credence

defmodule Credence.Pattern.NoDeadMapUpdate do
  @moduledoc """
  Detects a *no-op* `Map.update(key, literal, & &1) |> Map.drop([key])` (and the
  `Map.delete`/direct-call variants) where the updated value is immediately
  thrown away by dropping the same key.

  ## Safe core only

  In general `Map.update(map, key, default, fun)` runs `fun` on the **existing**
  value when `key` is present, and eagerly evaluates `default`. Removing the
  update would therefore drop any exception or side effect that `fun`/`default`
  produces — e.g. `%{prev: "x"} |> Map.update(:prev, 0, &(&1 - 1))` raises
  `ArithmeticError`, whereas `Map.drop(map, [:prev])` returns `%{}`. That is a
  behaviour change, so those forms are intentionally **not** flagged.

  This rule fires only when removing the update is provably output-identical:

  - `fun` is the identity capture `& &1` — `fun.(value) == value` for every
    value, so it can never raise or side-effect; and
  - `default` is a literal — a pure value whose eager evaluation has no
    observable effect.

  Under those two conditions the update is a genuine no-op before the key is
  dropped, so it can be removed for *any* input.

  ## Bad

      map |> Map.update(prev, 0, & &1) |> Map.drop([prev])
      Map.delete(Map.update(map, key, 0, & &1), key)

  ## Good

      Map.drop(map, [prev])
      Map.delete(map, key)

  ## Auto-fix

  Removes the dead identity `Map.update` call, keeping only the drop/delete.
  """

  use Credence.Pattern.Rule
  alias Credence.Issue

  @impl true
  def check(ast, _opts) do
    {_ast, issues} =
      Macro.prewalk(ast, [], fn node, acc ->
        case match_dead_update(node) do
          {:ok, meta} -> {node, [build_issue(meta) | acc]}
          :no_match -> {node, acc}
        end
      end)

    Enum.reverse(issues)
  end

  @impl true
  def fix_patches(ast, _opts) do
    Credence.RuleHelpers.patches_from_postwalk(ast, fn node ->
      case match_dead_update(node) do
        {:ok, _meta} -> simplify(node)
        :no_match -> node
      end
    end)
  end

  # Piped form: map |> Map.update(key, default, & &1) |> Map.drop([key_list])
  defp match_dead_update(
         {:|>, meta,
          [
            {:|>, _, [_map, update_call]},
            {{:., _, [{:__aliases__, _, [:Map]}, :drop]}, _, [drop_keys]}
          ]}
       ) do
    with {key, default, fun} <- piped_update_parts(update_call),
         true <- safe_core?(default, fun),
         true <- key_in_drop_list?(key, drop_keys) do
      {:ok, meta}
    else
      _ -> :no_match
    end
  end

  # Piped form: map |> Map.update(key, default, & &1) |> Map.delete(key)
  defp match_dead_update(
         {:|>, meta,
          [
            {:|>, _, [_map, update_call]},
            {{:., _, [{:__aliases__, _, [:Map]}, :delete]}, _, [del_key]}
          ]}
       ) do
    with {key, default, fun} <- piped_update_parts(update_call),
         true <- safe_core?(default, fun),
         true <- keys_match?(key, del_key) do
      {:ok, meta}
    else
      _ -> :no_match
    end
  end

  # Direct form: Map.drop(Map.update(map, key, default, & &1), [key_list])
  defp match_dead_update(
         {{:., meta, [{:__aliases__, _, [:Map]}, :drop]}, _,
          [
            {{:., _, [{:__aliases__, _, [:Map]}, :update]}, _, [_map, key, default, fun]},
            drop_keys
          ]}
       ) do
    if safe_core?(default, fun) and key_in_drop_list?(key, drop_keys) do
      {:ok, meta}
    else
      :no_match
    end
  end

  # Direct form: Map.delete(Map.update(map, key, default, & &1), key)
  defp match_dead_update(
         {{:., meta, [{:__aliases__, _, [:Map]}, :delete]}, _,
          [
            {{:., _, [{:__aliases__, _, [:Map]}, :update]}, _, [_map, key, default, fun]},
            del_key
          ]}
       ) do
    if safe_core?(default, fun) and keys_match?(key, del_key) do
      {:ok, meta}
    else
      :no_match
    end
  end

  defp match_dead_update(_), do: :no_match

  # Extract (key, default, fun) from a piped `Map.update(key, default, fun)`
  # call (the map is supplied by the pipe, so it is not in the arg list).
  defp piped_update_parts(
         {{:., _, [{:__aliases__, _, [:Map]}, :update]}, _, [key, default, fun]}
       ),
       do: {key, default, fun}

  defp piped_update_parts(_), do: :no_match

  # The two conditions that make removing the update output-identical for every
  # input: identity `fun` (never raises/side-effects) and a pure literal default.
  defp safe_core?(default, fun), do: identity_fun?(fun) and literal_default?(default)

  # `& &1` / `&(&1)` — the identity capture.
  defp identity_fun?({:&, _, [{:&, _, [1]}]}), do: true
  defp identity_fun?(_), do: false

  # Eager-evaluation-safe defaults: scalars (incl. negative numbers) and `[]`.
  defp literal_default?({:__block__, _, [[]]}), do: true

  defp literal_default?({:__block__, _, [v]}) when is_number(v) or is_atom(v) or is_binary(v),
    do: true

  defp literal_default?({:-, _, [{:__block__, _, [n]}]}) when is_number(n), do: true

  defp literal_default?(v) when is_number(v) or is_atom(v) or is_binary(v), do: true

  defp literal_default?(_), do: false

  # Replace the dead-update node with just the drop/delete, removing the update.
  defp simplify(
         {:|>, _,
          [
            {:|>, _, [map, _update_call]},
            {{:., _, [{:__aliases__, _, [:Map]}, :drop]}, _, [drop_keys]}
          ]}
       ) do
    {{:., [], [{:__aliases__, [], [:Map]}, :drop]}, [], [map, drop_keys]}
  end

  defp simplify(
         {:|>, _,
          [
            {:|>, _, [map, _update_call]},
            {{:., _, [{:__aliases__, _, [:Map]}, :delete]}, _, [del_key]}
          ]}
       ) do
    {{:., [], [{:__aliases__, [], [:Map]}, :delete]}, [], [map, del_key]}
  end

  defp simplify(
         {{:., _, [{:__aliases__, _, [:Map]}, :drop]}, _,
          [
            {{:., _, [{:__aliases__, _, [:Map]}, :update]}, _, [map | _]},
            drop_keys
          ]}
       ) do
    {{:., [], [{:__aliases__, [], [:Map]}, :drop]}, [], [map, drop_keys]}
  end

  defp simplify(
         {{:., _, [{:__aliases__, _, [:Map]}, :delete]}, _,
          [
            {{:., _, [{:__aliases__, _, [:Map]}, :update]}, _, [map | _]},
            del_key
          ]}
       ) do
    {{:., [], [{:__aliases__, [], [:Map]}, :delete]}, [], [map, del_key]}
  end

  defp simplify(node), do: node

  defp key_in_drop_list?(key, {:__block__, _, [list]}) when is_list(list) do
    Enum.any?(list, &keys_match?(key, &1))
  end

  defp key_in_drop_list?(key, list) when is_list(list) do
    Enum.any?(list, &keys_match?(key, &1))
  end

  defp key_in_drop_list?(_, _), do: false

  defp keys_match?({name, _, ctx}, {name, _, ctx2})
       when is_atom(name) and is_atom(ctx) and is_atom(ctx2),
       do: true

  defp keys_match?(literal, literal), do: true
  defp keys_match?(_, _), do: false

  defp build_issue(meta) do
    %Issue{
      rule: :no_dead_map_update,
      message:
        "`Map.update(key, literal, & &1) |> Map.drop([key])` discards a no-op update. " <>
          "Use `Map.drop(map, [key])` or `Map.delete(map, key)` directly.",
      meta: %{line: Keyword.get(meta, :line)}
    }
  end
end