defmodule ExToon.Decoder.Parser do
  @moduledoc false

  # Structural parser for TOON documents.
  #
  # Consumes a list of %Scanner{} structs and produces a flat list of stream events:
  #   %{type: :start_object}
  #   %{type: :end_object}
  #   %{type: :start_array, length: n}
  #   %{type: :end_array}
  #   %{type: :key, key: str}
  #   %{type: :primitive, value: val}
  #
  # Design:
  # - Recursive descent over the line list, tracking expected depth.
  # - Root form detection per §5: first depth-0 line determines array vs object vs primitive.
  # - Array forms: inline primitive, tabular rows, expanded list items.
  # - Object forms: key:value pairs at a given depth.

  alias ExToon.Decoder.Scanner
  alias ExToon.{DecodeError, LiteralUtils, StringUtils}

  @type event :: map()

  # Parse a list of Scanner structs into stream events.
  # Returns {:ok, [event()]} | {:error, term()}
  @spec parse([Scanner.t()], pos_integer(), boolean()) ::
          {:ok, [event()]} | {:error, term()}
  def parse(parsed_lines, indent_size, strict) do
    non_blank = Enum.reject(parsed_lines, fn l -> String.trim(l.raw) == "" end)

    # Strict-mode: blank lines inside arrays (depth > 0 context) are invalid.
    # We detect them by looking for blank lines that occur between two non-blank
    # lines where at least one surrounding non-blank line is at depth > 0.
    if strict do
      validate_no_blank_lines_in_arrays(parsed_lines)
    end

    # Strict-mode: multiple depth-0 non-kv lines with no structure = ambiguous root.
    if strict do
      depth0_unstructured = Enum.filter(non_blank, fn l ->
        l.depth == 0 and
          not match?({:array_header, _, _, _, _, _}, Scanner.parse_header(l.content)) and
          not is_kv_line?(l)
      end)

      if length(depth0_unstructured) > 1 do
        first_extra = Enum.at(depth0_unstructured, 1)
        throw({:decode_error, %DecodeError{
          line: first_extra.line_number,
          reason: :multiple_root_primitives,
          message:
            "multiple root-level primitives not allowed in strict mode at line #{first_extra.line_number}"
        }})
      end
    end

    case detect_root_form(non_blank) do
      :empty ->
        {:ok, [%{type: :start_object}, %{type: :end_object}]}

      {:root_primitive, line} ->
        {:ok, [%{type: :primitive, value: parse_primitive_token(line.content)}]}

      {:root_array, _line} ->
        parse_root_array(non_blank, indent_size, strict)

      :root_object ->
        parse_root_object(non_blank, indent_size, strict)
    end
  end

  # ---------------------------------------------------------------------------
  # Root form detection (§5)
  # ---------------------------------------------------------------------------

  defp detect_root_form([]), do: :empty

  defp detect_root_form(lines) do
    first = hd(lines)
    depth0_lines = Enum.filter(lines, &(&1.depth == 0))

    case Scanner.parse_header(first.content) do
      {:array_header, nil, _, _, _, _} ->
        # Keyless array header at root depth → root array
        {:root_array, first}

      {:array_header, _key, _, _, _, _} ->
        # Keyed array header at root → this is an object with an array-valued field
        :root_object

      _ ->
        if length(depth0_lines) == 1 and not is_kv_line?(first) and
             not String.starts_with?(first.content, "- ") do
          {:root_primitive, first}
        else
          :root_object
        end
    end
  end

  defp is_kv_line?(line) do
    Scanner.parse_kv(line.content) != :not_kv
  end

  # ---------------------------------------------------------------------------
  # Root array
  # ---------------------------------------------------------------------------

  defp parse_root_array([header_line | rest], indent_size, strict) do
    {:array_header, _nil_key, length, delimiter, fields, inline} =
      Scanner.parse_header(header_line.content)

    {body_events, _remaining} =
      parse_array_body(rest, 1, length, delimiter, fields, inline, indent_size, strict)

    {:ok,
     [%{type: :start_array, length: length}] ++ body_events ++ [%{type: :end_array}]}
  end

  # ---------------------------------------------------------------------------
  # Root object
  # ---------------------------------------------------------------------------

  defp parse_root_object(lines, indent_size, strict) do
    {events, _remaining} = parse_object_body(lines, 0, indent_size, strict)
    {:ok, [%{type: :start_object}] ++ events ++ [%{type: :end_object}]}
  end

  # ---------------------------------------------------------------------------
  # Object body: parse key-value pairs at the given depth
  # ---------------------------------------------------------------------------

  defp parse_object_body([], _depth, _indent_size, _strict), do: {[], []}

  defp parse_object_body([line | rest] = lines, depth, indent_size, strict) do
    if line.depth < depth do
      # Depth decreased — end of this object scope
      {[], lines}
    else
      case Scanner.parse_header(line.content) do
        {:array_header, key, length, delimiter, fields, inline} when key != nil ->
          # Array-valued field
          key_event = %{type: :key, key: key}
          start_event = %{type: :start_array, length: length}

          {body_events, remaining} =
            parse_array_body(rest, depth + 1, length, delimiter, fields, inline, indent_size, strict)

          end_event = %{type: :end_array}

          {more_events, final_remaining} =
            parse_object_body(remaining, depth, indent_size, strict)

          events =
            [key_event, start_event] ++
              body_events ++
              [end_event] ++
              more_events

          {events, final_remaining}

        _ ->
          case Scanner.parse_kv(line.content) do
            {:kv, key, value_str, was_quoted} ->
              key_event =
                if was_quoted,
                  do: %{type: :key, key: key, literal: true},
                  else: %{type: :key, key: key}

              {value_events, remaining} =
                parse_value(value_str, rest, depth, indent_size, strict)

              {more_events, final_remaining} =
                parse_object_body(remaining, depth, indent_size, strict)

              {[key_event] ++ value_events ++ more_events, final_remaining}

            :not_kv ->
              # In strict mode, an unrecognized line AT the exact object depth is an error.
              # Lines at deeper depth are silently skipped (they may belong to nested contexts
              # that the current pass doesn't own). In lenient mode, always skip.
              if strict and depth > 0 and line.depth == depth do
                throw({:decode_error, %DecodeError{
                  line: line.line_number,
                  reason: :syntax_error,
                  message:
                    "expected key:value or array header at line #{line.line_number}, got: #{inspect(line.content)}"
                }})
              else
                parse_object_body(rest, depth, indent_size, strict)
              end
          end
      end
    end
  end

  # ---------------------------------------------------------------------------
  # Value parsing (after "key:")
  # ---------------------------------------------------------------------------

  # Empty after colon — nested object or nested array
  defp parse_value("", rest, depth, indent_size, strict) do
    case rest do
      [] ->
        {[%{type: :start_object}, %{type: :end_object}], []}

      [next | _] ->
        if next.depth > depth do
          # Peek to decide: if next line is an array header without key, it's a nested array.
          # Otherwise it's a nested object.
          case Scanner.parse_header(next.content) do
            {:array_header, nil, length, delimiter, fields, inline} ->
              # Anonymous array header at child depth — nested array value
              [_header | after_header] = rest

              {body_events, remaining} =
                parse_array_body(
                  after_header,
                  depth + 2,
                  length,
                  delimiter,
                  fields,
                  inline,
                  indent_size,
                  strict
                )

              {[%{type: :start_array, length: length}] ++ body_events ++ [%{type: :end_array}],
               remaining}

            _ ->
              # Nested object
              {child_events, remaining} =
                parse_object_body(rest, depth + 1, indent_size, strict)

              {[%{type: :start_object}] ++ child_events ++ [%{type: :end_object}], remaining}
          end
        else
          {[%{type: :start_object}, %{type: :end_object}], rest}
        end
    end
  end

  # Inline primitive value after colon
  defp parse_value(value_str, rest, _depth, _indent_size, _strict) do
    value = parse_primitive_token(value_str)
    {[%{type: :primitive, value: value}], rest}
  end

  # ---------------------------------------------------------------------------
  # Array body dispatch
  # ---------------------------------------------------------------------------

  defp parse_array_body(rest, _depth, length, delimiter, nil, inline, _indent_size, strict)
       when inline != nil do
    # Inline primitive array — all values on the header line itself.
    # The first clause (fields == nil, inline != nil) handles cases where the
    # header had no field spec and all values are on the same line.
    values = parse_inline_values(inline, delimiter)

    if strict and length(values) != length do
      throw({:decode_error, %DecodeError{
        reason: :length_mismatch,
        message: "array declared length #{length} but found #{length(values)} inline values"
      }})
    end

    events = Enum.map(values, &%{type: :primitive, value: &1})
    {events, rest}
  end

  defp parse_array_body(rest, depth, length, delimiter, fields, inline, indent_size, strict) do
    cond do
      inline != nil ->
        values = parse_inline_values(inline, delimiter)

        if strict and length(values) != length do
          throw({:decode_error, %DecodeError{
            reason: :length_mismatch,
            message: "array declared length #{length} but found #{length(values)} inline values"
          }})
        end

        events = Enum.map(values, &%{type: :primitive, value: &1})
        {events, rest}

      fields != nil ->
        parse_tabular_rows(rest, depth, length, delimiter, fields, indent_size, strict)

      true ->
        parse_list_items(rest, depth, length, delimiter, indent_size, strict)
    end
  end

  # ---------------------------------------------------------------------------
  # Inline values: split by delimiter, parse each as primitive
  # ---------------------------------------------------------------------------

  defp parse_inline_values(str, delimiter) do
    str
    |> Scanner.split_by_delimiter(delimiter)
    |> Enum.map(fn token ->
      token
      |> String.trim()
      |> parse_primitive_token()
    end)
  end

  # ---------------------------------------------------------------------------
  # Tabular rows
  # ---------------------------------------------------------------------------

  defp parse_tabular_rows([], _depth, _length, _delimiter, _fields, _indent_size, _strict) do
    {[], []}
  end

  defp parse_tabular_rows(
         [line | rest] = lines,
         depth,
         length,
         delimiter,
         fields,
         indent_size,
         strict
       ) do
    # Blank line inside tabular array: strict mode throws, lenient mode skips.
    if String.trim(line.raw) == "" do
      if strict do
        throw({:decode_error, %DecodeError{
          line: line.line_number,
          reason: :blank_line_in_array,
          message: "blank line inside tabular array at line #{line.line_number}"
        }})
      else
        parse_tabular_rows(rest, depth, length, delimiter, fields, indent_size, strict)
      end
    else
      if line.depth != depth do
        # If we're in strict mode and there are MORE rows at this depth than declared,
        # we've already consumed what was expected; this line at a different depth ends the array.
        {[], lines}
      else
        # Strict mode: if we have already consumed the declared number of rows (length == 0),
        # but there is still a valid data row at the correct depth, that is a row-count mismatch.
        if strict and length <= 0 do
          throw({:decode_error, %DecodeError{
            line: line.line_number,
            reason: :row_count_mismatch,
            message:
              "tabular array has more rows than the declared count at line #{line.line_number}"
          }})
        end

        raw_values =
          line.content
          |> Scanner.split_by_delimiter(delimiter)
          |> Enum.map(&String.trim/1)

        values = Enum.map(raw_values, &parse_primitive_token/1)

        # Strict mode: validate that each row has exactly the right number of fields.
        if strict and length(values) != length(fields) do
          throw({:decode_error, %DecodeError{
            line: line.line_number,
            reason: :field_count_mismatch,
            message:
              "tabular row has #{length(values)} values but header declares #{length(fields)} fields at line #{line.line_number}"
          }})
        end

        field_events =
          fields
          |> Enum.zip(values)
          |> Enum.flat_map(fn {field, value} ->
            [%{type: :key, key: field}, %{type: :primitive, value: value}]
          end)

        row_events =
          [%{type: :start_object}] ++ field_events ++ [%{type: :end_object}]

        {more_events, remaining} =
          parse_tabular_rows(rest, depth, length - 1, delimiter, fields, indent_size, strict)

        {row_events ++ more_events, remaining}
      end
    end
  end

  # ---------------------------------------------------------------------------
  # Expanded list items
  # ---------------------------------------------------------------------------

  defp parse_list_items(lines, depth, expected_length, delimiter, indent_size, strict) do
    {events, remaining} = collect_items(lines, depth, delimiter, indent_size, strict, [])
    # Count items by counting :start_object/:primitive events at the item level
    # (before any nested object/array content). We approximate by counting parse results
    # that would come from the item-level loop. Instead, count by tracking in collect.
    # We pass expected_length to collect_items for strict validation.
    if strict do
      actual = count_items(events)
      if actual != expected_length do
        throw({:decode_error, %DecodeError{
          reason: :length_mismatch,
          message: "list array declared length #{expected_length} but found #{actual} items"
        }})
      end
    end
    {events, remaining}
  end

  # Count top-level items in the flattened event list.
  # Items are: start_object at depth 0, start_array at depth 0, or bare primitives at depth 0.
  defp count_items(events) do
    {count, _depth} =
      Enum.reduce(events, {0, 0}, fn event, {count, depth} ->
        case event do
          %{type: :start_object} when depth == 0 -> {count + 1, depth + 1}
          %{type: :start_object} -> {count, depth + 1}
          %{type: :end_object} -> {count, depth - 1}
          %{type: :start_array} when depth == 0 -> {count + 1, depth + 1}
          %{type: :start_array} -> {count, depth + 1}
          %{type: :end_array} -> {count, depth - 1}
          %{type: :primitive} when depth == 0 -> {count + 1, depth}
          _ -> {count, depth}
        end
      end)

    count
  end

  defp collect_items([], _depth, _delimiter, _indent_size, _strict, acc) do
    {acc |> Enum.reverse() |> List.flatten(), []}
  end

  defp collect_items([line | rest] = lines, depth, delimiter, indent_size, strict, acc) do
    # Blank line (or whitespace-only line) inside an expanded list array.
    if String.trim(line.raw) == "" do
      if strict and acc != [] do
        # Only error on blank lines that appear INSIDE the array (after at least one item).
        throw({:decode_error, %DecodeError{
          line: line.line_number,
          reason: :blank_line_in_array,
          message: "blank line inside list array at line #{line.line_number}"
        }})
      else
        collect_items(rest, depth, delimiter, indent_size, strict, acc)
      end
    else
      if line.depth < depth do
        {acc |> Enum.reverse() |> List.flatten(), lines}
      else
        content = line.content

        if String.starts_with?(content, "- ") or content == "-" do
          item_content =
            if content == "-", do: "", else: String.slice(content, 2, String.length(content))

          {item_events, remaining} =
            parse_list_item_content(item_content, rest, depth, delimiter, indent_size, strict)

          collect_items(remaining, depth, delimiter, indent_size, strict, [item_events | acc])
        else
          # Not a list item — stop collecting
          {acc |> Enum.reverse() |> List.flatten(), lines}
        end
      end
    end
  end

  # ---------------------------------------------------------------------------
  # List item content dispatch
  # ---------------------------------------------------------------------------

  # Bare hyphen — check for nested object at depth+1
  defp parse_list_item_content("", rest, depth, _delimiter, indent_size, strict) do
    case rest do
      [] ->
        {[%{type: :start_object}, %{type: :end_object}], []}

      [next | _] ->
        if next.depth > depth do
          {child_events, remaining} = parse_object_body(rest, depth + 1, indent_size, strict)
          {[%{type: :start_object}] ++ child_events ++ [%{type: :end_object}], remaining}
        else
          {[%{type: :start_object}, %{type: :end_object}], rest}
        end
    end
  end

  defp parse_list_item_content(item_content, rest, depth, _delimiter, indent_size, strict) do
    # Try array header first (for "- [N]:" or "- key[N]:" patterns)
    case Scanner.parse_header(item_content) do
      {:array_header, key, length, delim, fields, inline} when key != nil ->
        # Object with an array field on the hyphen line: "- key[N]{...}:"
        # Per spec: tabular/list body is at depth+2 (one for the item scope, one for the
        # array body level), while sibling object fields are at depth+1.
        key_event = %{type: :key, key: key}
        start_event = %{type: :start_array, length: length}

        {array_body, after_array} =
          parse_array_body(rest, depth + 2, length, delim, fields, inline, indent_size, strict)

        {more_obj, final_remaining} =
          parse_object_body(after_array, depth + 1, indent_size, strict)

        events =
          [%{type: :start_object}, key_event, start_event] ++
            array_body ++
            [%{type: :end_array}] ++
            more_obj ++
            [%{type: :end_object}]

        {events, final_remaining}

      {:array_header, nil, length, delim, fields, inline} ->
        # Root-style array as a list item: "- [N]:"
        start_event = %{type: :start_array, length: length}

        {array_body, remaining} =
          parse_array_body(rest, depth + 1, length, delim, fields, inline, indent_size, strict)

        {[start_event] ++ array_body ++ [%{type: :end_array}], remaining}

      :not_header ->
        # Try key:value (object starting on the hyphen line)
        case Scanner.parse_kv(item_content) do
          {:kv, key, value_str, was_quoted} ->
            key_event =
              if was_quoted,
                do: %{type: :key, key: key, literal: true},
                else: %{type: :key, key: key}

            {value_events, after_value} =
              parse_value(value_str, rest, depth, indent_size, strict)

            {more_events, remaining} =
              parse_object_body(after_value, depth + 1, indent_size, strict)

            events =
              [%{type: :start_object}, key_event] ++
                value_events ++
                more_events ++
                [%{type: :end_object}]

            {events, remaining}

          :not_kv ->
            # Plain primitive list item
            value = parse_primitive_token(String.trim(item_content))
            {[%{type: :primitive, value: value}], rest}
        end
    end
  end

  # ---------------------------------------------------------------------------
  # Primitive token parsing
  # ---------------------------------------------------------------------------

  # Quoted string — unescape inner content
  defp parse_primitive_token("\"" <> _ = quoted) do
    len = String.length(quoted)

    if len >= 2 and String.last(quoted) == "\"" do
      inner = String.slice(quoted, 1, len - 2)

      case StringUtils.unescape(inner) do
        {:ok, s} -> s
        # Return raw on bad escape — strict mode catches this separately
        {:error, _} -> quoted
      end
    else
      quoted
    end
  end

  defp parse_primitive_token(token), do: LiteralUtils.parse_primitive(token)

  # ---------------------------------------------------------------------------
  # Blank line validation (strict mode)
  # ---------------------------------------------------------------------------

  # In strict mode, blank lines inside arrays (between list items or tabular rows)
  # are forbidden. Blank lines between object fields are allowed.
  # Detection: a blank line is inside an array if it is preceded by a line that
  # looks like an array body line (starts with "- " or "- " indicator OR is a
  # tabular row at depth > 0 — i.e., not a kv pair and not an array header).
  defp validate_no_blank_lines_in_arrays(parsed_lines) do
    do_validate_blank_lines(parsed_lines, nil)
  end

  defp do_validate_blank_lines([], _prev_line), do: :ok

  defp do_validate_blank_lines([line | rest], prev_line) do
    if String.trim(line.raw) == "" do
      # Check surrounding non-blank context to decide if we're inside an array.
      next_line = find_next_non_blank(rest)

      # Only throw if BOTH surrounding lines indicate we're inside an array scope.
      # If the next line is at depth 0 (or nil), the blank line is outside the array.
      prev_in_array = is_array_body_line?(prev_line)
      next_in_array = is_array_body_line?(next_line)
      next_depth = if next_line, do: next_line.depth, else: 0
      prev_depth = if prev_line, do: prev_line.depth, else: 0

      if prev_in_array and next_in_array and next_depth == prev_depth do
        throw({:decode_error, %DecodeError{
          line: line.line_number,
          reason: :blank_line_in_array,
          message: "blank line inside array at line #{line.line_number}"
        }})
      else
        do_validate_blank_lines(rest, prev_line)
      end
    else
      do_validate_blank_lines(rest, line)
    end
  end

  defp find_next_non_blank([]), do: nil

  defp find_next_non_blank([line | rest]) do
    if String.trim(line.raw) == "", do: find_next_non_blank(rest), else: line
  end

  # An array body line is a list item ("- " prefix or bare "-") OR
  # a tabular row (at depth > 0, not a kv pair, not an array header).
  defp is_array_body_line?(nil), do: false

  defp is_array_body_line?(line) do
    cond do
      String.starts_with?(line.content, "- ") or line.content == "-" -> true
      line.depth > 0 and
          Scanner.parse_kv(line.content) == :not_kv and
          Scanner.parse_header(line.content) == :not_header -> true
      true -> false
    end
  end
end