defmodule BrimstoneConditional do
  @external_resource Path.expand("./README.md")
  @moduledoc File.read!(Path.expand("./README.md"))
             |> String.split("<!-- INTRODUCTION START -->")
             |> Enum.at(1)
             |> String.split("<!-- INTRODUCTION END -->")
             |> List.first()

  require Integer

  @module_name String.Chars.to_string(__MODULE__)

  @valid_condition_args ~w{
    and nand or xor not nor xnor
    eq neq gt ge lt le in match
    cond fn var count each sum
    cat interpolate
  }a

  defstruct Enum.map(@valid_condition_args, &{&1, nil})

  @doc [
         "Evaluate conditions defined in a condition structure, returning the computed value.",
         File.read!(Path.expand("./README.md"))
         |> String.split("<!-- USAGE START -->")
         |> Enum.at(1)
         |> String.split("<!-- USAGE END -->")
         |> List.first()
       ]
       |> Enum.join("\n\n")
  def evaluate(%__MODULE__{} = condition, %{} = state \\ %{}),
    do: digest(condition, state, false)

  defp is_met?(%__MODULE__{} = condition, state),
    do: Enum.all?(digest(condition, state), &is_met?(&1, state))

  defp is_met?({key, nil}, _state) when key in @valid_condition_args,
    do: true

  defp is_met?({key, value}, state) when is_bitstring(key),
    do: is_met?({String.to_existing_atom(key), value}, state)

  defp is_met?({:not, item}, state) when not is_list(item),
    do: !is_met?(item, state)

  defp is_met?({:and, list}, state) when is_list(list),
    do: Enum.all?(digest(list, state), &is_met?(&1, state))

  defp is_met?({:nand, list}, state) when is_list(list),
    do: !Enum.all?(digest(list, state), &is_met?(&1, state))

  defp is_met?({:or, list}, state) when is_list(list),
    do: Enum.any?(digest(list, state), &is_met?(&1, state))

  defp is_met?({:nor, list}, state) when is_list(list),
    do: !Enum.any?(digest(list, state), &is_met?(&1, state))

  defp is_met?({:xor, list}, state) when is_list(list),
    do: Enum.reduce(digest(list, state), false, &if(is_met?(&1, state), do: !&2, else: &2))

  defp is_met?({:xnor, list}, state) when is_list(list) do
    list
    |> digest(state)
    |> Enum.map(&is_met?(&1, state))
    |> Enum.frequencies()
    |> Map.get(true, 0)
    |> Integer.is_even()
  end

  defp is_met?({key, list}, state) when (is_atom(key) or is_bitstring(key)) and is_list(list),
    do: Enum.all?(digest(list, state), &is_met?({key, &1}, state))

  defp is_met?(anything_else, _state),
    do: anything_else

  defp is_in?(content, container) when is_bitstring(container) and is_bitstring(content),
    do: String.contains?(container, content)

  defp is_in?(content, container) when is_map(container) and is_map(content),
    do: MapSet.subset?(MapSet.new(content), MapSet.new(container))

  defp is_in?(content, container) when is_list(container),
    do: Enum.member?(container, content)

  # Replace any `cond: [{conditions, value}, ...]` occurence with the first valid
  # option in maps and lists, recursively. Return any other value unchanged.
  defp apply_cond_switch({:cond, map}, state) when is_map(map) do
    map
    |> Map.to_list()
    |> apply_cond_switch(state)
    |> Map.new()
  end

  defp apply_cond_switch(list, state) when is_list(list),
    do: Enum.map(list, &apply_cond_switch(&1, state))

  defp apply_cond_switch({:cond, items}, state) when is_list(items) do
    Enum.find(items, {nil, nil}, &is_met?(elem(&1, 0), state))
    |> elem(1)
    |> apply_cond_switch(state)
  end

  defp apply_cond_switch({:cond, item}, state),
    do: apply_cond_switch({:cond, [item]}, state)

  defp apply_cond_switch({key, value}, state),
    do: {key, apply_cond_switch(value, state)}

  defp apply_cond_switch(value, _state),
    do: value

  defp digest(item, state, dont_recurse \\ false)

  defp digest(%__MODULE__{} = condition, state, false) do
    condition =
      condition
      |> Map.from_struct()
      |> Enum.reject(&is_nil(elem(&1, 1)))

    cond do
      Enum.count(condition) == 1 && is_tuple(List.first(condition)) &&
          elem(List.first(condition), 0) in @valid_condition_args ->
        {head, tail} = List.first(condition)
        digest({head, tail}, state)

      true ->
        digest(condition, state)
    end
  end

  defp digest(map, state, false) when is_map(map) do
    list =
      map
      |> Map.to_list()
      |> digest(state)

    if is_list(list) &&
         Enum.all?(list, &is_tuple/1) &&
         Enum.all?(list, &(tuple_size(&1) == 2)) &&
         Enum.all?(list, &(is_atom(elem(&1, 0)) || is_bitstring(elem(&1, 0)))),
       do: Map.new(list),
       else: list
  end

  defp digest({:match, [other | patterns]}, state, false) when not is_bitstring(other),
    do: digest({:match, ["#{other}" | patterns]}, state)

  defp digest({:match, [binary | patterns]}, state, false) when is_bitstring(binary) do
    Enum.reduce(
      digest(patterns, state),
      true,
      &if(
        &2 &&
          String.match?(
            binary,
            cond do
              match?(%Regex{}, &1) -> &1
              is_bitstring(&1) -> Regex.compile!(&1)
              is_tuple(&1) -> Regex.compile!(elem(&1, 0), elem(&1, 1))
              true -> Regex.compile!("^#{&1}$")
            end
          ),
        do: true,
        else: false
      )
    )
  end

  defp digest({:match, anything_else}, state, false),
    do: digest({:match, digest(anything_else, state)}, state)

  defp digest({:eq, list}, state, false) do
    list = digest(list, state)

    Enum.reduce_while(list, true, fn item, _ ->
      if(List.first(list) == item, do: {:cont, true}, else: {:halt, false})
    end)
  end

  defp digest({:neq, list}, state, false) do
    [head | tail] = digest(list, state)
    Enum.all?(digest(tail, state), &(digest(head, state) != &1))
  end

  defp digest({:gt, list}, state, false) do
    [head | tail] = digest(list, state)
    Enum.all?(digest(tail, state), &(digest(head, state) > &1))
  end

  defp digest({:ge, list}, state, false) do
    [head | tail] = digest(list, state)
    Enum.all?(digest(tail, state), &(digest(head, state) >= &1))
  end

  defp digest({:lt, list}, state, false) do
    [head | tail] = digest(list, state)
    Enum.all?(digest(tail, state), &(digest(head, state) < &1))
  end

  defp digest({:le, list}, state, false) do
    [head | tail] = digest(list, state)
    Enum.all?(digest(tail, state), &(digest(head, state) <= &1))
  end

  defp digest({:in, list}, state, false) do
    [head | tail] = digest(list, state)
    Enum.all?(digest(tail, state), &is_in?(digest(head, state), &1))
  end

  defp digest({key, nil}, state, false),
    do: {digest(key, state), nil}

  defp digest({:var, key}, state, false)
       when is_map(state) and (is_atom(key) or is_bitstring(key)),
       do: Map.get(state, digest(key, state))

  defp digest({:count, variables}, state, false),
    do: Enum.count(digest(variables, state))

  defp digest({:cat, variables}, state, false) do
    Enum.reduce(digest(variables, state), fn link, chain ->
      cond do
        is_nil(link) ->
          chain

        is_nil(chain) ->
          link

        is_boolean(chain) ->
          chain && link

        is_bitstring(chain) ->
          "#{chain}#{link}"

        is_list(chain) && is_list(link) ->
          chain ++ link

        is_list(chain) ->
          chain ++ [link]

        is_map(chain) && is_map(link) ->
          Map.merge(chain, link)

        is_map(chain) && Keyword.keyword?(link) ->
          Map.merge(chain, Map.new(link))

        is_integer(chain) && is_integer(link) ->
          String.to_integer("#{chain}#{link}")

        is_float(chain) && is_integer(link) ->
          String.to_float("#{chain}#{link}")

        is_float(chain) && is_float(link) ->
          String.to_float("#{chain}" <> String.replace("#{link}", ".", ""))

        is_tuple(chain) && is_tuple(link) ->
          List.to_tuple(Tuple.to_list(chain) ++ Tuple.to_list(link))

        is_tuple(chain) ->
          Tuple.append(chain, link)

        true ->
          "#{chain}#{link}"
      end
    end)
  end

  defp digest({:interpolate, variables}, state, false) do
    case digest(variables, state) do
      [string | replacements] ->
        Enum.reduce(
          digest(replacements, state),
          %{output: digest(string, state), free_variables_seen: 0},
          fn variable, acc ->
            case variable do
              {key, value} ->
                new_output = String.replace(acc[:output], "%{#{key}}", "#{value}")
                %{acc | output: new_output}

              value ->
                new_output =
                  String.replace(acc[:output], "%{#{acc.free_variables_seen + 1}}", "#{value}")

                %{acc | output: new_output, free_variables_seen: acc.free_variables_seen + 1}
            end
          end
        )
        |> Map.get(:output)

      anything_else ->
        anything_else
    end
  end

  defp digest({:each, key}, state, false)
       when is_map(state) and (is_atom(key) or is_bitstring(key)) do
    state
    |> Enum.filter(
      &String.match?("#{elem(&1, 0)}", Regex.compile!("#{digest(key, state)}\[[0-9]+\]$"))
    )
    |> Enum.map(&digest(elem(&1, 1), state))
  end

  defp digest({:sum, variables}, state, false),
    do: Enum.sum(Enum.map(digest(variables, state), &digest(&1, state)))

  defp digest({:fn, {module, name}}, state, false) when is_atom(module) and is_atom(name),
    do: digest({:fn, {module, name, []}}, state)

  defp digest({:fn, {module, name, args}}, state, false)
       when is_atom(module) and is_atom(name) and is_list(args) do
    arity = Enum.count(args)

    arity =
      if Code.ensure_loaded?(module) && function_exported?(module, name, arity + 1),
        do: arity + 1,
        else: arity

    digest({:fn, [Function.capture(module, name, arity)] ++ args}, state)
  end

  defp digest({:fn, function}, state, false) when is_function(function) or is_bitstring(function),
    do: digest({:fn, [function]}, state)

  defp digest({:fn, [function | args]}, state, false)
       when is_bitstring(function) and is_list(args),
       do:
         digest(
           {:fn, [elem(Code.eval_string("fn " <> function <> " end"), 0) | args]},
           state,
           false
         )

  defp digest({:fn, [function | args]}, state, false)
       when is_function(function) and is_list(args) do
    args = Enum.map(args, &digest(&1, state))

    if(:erlang.fun_info(function)[:arity] == Enum.count(args),
      do: apply(function, args),
      else: apply(function, [state] ++ args)
    )
  end

  defp digest({:cond, value}, state, false),
    do: apply_cond_switch({:cond, digest(value, state)}, state)

  defp digest({:not, item}, state, false),
    do: is_met?({:not, item}, state)

  defp digest({:and, list}, state, false),
    do: is_met?({:and, list}, state)

  defp digest({:nand, list}, state, false),
    do: is_met?({:nand, list}, state)

  defp digest({:or, list}, state, false),
    do: is_met?({:or, list}, state)

  defp digest({:nor, list}, state, false),
    do: is_met?({:nor, list}, state)

  defp digest({:xnor, list}, state, false),
    do: is_met?({:xnor, list}, state)

  defp digest({:xor, list}, state, false),
    do: is_met?({:xor, list}, state)

  defp digest({key, value}, state, false),
    do: {digest(key, state), digest(value, state)}

  defp digest(list, state, false) when is_list(list),
    do: digest(Enum.map(list, &digest(&1, state)), state, true)

  defp digest(anything_else, _state, _),
    do: anything_else

  @doc """
  Restore a conditional struct from its string form.
  """
  def from_string("module:#{@module_name};version:0.1.0;" <> data64),
    do: :erlang.binary_to_term(Base.decode64!(data64))

  defimpl String.Chars, for: __MODULE__ do
    @module_name String.replace("#{__MODULE__}", "Elixir.String.Chars", "Elixir")
    @current_struct_version "0.1.0"

    def to_string(instance),
      do:
        "module:#{@module_name};" <>
          "version:#{@current_struct_version};" <>
          Base.encode64(:erlang.term_to_binary(instance))
  end
end