defmodule Squirrelix.Postgres do
  @moduledoc """
  Postgrex-backed query inferrer for Squirrelix inference.
  """

  alias Squirrelix.Column
  alias Squirrelix.ConnectionOptions
  alias Squirrelix.Error
  alias Squirrelix.Error.MissingPostgresColumn
  alias Squirrelix.Error.MissingPostgresTable
  alias Squirrelix.Error.PostgresInferenceError
  alias Squirrelix.Error.PostgresSyntaxError
  alias Squirrelix.Error.QueryHasInvalidEnum
  alias Squirrelix.Query
  alias Squirrelix.SQL
  alias Squirrelix.TypeMapper

  require Logger

  # $1 = relation name, $2 = column name, $3 = schema (NULL = search_path / temp).
  @column_nullability_query """
  select a.attnotnull
  from pg_attribute a
  join pg_class c on a.attrelid = c.oid
  join pg_namespace n on c.relnamespace = n.oid
  where c.relname = $1
    and a.attname = $2
    and a.attnum > 0
    and not a.attisdropped
    and (
      case
        when $3::text is null then n.nspname = any(current_schemas(true))
        when $3::text = 'pg_temp' or $3::text like 'pg_temp_%' then
          n.oid = pg_my_temp_schema()
        else n.nspname = $3
      end
    )
  limit 1
  """

  @type_lookup_query """
  with recursive types(oid, name, elem, kind, base, array_dimensions, jumps) as (
    select
      pg_type.oid as oid,
      pg_type.typname as name,
      pg_type.typelem as elem,
      pg_type.typtype as kind,
      pg_type.typbasetype as base,
      0 as array_dimensions,
      0 as jumps
    from pg_type
    where pg_type.oid = $1::oid
  union all
    select
      pg_type.oid as oid,
      pg_type.typname as name,
      pg_type.typelem as elem,
      pg_type.typtype as kind,
      pg_type.typbasetype as base,
      next_type.array_dimensions as array_dimensions,
      types.jumps + 1 as jumps
    from types
    join lateral (
      values
        (case when types.elem != 0 and types.name not in ('name', 'point') then types.elem end, types.array_dimensions + 1),
        (case when types.kind = 'd' then types.base end, types.array_dimensions)
    ) as next_type(oid, array_dimensions)
      on next_type.oid is not null
    join pg_type
      on pg_type.oid = next_type.oid
  )
  select types.name, types.kind, types.array_dimensions, types.oid
  from types
  order by types.jumps desc
  limit 1
  """

  @enum_variants_query """
  select enumlabel
  from pg_enum
  where enumtypid = $1::oid
  order by enumsortorder asc
  """

  @doc """
  Opens a Postgrex connection after a synchronous probe so connection failures
  and timeouts become structured Squirrelix errors.
  """
  @spec connect(ConnectionOptions.t()) :: {:ok, pid()} | {:error, struct()}
  def connect(%ConnectionOptions{} = connection_options) do
    postgrex_opts = postgrex_opts(connection_options)

    case probe_connection(postgrex_opts) do
      :ok ->
        case Postgrex.start_link(postgrex_opts) do
          {:ok, conn} ->
            {:ok, conn}

          {:error, reason} ->
            {:error, Error.connection_error(reason, connection_options)}
        end

      {:error, reason} ->
        {:error, Error.connection_error(reason, connection_options)}
    end
  end

  @spec inferrer(Postgrex.conn()) :: Squirrelix.Inference.inferrer()
  def inferrer(conn) do
    &infer(conn, &1)
  end

  @spec infer(Postgrex.conn(), Query.t()) :: {:ok, keyword()} | {:error, struct()}
  def infer(conn, %Query{} = query) do
    with {:ok, prepared_query} <- prepare(conn, query),
         {:ok, params} <- describe_oids(conn, prepared_query.param_oids || [], query),
         {:ok, returns} <- describe_returns(conn, prepared_query, query) do
      {:ok, [params: params, returns: returns]}
    end
  end

  defp postgrex_opts(%ConnectionOptions{} = connection_options) do
    [
      hostname: connection_options.host,
      port: connection_options.port,
      username: connection_options.user,
      password: connection_options.password,
      database: connection_options.database,
      timeout: connection_options.timeout_seconds * 1000,
      connect_timeout: connection_options.timeout_seconds * 1000,
      types: Postgrex.DefaultTypes
    ]
    |> Enum.reject(fn {_key, value} -> is_nil(value) end)
  end

  defp probe_connection(opts) do
    case Postgrex.Protocol.connect(opts) do
      {:ok, state} ->
        _ = Postgrex.Protocol.disconnect(:normal, state)
        :ok

      {:error, reason} ->
        {:error, reason}
    end
  end

  defp prepare(conn, query) do
    case Postgrex.prepare(conn, "", query.content) do
      {:ok, prepared_query} -> {:ok, prepared_query}
      {:error, %Postgrex.Error{} = error} -> {:error, postgres_error(query, error)}
    end
  end

  defp postgres_error(query, %Postgrex.Error{postgres: %{code: :syntax_error} = postgres}) do
    %PostgresSyntaxError{
      file: query.file,
      starting_line: query.starting_line,
      content: query.content,
      message: postgres.message,
      position: parse_position(postgres)
    }
  end

  defp postgres_error(query, %Postgrex.Error{postgres: %{code: :undefined_table} = postgres}) do
    %MissingPostgresTable{
      file: query.file,
      starting_line: query.starting_line,
      content: query.content,
      message: postgres.message,
      table: quoted_identifier(postgres.message),
      position: parse_position(postgres)
    }
  end

  defp postgres_error(query, %Postgrex.Error{postgres: %{code: :undefined_column} = postgres}) do
    %MissingPostgresColumn{
      file: query.file,
      starting_line: query.starting_line,
      content: query.content,
      message: postgres.message,
      column: quoted_identifier(postgres.message),
      position: parse_position(postgres)
    }
  end

  defp postgres_error(query, %Postgrex.Error{postgres: postgres}) do
    %PostgresInferenceError{
      file: query.file,
      starting_line: query.starting_line,
      content: query.content,
      message: Map.get(postgres, :message, "Postgres rejected query inference"),
      code: Map.get(postgres, :code),
      position: parse_position(postgres)
    }
  end

  defp parse_position(%{position: position}) when is_binary(position) do
    case Integer.parse(position) do
      {value, ""} -> value
      _invalid -> nil
    end
  end

  defp parse_position(_postgres), do: nil

  defp quoted_identifier(message) when is_binary(message) do
    case Regex.run(~r/"([^"]+)"/, message) do
      [_match, identifier] -> identifier
      nil -> nil
    end
  end

  defp quoted_identifier(_message), do: nil

  defp describe_returns(conn, prepared_query, query) do
    columns = prepared_query.columns || []
    result_oids = prepared_query.result_oids || []

    {plan_available?, plan_nullables, column_sources} =
      infer_nullability(conn, query, prepared_query)

    with {:ok, types} <- describe_oids(conn, result_oids, query) do
      returns =
        columns
        |> Enum.zip(types)
        |> Enum.with_index()
        |> Enum.map(fn {{name, type}, index} ->
          nullable? =
            column_nullable?(
              conn,
              name,
              index,
              plan_nullables,
              column_sources,
              plan_available?
            )

          %Column{name: name, type: type, nullable?: nullable?}
        end)

      {:ok, returns}
    end
  end

  # -- Plan-based nullability inference (mirrors upstream Gleam squirrel) --

  defp infer_nullability(conn, query, _prepared_query) do
    case query_plan(conn, query) do
      {:ok, plan} ->
        {true, nullables_from_plan(plan), column_sources_from_plan(plan)}

      :error ->
        {false, MapSet.new(), []}
    end
  end

  defp query_plan(conn, query) do
    content = explainable_query_content(query.content)

    if SQL.single_statement?(content) do
      # Simple protocol is required so `$n` placeholders work with
      # `generic_plan` without binding params. Multi-statement SQL is rejected
      # above; prepare/2 already validated the query as a single statement.
      explain_query = "explain (format json, verbose, generic_plan) " <> content

      try do
        with {:ok, %Postgrex.Result{rows: [[plan_json]]}} <-
               Postgrex.query(conn, explain_query, [], query_type: :text),
             {:ok, root_plan} <- decode_plan_json(plan_json) do
          {:ok, parse_plan(root_plan)}
        else
          _ ->
            warn_explain_unavailable(query.file)
            :error
        end
      rescue
        _ ->
          warn_explain_unavailable(query.file)
          :error
      end
    else
      Logger.warning(
        "Squirrelix: skipping EXPLAIN nullability for #{query.file} because the SQL is not a single statement"
      )

      :error
    end
  end

  defp warn_explain_unavailable(file) do
    Logger.warning(
      "Squirrelix: EXPLAIN nullability unavailable for #{file}; treating unknown columns as nullable"
    )
  end

  defp explainable_query_content(content) do
    content
    |> String.trim_leading()
    |> String.replace(~r/^;+\s*/, "")
  end

  defp decode_plan_json(plan_json) when is_binary(plan_json) do
    with {:ok, data} <- JSON.decode(plan_json) do
      decode_plan_json(data)
    end
  end

  defp decode_plan_json([%{"Plan" => root_plan} | _]), do: {:ok, root_plan}
  defp decode_plan_json(%{"Plan" => root_plan}), do: {:ok, root_plan}
  defp decode_plan_json(_), do: :error

  defp parse_plan(plan_map) do
    %{
      join_type: Map.get(plan_map, "Join Type"),
      output: Map.get(plan_map, "Output", []),
      relation: Map.get(plan_map, "Relation Name"),
      schema: Map.get(plan_map, "Schema"),
      plans: plan_map |> Map.get("Plans", []) |> Enum.map(&parse_plan/1)
    }
  end

  defp nullables_from_plan(plan) do
    outputs =
      plan.output
      |> Enum.with_index()
      |> Map.new(fn {expr, idx} -> {expr, idx} end)

    do_nullables_from_plan(plan, outputs, MapSet.new())
  end

  defp do_nullables_from_plan(plan, query_outputs, nullables) do
    case {plan.join_type, plan.plans} do
      {"Full", _} ->
        plan_outputs_indices(plan, query_outputs)
        |> MapSet.union(nullables)

      {"Right", [left, right]} ->
        nullables =
          plan_outputs_indices(left, query_outputs)
          |> MapSet.union(nullables)

        do_nullables_from_plan(right, query_outputs, nullables)

      {"Left", [left, right]} ->
        nullables =
          plan_outputs_indices(right, query_outputs)
          |> MapSet.union(nullables)

        do_nullables_from_plan(left, query_outputs, nullables)

      {"Semi", [left, right]} ->
        nullables =
          plan_outputs_indices(right, query_outputs)
          |> MapSet.union(nullables)

        do_nullables_from_plan(left, query_outputs, nullables)

      {"Inner", plans} ->
        Enum.reduce(plans, nullables, fn child, acc ->
          do_nullables_from_plan(child, query_outputs, acc)
        end)

      {_, plans} ->
        Enum.reduce(plans, nullables, fn child, acc ->
          do_nullables_from_plan(child, query_outputs, acc)
        end)
    end
  end

  defp plan_outputs_indices(plan, query_outputs) do
    Enum.reduce(plan.output, MapSet.new(), fn output, acc ->
      case Map.fetch(query_outputs, output) do
        {:ok, idx} -> MapSet.put(acc, idx)
        :error -> acc
      end
    end)
  end

  defp column_sources_from_plan(plan) do
    expr_to_source = collect_expr_sources(plan)

    Enum.map(plan.output, fn expr ->
      Map.get(expr_to_source, expr) || classify_output_expr(expr, nil, nil)
    end)
  end

  defp collect_expr_sources(%{relation: relation, schema: schema, output: output, plans: plans})
       when is_binary(relation) and relation != "" do
    own =
      output
      |> Enum.map(fn expr -> {expr, classify_output_expr(expr, schema, relation)} end)
      |> Map.new()

    Enum.reduce(plans, own, fn child, acc ->
      Map.merge(acc, collect_expr_sources(child))
    end)
  end

  defp collect_expr_sources(%{plans: plans}) do
    Enum.reduce(plans, %{}, fn child, acc ->
      Map.merge(acc, collect_expr_sources(child))
    end)
  end

  # Classify EXPLAIN "Output" entries into table columns, scalar subplans, or
  # expression-derived values. Matches Gleam squirrel's table_oid=0 behaviour
  # for expressions (non-nullable) while treating SubPlan outputs as nullable.
  defp classify_output_expr(expr, schema, relation) when is_binary(expr) do
    if Regex.match?(~r/^\(SubPlan \d+\)$/, expr) do
      :subquery
    else
      table_column_source(expr, schema, relation) || :expression
    end
  end

  defp classify_output_expr(_expr, _schema, _relation), do: :expression

  defp table_column_source(expr, schema, relation) do
    with {:ok, column} <- parse_column_ref(expr),
         table when is_binary(table) <- relation || table_from_qualified_expr(expr) do
      {:table_column, schema, table, column}
    else
      _ -> nil
    end
  end

  defp parse_column_ref(expr) do
    # Optional alias/table qualifier, then a single identifier (quoted or bare).
    # Anything else (operators, function calls, casts) is an expression.
    case Regex.run(
           ~r/^((?:[A-Za-z_][A-Za-z0-9_]*|"[^"]+")\.)?([A-Za-z_][A-Za-z0-9_]*|"[^"]+")$/,
           expr
         ) do
      [_match, _qualifier, column] -> {:ok, unquote_ident(column)}
      nil -> :error
    end
  end

  defp table_from_qualified_expr(expr) do
    case String.split(expr, ".", parts: 2) do
      [table, _column] -> unquote_ident(table)
      _ -> nil
    end
  end

  defp unquote_ident(<<"\"", rest::binary>>) do
    String.trim_trailing(rest, "\"")
  end

  defp unquote_ident(ident), do: ident

  defp column_nullable?(conn, name, index, plan_nullables, column_sources, plan_available?) do
    cond do
      String.ends_with?(name, "!") ->
        false

      String.ends_with?(name, "?") ->
        true

      MapSet.member?(plan_nullables, index) ->
        true

      true ->
        source_nullable?(conn, Enum.at(column_sources, index), plan_available?)
    end
  end

  defp source_nullable?(_conn, :subquery, _plan_available?), do: true
  defp source_nullable?(_conn, :expression, _plan_available?), do: false

  defp source_nullable?(conn, {:table_column, schema, table, column}, _plan_available?)
       when is_binary(table) and is_binary(column) do
    !column_has_not_null_constraint?(conn, schema, table, column)
  end

  defp source_nullable?(_conn, _other, plan_available?), do: !plan_available?

  defp column_has_not_null_constraint?(conn, schema, table, column) do
    case Postgrex.query(conn, @column_nullability_query, [table, column, schema]) do
      {:ok, %Postgrex.Result{rows: [[true]]}} -> true
      {:ok, %Postgrex.Result{rows: [[false]]}} -> false
      _ -> false
    end
  end

  defp describe_oids(conn, oids, query) do
    Enum.reduce_while(oids, {:ok, []}, fn oid, {:ok, types} ->
      case describe_oid(conn, oid, query) do
        {:ok, type} -> {:cont, {:ok, [type | types]}}
        {:error, error} -> {:halt, {:error, error}}
      end
    end)
    |> case do
      {:ok, types} -> {:ok, Enum.reverse(types)}
      {:error, error} -> {:error, error}
    end
  end

  defp describe_oid(conn, oid, query) do
    with {:ok, %Postgrex.Result{rows: [[name, kind, array_dimensions, type_oid]]}} <-
           Postgrex.query(conn, @type_lookup_query, [oid]) do
      resolve_postgres_type(conn, type_oid, name, kind, array_dimensions, query)
    end
  end

  defp resolve_postgres_type(conn, oid, name, "e", array_dimensions, query) do
    with {:ok, variants} <- enum_variants(conn, oid),
         :ok <- TypeMapper.validate_enum(name, variants),
         {:ok, type} <-
           TypeMapper.from_postgres(name, kind: "e", array_dimensions: array_dimensions) do
      {:ok, type}
    else
      {:error, :no_variants} ->
        {:error, invalid_enum_error(query, name, :no_variants)}

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

  defp resolve_postgres_type(_conn, _oid, name, kind, array_dimensions, _query) do
    TypeMapper.from_postgres(name, kind: kind, array_dimensions: array_dimensions)
  end

  defp enum_variants(conn, oid) do
    case Postgrex.query(conn, @enum_variants_query, [oid]) do
      {:ok, %Postgrex.Result{rows: rows}} ->
        {:ok, Enum.map(rows, fn [variant] -> variant end)}

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

  defp invalid_enum_error(query, enum_name, reason) do
    %QueryHasInvalidEnum{
      file: query.file,
      starting_line: query.starting_line,
      content: query.content,
      enum_name: enum_name,
      reason: reason
    }
  end
end