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lib/external_configs/cursor_rule.ex
defmodule ExternalConfigs.CursorRule do
@moduledoc """
A parsed Cursor rule (`.mdc` file or legacy `.cursorrules`).
Cursor rules have four application modes determined by which frontmatter
fields are set:
* `:always` - `alwaysApply: true`. Injected unconditionally.
* `:auto_attached` - `globs` non-empty. Injected when a file already
in the chat context matches one of the globs (fnord triggers on file
read/write).
* `:agent_requested` - `description` set, no globs, not always. The
model sees the description and chooses whether to fetch.
* `:manual` - none of the above. Only fetched on explicit
request.
Legacy `.cursorrules` files at the project root are treated as a single
`:always` rule.
"""
defstruct [
:name,
:description,
:globs,
:always_apply,
:body,
:path,
:source,
:mode
]
@type source :: :global | :project | :legacy
@type mode :: :always | :auto_attached | :agent_requested | :manual
@type t :: %__MODULE__{
name: String.t(),
description: String.t() | nil,
globs: [String.t()],
always_apply: boolean(),
body: String.t(),
path: String.t(),
source: source(),
mode: mode()
}
@doc """
Load a `.mdc` rule file. The rule name is derived from the file basename
(without the `.mdc` extension).
"""
@spec from_file(String.t(), source()) :: {:ok, t} | {:error, term()}
def from_file(path, source) when is_binary(path) do
with {:ok, content} <- File.read(path),
{:ok, %{frontmatter: fm, body: body}} <-
ExternalConfigs.Frontmatter.parse(content) do
name = derive_name(path)
description = fetch_string(fm, "description")
globs = fetch_globs(fm, "globs")
always_apply = fetch_bool(fm, "alwaysApply", false)
{:ok,
%__MODULE__{
name: name,
description: description,
globs: globs,
always_apply: always_apply,
body: String.trim(body),
path: path,
source: source,
mode: classify(always_apply, globs, description)
}}
end
end
@doc """
Load a legacy `.cursorrules` file as a single `:always` rule. The body is
the full file contents (there is no frontmatter in the legacy format).
"""
@spec from_legacy_file(String.t()) :: {:ok, t} | {:error, term()}
def from_legacy_file(path) when is_binary(path) do
with {:ok, content} <- File.read(path) do
{:ok,
%__MODULE__{
name: ".cursorrules",
description: nil,
globs: [],
always_apply: true,
body: String.trim(content),
path: path,
source: :legacy,
mode: :always
}}
end
end
@doc """
Returns true when any of the rule's globs match the given path (relative
to the project source root).
"""
@spec matches_path?(t, String.t()) :: boolean()
def matches_path?(%__MODULE__{globs: []}, _path), do: false
def matches_path?(%__MODULE__{globs: globs}, path) when is_binary(path) do
Enum.any?(globs, &glob_match?(&1, path))
end
defp classify(true, _globs, _desc), do: :always
defp classify(false, globs, _desc) when globs != [], do: :auto_attached
defp classify(false, [], desc) when is_binary(desc) and desc != "", do: :agent_requested
defp classify(_, _, _), do: :manual
defp derive_name(path) do
path
|> Path.basename()
|> String.replace_suffix(".mdc", "")
end
defp fetch_string(fm, key) do
case Map.get(fm, key) do
v when is_binary(v) ->
case String.trim(v) do
"" -> nil
trimmed -> trimmed
end
_ ->
nil
end
end
defp fetch_bool(fm, key, default) do
case Map.get(fm, key) do
true -> true
false -> false
"true" -> true
"false" -> false
_ -> default
end
end
# Cursor stores `globs` as a comma-separated string by convention, but the
# YAML flow also allows a list. Accept either.
defp fetch_globs(fm, key) do
case Map.get(fm, key) do
nil ->
[]
list when is_list(list) ->
list
|> Enum.filter(&is_binary/1)
|> Enum.map(&String.trim/1)
|> Enum.reject(&(&1 == ""))
str when is_binary(str) ->
str
|> String.split(",")
|> Enum.map(&String.trim/1)
|> Enum.reject(&(&1 == ""))
_ ->
[]
end
end
# Path.wildcard compares against the filesystem; we need a pure pattern
# match against a string. Convert glob -> regex and test.
#
# Supported glob syntax:
# * matches any run of non-slash characters (does NOT cross /)
# ** matches anything including slashes (tail-position)
# **/ matches zero or more path segments (including empty)
# ? matches a single non-slash character
#
# Not supported (fall through as literal regex-escaped chars):
# {a,b,c} brace alternation
# [abc] character classes
# extglob +(...), @(...), !(...), ?(...)
#
# Cursor's own .mdc globs use the same subset in practice, so any rule
# that works in Cursor will match here.
defp glob_match?(glob, path) do
regex = glob_to_regex(glob)
Regex.match?(regex, path)
end
@spec glob_to_regex(String.t()) :: Regex.t()
defp glob_to_regex(glob) do
# Strip any leading "./" since paths we test are already project-relative.
glob =
glob
|> String.trim_leading("./")
|> String.trim_leading("/")
pattern = translate(glob, "")
Regex.compile!("^" <> pattern <> "$")
end
defp translate("", acc), do: acc
defp translate("**/" <> rest, acc), do: translate(rest, acc <> "(?:.*/)?")
defp translate("**", acc), do: acc <> ".*"
defp translate("*" <> rest, acc), do: translate(rest, acc <> "[^/]*")
defp translate("?" <> rest, acc), do: translate(rest, acc <> "[^/]")
defp translate(<<c::utf8, rest::binary>>, acc) do
translate(rest, acc <> escape_char(<<c::utf8>>))
end
defp escape_char(c) when c in [".", "+", "(", ")", "|", "^", "$", "{", "}", "[", "]", "\\"],
do: "\\" <> c
defp escape_char(c), do: c
end