hl7v2

defmodule HL7v2.Access do
  @moduledoc """
  Path-based access to HL7v2 message fields.

  Provides `get/2`, `get/3`, and `fetch/2` for extracting values from typed
  messages using path strings like `"PID-5"` or `"MSH-9.1"`.

  `get/2` returns nil on any resolution failure (silent). `fetch/2` returns
  `{:ok, value}` or `{:error, reason}` for explicit error handling.

  ## Path Syntax

      "PID"          — first PID segment struct
      "PID-5"        — PID field 5 (patient_name)
      "PID-5.1"      — first component of PID-5
      "PID-3[2]"     — second repetition of PID-3
      "MSH-9.1"      — message code from MSH-9
      "OBX[*]-5"     — field 5 from ALL OBX segments (returns list)
      "OBX[2]-5"     — field 5 from the 2nd OBX segment
      "PID-3[*]"     — ALL repetitions of PID-3 (returns list)

  ## Examples

      {:ok, msg} = HL7v2.parse(text, mode: :typed)
      patient_name = HL7v2.Access.get(msg, "PID-5")
      mrn = HL7v2.Access.get(msg, "PID-3.1")
      all_obx_values = HL7v2.Access.get(msg, "OBX[*]-5")

  """

  alias HL7v2.{Path, TypedMessage}

  @type path :: %{
          segment: binary(),
          segment_index: pos_integer() | :all | nil,
          field: pos_integer() | nil,
          component: pos_integer() | nil,
          repetition: pos_integer() | nil
        }

  # Regex groups:
  #   1: segment ID  (e.g. "OBX")
  #   2: segment index  (e.g. "2" or "*")        — optional [N] or [*] after segment
  #   3: field sequence  (e.g. "5")               — after "-"
  #   4: component  (e.g. "1")                    — after "."
  #   5: repetition index  (e.g. "2" or "*")      — trailing [N] or [*]
  @path_regex ~r/^([A-Z][A-Z0-9]{2})(?:\[(\d+|\*)\])?(?:-(\d+)(?:\.(\d+))?(?:\[(\d+|\*)\])?)?$/

  @doc """
  Gets a value from a typed message using a path string or `%Path{}` struct.

  Returns `nil` if the path doesn't resolve.
  """
  @spec get(TypedMessage.t(), binary() | Path.t()) :: term()
  def get(%TypedMessage{} = msg, %Path{} = path) do
    case resolve(msg, path_to_map(path)) do
      :invalid_repetition -> nil
      :invalid_component -> nil
      value -> value
    end
  end

  def get(%TypedMessage{} = msg, path) when is_binary(path) do
    case parse_path(path) do
      {:ok, parsed} ->
        case resolve(msg, parsed) do
          :invalid_repetition -> nil
          :invalid_component -> nil
          value -> value
        end

      {:error, _} ->
        nil
    end
  end

  @doc """
  Gets a value from a typed message with a default.
  """
  @spec get(TypedMessage.t(), binary() | Path.t(), term()) :: term()
  def get(%TypedMessage{} = msg, %Path{} = path, default) do
    case fetch(msg, path) do
      {:ok, value} -> value
      {:error, _} -> default
    end
  end

  def get(%TypedMessage{} = msg, path, default) when is_binary(path) do
    case fetch(msg, path) do
      {:ok, value} -> value
      {:error, _} -> default
    end
  end

  @doc """
  Fetches a value from a typed message, returning `{:ok, value}` or `{:error, reason}`.

  Unlike `get/2`, this function distinguishes between a nil field value and
  a resolution failure (unknown segment, invalid path, unknown field).

  Accepts both string paths and `%HL7v2.Path{}` structs.

  ## Examples

      {:ok, pid} = HL7v2.Access.fetch(msg, "PID")
      {:error, :segment_not_found} = HL7v2.Access.fetch(msg, "ZZZ")
      {:error, :invalid_path} = HL7v2.Access.fetch(msg, "not a path")
      {:error, :field_not_found} = HL7v2.Access.fetch(msg, "PID-99")

  """
  @spec fetch(TypedMessage.t(), binary() | Path.t()) :: {:ok, term()} | {:error, atom()}
  def fetch(%TypedMessage{} = msg, %Path{} = path) do
    resolve_with_error(msg, path_to_map(path))
  end

  def fetch(%TypedMessage{} = msg, path) when is_binary(path) do
    case parse_path(path) do
      {:ok, parsed} -> resolve_with_error(msg, parsed)
      {:error, _} = err -> err
    end
  end

  defp path_to_map(%Path{} = p) do
    %{
      segment: p.segment,
      segment_index: p.segment_index,
      field: p.field,
      component: p.component,
      repetition: p.repetition
    }
  end

  # -- Path Parsing --

  @doc false
  @spec parse_path(binary()) :: {:ok, path()} | {:error, :invalid_path}
  def parse_path(path) when is_binary(path) do
    case Regex.run(@path_regex, path) do
      nil ->
        {:error, :invalid_path}

      captures ->
        build_parsed(captures)
    end
  end

  defp build_parsed(captures) do
    segment = Enum.at(captures, 1)
    seg_idx = parse_index(Enum.at(captures, 2))
    field = parse_int(Enum.at(captures, 3))
    comp = parse_int(Enum.at(captures, 4))
    rep = parse_index(Enum.at(captures, 5))

    if seg_idx == :invalid_index or rep == :invalid_index do
      {:error, :invalid_path}
    else
      {:ok,
       %{
         segment: segment,
         segment_index: seg_idx,
         field: field,
         component: comp,
         repetition: rep
       }}
    end
  end

  defp parse_int(nil), do: nil
  defp parse_int(""), do: nil
  defp parse_int(s), do: String.to_integer(s)

  defp parse_index(nil), do: nil
  defp parse_index(""), do: nil
  defp parse_index("*"), do: :all
  defp parse_index("0"), do: :invalid_index
  defp parse_index(s), do: String.to_integer(s)

  # -- Resolution --

  # Segment wildcard: OBX[*] (no field)
  defp resolve(msg, %{segment_index: :all, field: nil} = path) do
    find_all_segments(msg, path.segment)
  end

  # Segment wildcard with field: OBX[*]-5
  defp resolve(msg, %{segment_index: :all} = path) do
    msg
    |> find_all_segments(path.segment)
    |> Enum.map(&resolve_field(&1, path))
  end

  # Specific segment index: OBX[2] (no field)
  defp resolve(msg, %{segment_index: idx, field: nil} = path) when is_integer(idx) do
    msg
    |> find_all_segments(path.segment)
    |> Enum.at(idx - 1)
  end

  # Specific segment index with field: OBX[2]-5
  defp resolve(msg, %{segment_index: idx} = path) when is_integer(idx) do
    case msg |> find_all_segments(path.segment) |> Enum.at(idx - 1) do
      nil -> nil
      segment -> resolve_field(segment, path)
    end
  end

  # No segment index (nil): first match — backwards compatible
  defp resolve(msg, %{segment_index: nil, field: nil} = path) do
    find_segment(msg, path.segment)
  end

  defp resolve(msg, %{segment_index: nil} = path) do
    case find_segment(msg, path.segment) do
      nil -> nil
      segment -> resolve_field(segment, path)
    end
  end

  defp find_segment(%TypedMessage{segments: segments}, seg_id) do
    Enum.find(segments, &segment_match?(&1, seg_id))
  end

  defp find_all_segments(%TypedMessage{segments: segments}, seg_id) do
    Enum.filter(segments, &segment_match?(&1, seg_id))
  end

  defp segment_match?(%HL7v2.Segment.ZXX{segment_id: id}, seg_id), do: id == seg_id
  defp segment_match?(%{__struct__: module}, seg_id), do: module.segment_id() == seg_id
  defp segment_match?({name, _fields}, seg_id), do: name == seg_id
  defp segment_match?(_, _seg_id), do: false

  defp resolve_field({_name, raw_fields}, %{field: field_seq}) when is_list(raw_fields) do
    # Raw tuple segment — return the field by position (1-indexed), no type awareness
    Enum.at(raw_fields, field_seq - 1)
  end

  defp resolve_field(segment, %{field: field_seq} = path) when is_struct(segment) do
    module = segment.__struct__
    fields = module.fields()

    case Enum.find(fields, fn {seq, _, _, _, _} -> seq == field_seq end) do
      {_, field_name, _, _, max_reps} ->
        value = Map.get(segment, field_name)
        unwrap_and_select(value, max_reps, path)

      nil ->
        nil
    end
  end

  defp resolve_field(_, _), do: nil

  # Unwrap repeating fields and select component
  defp unwrap_and_select(nil, max_reps, %{repetition: rep, component: comp}) do
    # Even for nil values, validate that selectors are legal
    case select_repetition(nil, max_reps, rep) do
      :invalid_repetition ->
        :invalid_repetition

      _ ->
        case select_component(nil, comp) do
          :invalid_component -> :invalid_component
          _ -> nil
        end
    end
  end

  defp unwrap_and_select(value, max_reps, %{repetition: rep, component: comp}) do
    case select_repetition(value, max_reps, rep) do
      :invalid_repetition -> :invalid_repetition
      selected -> select_component(selected, comp)
    end
  end

  # Non-repeating field: only nil (no selector) is valid
  defp select_repetition(value, 1, nil), do: value
  defp select_repetition(_value, 1, _rep), do: :invalid_repetition

  # Repetition wildcard: return all repetitions
  defp select_repetition(value, _max_reps, :all) when is_list(value), do: value

  defp select_repetition(value, _max_reps, rep) when is_list(value) do
    if rep != nil do
      Enum.at(value, rep - 1)
    else
      List.first(value)
    end
  end

  defp select_repetition(value, _max_reps, :all), do: [value]
  defp select_repetition(value, _max_reps, _rep), do: value

  defp select_component(nil, _), do: nil
  defp select_component(value, nil), do: value

  defp select_component(values, comp) when is_list(values) do
    Enum.map(values, &select_component(&1, comp))
  end

  defp select_component(value, comp) when is_struct(value) do
    if HL7v2.Segment.composite_type?(value.__struct__) do
      ordered_keys =
        value.__struct__.__info__(:struct)
        |> Enum.map(& &1.field)

      case Enum.at(ordered_keys, comp - 1) do
        nil -> nil
        key -> Map.get(value, key)
      end
    else
      # Primitive type (NM, SI, DT, etc.) — component access is invalid
      :invalid_component
    end
  end

  # Scalar/primitive value with component selector — not a composite type
  defp select_component(_value, _comp), do: :invalid_component

  # -- Error-returning resolution for fetch/2 --

  # Segment wildcard: OBX[*] (no field)
  defp resolve_with_error(msg, %{segment_index: :all, field: nil} = path) do
    case find_all_segments(msg, path.segment) do
      [] -> {:error, :segment_not_found}
      segments -> {:ok, segments}
    end
  end

  # Segment wildcard with field: OBX[*]-5
  defp resolve_with_error(msg, %{segment_index: :all} = path) do
    case find_all_segments(msg, path.segment) do
      [] ->
        {:error, :segment_not_found}

      segments ->
        results = Enum.map(segments, &resolve_field_with_error(&1, path))

        case Enum.find(results, &match?({:error, _}, &1)) do
          {:error, _} = err -> err
          nil -> {:ok, Enum.map(results, fn {:ok, v} -> v end)}
        end
    end
  end

  # Specific segment index: OBX[2] (no field)
  defp resolve_with_error(msg, %{segment_index: idx, field: nil} = path) when is_integer(idx) do
    case msg |> find_all_segments(path.segment) |> Enum.at(idx - 1) do
      nil -> {:error, :segment_not_found}
      seg -> {:ok, seg}
    end
  end

  # Specific segment index with field: OBX[2]-5
  defp resolve_with_error(msg, %{segment_index: idx} = path) when is_integer(idx) do
    case msg |> find_all_segments(path.segment) |> Enum.at(idx - 1) do
      nil -> {:error, :segment_not_found}
      segment -> resolve_field_with_error(segment, path)
    end
  end

  # No segment index: first match — backwards compatible
  defp resolve_with_error(msg, %{segment_index: nil, field: nil} = path) do
    case find_segment(msg, path.segment) do
      nil -> {:error, :segment_not_found}
      seg -> {:ok, seg}
    end
  end

  defp resolve_with_error(msg, %{segment_index: nil} = path) do
    case find_segment(msg, path.segment) do
      nil -> {:error, :segment_not_found}
      segment -> resolve_field_with_error(segment, path)
    end
  end

  defp resolve_field_with_error({_name, raw_fields}, %{field: field_seq})
       when is_list(raw_fields) do
    if field_seq > length(raw_fields) do
      {:error, :field_not_found}
    else
      case Enum.at(raw_fields, field_seq - 1) do
        nil -> {:ok, nil}
        value -> {:ok, value}
      end
    end
  end

  defp resolve_field_with_error(segment, %{field: field_seq} = path)
       when is_struct(segment) do
    module = segment.__struct__
    fields = module.fields()

    case Enum.find(fields, fn {seq, _, _, _, _} -> seq == field_seq end) do
      {_, field_name, _, _, max_reps} ->
        value = Map.get(segment, field_name)
        result = unwrap_and_select(value, max_reps, path)

        case result do
          :invalid_repetition -> {:error, :invalid_repetition}
          :invalid_component -> {:error, :invalid_component}
          nil when path.component != nil and value != nil -> {:error, :component_not_found}
          _ -> {:ok, result}
        end

      nil ->
        {:error, :field_not_found}
    end
  end
end