neiam/gitgud: lib/git_gud/pull_requests/suggestion.ex

defmodule GitGud.PullRequests.Suggestion do
  @moduledoc """
  A ```suggestion fenced block inside a line comment: the reviewer's
  proposed replacement for the line they commented on.

  Parsing lives here rather than in the Markdown renderer because
  applying one needs the raw replacement text, not its rendered HTML.
  """

  @fence ~r/^```suggestion[ \t]*\r?\n(?<body>.*?)\r?\n?^```[ \t]*$/ms

  @doc """
  The suggested replacement in `body`, or nil when there isn't one.

  An empty suggestion block is a real thing — it means "delete this
  line" — so it returns `""` rather than nil.
  """
  def parse(body) when is_binary(body) do
    case Regex.named_captures(@fence, body) do
      %{"body" => text} -> text
      _ -> nil
    end
  end

  def parse(_), do: nil

  @doc "True when `body` carries a suggestion block."
  def suggestion?(body), do: not is_nil(parse(body))

  @doc """
  The comment text with the suggestion block stripped out — what the
  reviewer said *about* the change, as opposed to the change itself.
  """
  def prose(body) when is_binary(body) do
    body
    |> String.replace(@fence, "")
    |> String.trim()
  end

  def prose(_), do: ""

  @doc """
  Replace 1-indexed `line` of `content` with `replacement`.

  A replacement spanning several lines expands in place; an empty one
  removes the line. Returns `{:error, :out_of_range}` when the file no
  longer has that line, which is how a stale suggestion surfaces.
  """
  def apply_to(content, line, replacement)
      when is_binary(content) and is_integer(line) and is_binary(replacement) do
    lines = String.split(content, "\n")

    if line < 1 or line > length(lines) do
      {:error, :out_of_range}
    else
      idx = line - 1
      replacement_lines = if replacement == "", do: [], else: String.split(replacement, "\n")

      updated =
        Enum.slice(lines, 0, idx) ++
          replacement_lines ++
          Enum.slice(lines, (idx + 1)..-1//1)

      {:ok, Enum.join(updated, "\n")}
    end
  end
end