {p}, {o}} -> fn solution -> {solution, {s, solution[p], solution[o]}} end
        {s, {p}, o} -> fn solution -> {solution, {s, solution[p], o}} end
        {s, p, {o}} -> fn solution -> {solution, {s, p, solution[o]}} end
        _ -> nil
      end

    if apply_solution do
      Stream.map(solutions, apply_solution)
    else
      solutions
    end
  end

  defp merging_match({dependent_solution, triple_pattern}, graph) do
    case match(graph, triple_pattern) do
      nil ->
        []

      solutions ->
        Enum.map(solutions, fn solution ->
          Map.merge(dependent_solution, solution)
        end)
    end
  end

  defp match(%Graph{descriptions: descriptions}, {subject_variable, _, _} = triple_pattern)
       when is_atom(subject_variable) do
    Enum.reduce(descriptions, [], fn {subject, description}, acc ->
      case match(description, solve_variables(subject_variable, subject, triple_pattern)) do
        nil ->
          acc

        solutions ->
          Enum.map(solutions, fn solution ->
            Map.put(solution, subject_variable, subject)
          end) ++ acc
      end
    end)
  end

  defp match(%Graph{} = graph, {subject, _, _} = triple_pattern) do
    case graph[subject] do
      nil -> []
      description -> match(description, triple_pattern)
    end
  end

  defp match(%Description{predications: predications}, {_, variable, variable})
       when is_atom(variable) do
    Enum.reduce(predications, [], fn {predicate, objects}, solutions ->
      if Map.has_key?(objects, predicate) do
        [%{variable => predicate} | solutions]
      else
        solutions
      end
    end)
  end

  defp match(%Description{predications: predications}, {_, predicate_variable, object_variable})
       when is_atom(predicate_variable) and is_atom(object_variable) do
    Enum.reduce(predications, [], fn {predicate, objects}, solutions ->
      solutions ++
        Enum.map(objects, fn {object, _} ->
          %{predicate_variable => predicate, object_variable => object}
        end)
    end)
  end

  defp match(
         %Description{predications: predications},
         {_, predicate_variable, object}
       )
       when is_atom(predicate_variable) do
    Enum.reduce(predications, [], fn {predicate, objects}, solutions ->
      if Map.has_key?(objects, object) do
        [%{predicate_variable => predicate} | solutions]
      else
        solutions
      end
    end)
  end

  defp match(
         %Description{predications: predications},
         {_, predicate, object_or_variable}
       ) do
    case predications[predicate] do
      nil ->
        []

      objects ->
        cond do
          # object_or_variable is a variable
          is_atom(object_or_variable) ->
            Enum.map(objects, fn {object, _} ->
              %{object_or_variable => object}
            end)

          # object_or_variable is a object
          Map.has_key?(objects, object_or_variable) ->
            [%{}]

          # else
          true ->
            []
        end
    end
  end

  defp solve_variables(var, val, {var, var, var}), do: {val, val, val}
  defp solve_variables(var, val, {s, var, var}), do: {s, val, val}
  defp solve_variables(var, val, {var, p, var}), do: {val, p, val}
  defp solve_variables(var, val, {var, var, o}), do: {val, val, o}
  defp solve_variables(var, val, {var, p, o}), do: {val, p, o}
  defp solve_variables(var, val, {s, var, o}), do: {s, val, o}
  defp solve_variables(var, val, {s, p, var}), do: {s, p, val}
  defp solve_variables(_, _, pattern), do: pattern
end