import filepath
import gleam/bool
import gleam/int
import gleam/list
import gleam/order
import gleam/result
import gleam/string
import repeatedly

// -- TYPES --------------------------------------------------------------------

/// A filesystem event.
/// 
/// Polly is careful when handing you events, and makes sure that you always get
/// them in the right order, so if a dictionary with some files was created,
/// you will get the dictionary event first, before any children.
pub type Event {
  /// A new file or dictionary was created!
  Created(String)
  /// A file got modified!
  Changed(String)
  /// A file or dictionary got deleted :(
  Deleted(String)
}

/// Some POSIX file system errors.
/// 
/// Only a handful of errors are handled by Polly, and everything else is collapsed into `EUnknown`.
pub type FsError {
  Eacces
  Enoent
  Enotdir
  EUnknown(String)
}

/// Polly uses the builder pattern to construct a watcher.
/// 
/// ```gleam
/// let options: Options = polly.new()
///     |> polly.add_dir("src")
///     |> polly.interval(3000)
///     |> polly.max_depth(10)
///     |> polly.filter(polly.default_filter)
/// ```
pub opaque type Options {
  Options(
    interval: Int,
    paths: List(String),
    max_depth: Int,
    filter: fn(String) -> Bool,
  )
}

/// A polling file system watcher, built from options.
/// 
/// Watchers are automatically started, and can by stopped by calling `stop`.
pub opaque type Watcher(state) {
  Watcher(repeater: repeatedly.Repeater(State(state)))
}

// -- OPTIONS BUILDER ----------------------------------------------------------

/// Start creating a new configuration using the default options.
/// 
/// By default, and interval of 1 second is set, and the `default_filter` is used.
pub fn new() -> Options {
  Options(interval: 1000, paths: [], max_depth: -1, filter: default_filter)
}

/// Tell Polly which directory to watch. If it does not exist, `watch` will return an error.
/// 
/// Paths are not expanded by default, so the paths reported by events and passed
/// to the filter function will be prefixed with whatever you specified here.
pub fn add_dir(options: Options, path: String) -> Options {
  Options(..options, paths: [path, ..options.paths])
}

/// Limit the maximum depth that Polly will walk each directory.
///
/// There is no limit by default, but setting a limit might be a good to
/// better control resource usage of the watcher.
pub fn max_depth(options: Options, max_depth: Int) -> Options {
  case options.max_depth < 0, options.max_depth > max_depth {
    True, _ | False, True -> Options(..options, max_depth: max_depth)
    False, False -> options
  }
}

/// Set the interval in-between file-system polls, in milliseconds.
/// 
/// This options is passed through to [repeatedly.call](https://hexdocs.pm/repeatedly/repeatedly.html#call).
pub fn interval(options: Options, interval: Int) -> Options {
  case interval > 0 {
    True -> Options(..options, interval: interval)
    False -> options
  }
}

/// Filter files using the given predicate.
/// 
/// Polly will ignore files and directories for which the predicate returns `False`
/// completely, and any event happening for them or for a contained file of them
/// will not get reported.
/// 
/// By default, all hidden files are ignored.
pub fn filter(options: Options, by filter: fn(String) -> Bool) -> Options {
  Options(..options, filter: filter)
}

/// The default filter function, ignoring hidden files starting with a colon `"."`
pub fn default_filter(path: String) -> Bool {
  let is_hidden = filepath.base_name(path) |> string.starts_with(".")
  is_hidden == False
}

// -- WATCH API ----------------------------------------------------------------

type State(state) =
  #(List(#(String, Vfs)), state)

/// Tell Polly to start watching all the specified directories for changes.
/// 
/// The callback is called synchronously after collecting all change events since
/// the last run. It is adviseable to move heavier cpu-bound tasks from this
/// callback into their own processes/threads.
/// 
/// On Erlang, the callback will be called in the same process as the watcher runs in.
pub fn watch(
  options: Options,
  with callback: fn(Event) -> ignored,
) -> Result(Watcher(Nil), #(String, FsError)) {
  watch_with(options, Nil, fn(_, event) {
    callback(event)
    Nil
  })
}

/// Like `watch`, but similar to `list.fold` Polly will also keep some state
/// around for you and pass it back on each invocation.
pub fn watch_with(
  options: Options,
  from initial: state,
  with callback: fn(state, Event) -> state,
) -> Result(Watcher(state), #(String, FsError)) {
  use roots <- result.try({
    use path <- list.try_map(options.paths)
    use #(vfs, _) <- result.map(
      hydrate(options.filter, options.max_depth, path, "") |> to_result(path),
    )
    #(path, vfs)
  })

  let state: State(state) = #(roots, initial)

  let repeater =
    repeatedly.call(options.interval, state, fn(state, _) {
      let #(roots, user_state) = state

      use #(new_roots, user_state), #(path, vfs) <- list.fold(roots, #(
        [],
        user_state,
      ))

      let assert Success(vfs, events) =
        diff(options.filter, options.max_depth, path, vfs, [])

      let user_state = list.fold(events, from: user_state, with: callback)
      #([#(path, vfs), ..new_roots], user_state)
    })

  Ok(Watcher(repeater))
}

/// Stop this watcher.
/// 
/// On Erlang, pending events may still be reported.
/// See the documentation of [repeatedly.stop](https://hexdocs.pm/repeatedly/repeatedly.html#stop) for more information.
pub fn stop(watcher: Watcher(state)) -> Nil {
  repeatedly.stop(watcher.repeater)
}

fn to_result(
  result: HydrationResult,
  path: String,
) -> Result(#(Vfs, List(Event)), #(String, FsError)) {
  case result {
    Success(vfs, events) -> Ok(#(vfs, events))
    Skipped -> Error(#(path, Enoent))
    Errored(path, err) -> Error(#(path, err))
  }
}

// -- IMPLEMENTATION -----------------------------------------------------------

type Vfs {
  File(name: String, modkey: Int)
  Folder(name: String, modkey: Int, children: List(Vfs))
}

type HydrationResult {
  Success(Vfs, List(Event))
  Skipped
  Errored(path: String, reason: FsError)
}

fn hydrate(
  filter: fn(String) -> Bool,
  depth: Int,
  path: String,
  name: String,
) -> HydrationResult {
  let full_path = filepath.join(path, name)
  use <- bool.guard(when: !filter(full_path), return: Skipped)

  case lstat(full_path) {
    Ok(stat) -> hydrate_stat(filter, depth, name, full_path, stat)
    Error(Eacces) | Error(Enoent) -> Skipped
    Error(e) -> Errored(full_path, e)
  }
}

fn try(
  path: String,
  result: Result(a, FsError),
  then: fn(a) -> HydrationResult,
) -> HydrationResult {
  case result {
    Ok(value) -> then(value)
    Error(err) -> Errored(path, err)
  }
}

fn hydrate_stat(
  filter: fn(String) -> Bool,
  depth: Int,
  name: String,
  full_path: String,
  stat: Stat,
) -> HydrationResult {
  // Only watch writable files, what are doing watching files that cannot change?
  use <- bool.guard(when: stat.access != ReadWrite, return: Skipped)

  case stat.type_ {
    Regular -> Success(File(name, get_modkey(stat)), [])

    Directory if depth == 0 -> {
      Success(Folder(name, get_modkey(stat), []), [])
    }

    Directory if depth != 0 -> {
      use entries <- try(full_path, readdir(full_path))

      case hydrate_children(filter, depth - 1, full_path, entries, []) {
        Ok(children) -> Success(Folder(name, get_modkey(stat), children), [])
        Error(#(path, reason)) -> Errored(path, reason)
      }
    }

    _ -> Skipped
  }
}

fn hydrate_children(
  filter: fn(String) -> Bool,
  depth: Int,
  path: String,
  children: List(String),
  acc: List(Vfs),
) -> Result(List(Vfs), #(String, FsError)) {
  case children {
    [] -> Ok(list.reverse(acc))
    [first, ..rest] ->
      case hydrate(filter, depth, path, first) {
        Success(vfs, _) ->
          hydrate_children(filter, depth, path, rest, [vfs, ..acc])
        Skipped -> hydrate_children(filter, depth, path, rest, acc)
        Errored(path, err) -> Error(#(path, err))
      }
  }
}

fn diff(
  filter: fn(String) -> Bool,
  depth: Int,
  path: String,
  vfs: Vfs,
  events: List(Event),
) -> HydrationResult {
  let full_path = filepath.join(path, vfs.name)

  use <- bool.guard(when: !filter(full_path), return: Skipped)

  case lstat(full_path) {
    Ok(stat) -> {
      // Only watch writable files, what are doing watching files that cannot change?
      use <- bool.guard(when: stat.access != ReadWrite, return: Skipped)

      case stat.type_, vfs {
        Regular, File(name:, modkey: old_key) -> {
          let new_key = get_modkey(stat)
          case new_key == old_key {
            True -> Success(vfs, events)
            // hurray for reuse
            False ->
              Success(File(name:, modkey: new_key), [
                Changed(full_path),
                ..events
              ])
          }
        }

        Directory, Folder(..) if depth == 0 -> {
          // TODO: we do not send change events for directories, so there is nothing to do.
          Success(vfs, events)
        }

        Directory, Folder(name:, modkey: _old_key, children: old_children)
          if depth != 0
        -> {
          let new_key = get_modkey(stat)
          // TODO: do we even want change events from dictionaries??
          use new_entries <- try(full_path, readdir(full_path))

          diff_children(
            filter,
            depth - 1,
            full_path,
            name,
            new_key,
            old_children,
            new_entries,
            [],
            events,
          )
        }

        _, _ ->
          case hydrate_stat(filter, depth, vfs.name, full_path, stat) {
            Success(new_vfs, _) -> {
              let events =
                list.append(deleted(path, vfs), created(path, new_vfs))
              Success(new_vfs, events)
            }

            _ as res -> res
          }
      }
    }

    // enoent might happen if the file is deleted while we are scanning
    Error(Eacces) | Error(Enoent) -> Skipped

    Error(e) -> {
      Errored(path, e)
    }
  }
}

fn diff_children(
  filter: fn(String) -> Bool,
  depth: Int,
  path: String,
  name: String,
  modkey: Int,
  old_children: List(Vfs),
  new_entries: List(String),
  new_children: List(Vfs),
  events: List(Event),
) -> HydrationResult {
  case old_children, new_entries {
    [], [] -> Success(Folder(name, modkey, list.reverse(new_children)), events)
    [first_old, ..rest_old], [first_new, ..rest_new] ->
      case string.compare(first_old.name, first_new) {
        order.Eq -> {
          case diff(filter, depth, path, first_old, events) {
            Success(new_vfs, events) -> {
              let new_children = [new_vfs, ..new_children]
              diff_children(
                filter,
                depth,
                path,
                name,
                modkey,
                rest_old,
                rest_new,
                new_children,
                events,
              )
            }

            Skipped -> {
              // the old file was here, but now we should skip it?
              // we treat this the same as if the file got deleted.
              let events = list.append(deleted(path, first_old), events)

              diff_children(
                filter,
                depth,
                path,
                name,
                modkey,
                rest_old,
                rest_new,
                new_children,
                events,
              )
            }

            Errored(_, _) as err -> err
          }
        }

        order.Gt -> {
          // created a file
          case hydrate(filter, depth, path, first_new) {
            Success(new_vfs, _) -> {
              let events = list.append(created(path, new_vfs), events)
              let new_children = [new_vfs, ..new_children]
              diff_children(
                filter,
                depth,
                path,
                name,
                modkey,
                old_children,
                rest_new,
                new_children,
                events,
              )
            }

            Skipped ->
              diff_children(
                filter,
                depth,
                path,
                name,
                modkey,
                old_children,
                rest_new,
                new_children,
                events,
              )
            Errored(_, _) as err -> err
          }
        }

        order.Lt -> {
          // deleted a file
          let events = list.append(deleted(path, first_old), events)
          diff_children(
            filter,
            depth,
            path,
            name,
            modkey,
            rest_old,
            new_entries,
            new_children,
            events,
          )
        }
      }

    [], [first_new, ..rest_new] -> {
      case hydrate(filter, depth, path, first_new) {
        Success(new_vfs, _) -> {
          let events = list.append(created(path, new_vfs), events)
          let new_children = [new_vfs, ..new_children]
          diff_children(
            filter,
            depth,
            path,
            name,
            modkey,
            old_children,
            rest_new,
            new_children,
            events,
          )
        }

        Skipped ->
          diff_children(
            filter,
            depth,
            path,
            name,
            modkey,
            old_children,
            rest_new,
            new_children,
            events,
          )
        Errored(_, _) as err -> err
      }
    }

    [first_old, ..rest_old], [] -> {
      // deleted all the remaining old ones
      let events = list.append(deleted(path, first_old), events)
      diff_children(
        filter,
        depth,
        path,
        name,
        modkey,
        rest_old,
        new_entries,
        new_children,
        events,
      )
    }
  }
}

fn created(path: String, vfs: Vfs) -> List(Event) {
  list.reverse(do_created(path, vfs, []))
}

fn do_created(path: String, vfs: Vfs, acc: List(Event)) -> List(Event) {
  let full_path = filepath.join(path, vfs.name)
  let acc = [Created(full_path), ..acc]
  case vfs {
    File(..) -> acc
    Folder(children:, ..) -> {
      use acc, child <- list.fold(children, acc)
      do_created(full_path, child, acc)
    }
  }
}

fn deleted(path: String, vfs: Vfs) -> List(Event) {
  // no list.reverse - deleted events are in reverse order from created
  do_deleted(path, vfs, [])
}

fn do_deleted(path: String, vfs: Vfs, acc: List(Event)) -> List(Event) {
  let full_path = filepath.join(path, vfs.name)
  let acc = [Deleted(full_path), ..acc]
  case vfs {
    File(..) -> acc
    Folder(children:, ..) -> {
      use acc, child <- list.fold(children, acc)
      do_deleted(full_path, child, acc)
    }
  }
}

fn get_modkey(stat: Stat) -> Int {
  int.max(stat.mtime, stat.ctime)
}

// -- EXTERNAL -----------------------------------------------------------------

@internal
pub type Stat {
  Stat(
    size: Int,
    type_: FileType,
    access: FileAccess,
    atime: Int,
    mtime: Int,
    ctime: Int,
    mode: Int,
    links: Int,
    major_device: Int,
    // character devices on unix
    minor_device: Int,
    // not on windows
    inode: Int,
    // not on windows
    uid: Int,
    // not on windows
    gid: Int,
  )
}

@internal
pub type FileType {
  Device
  Directory
  Symlink
  Other
  Regular
}

@internal
pub type FileAccess {
  Read
  Write
  ReadWrite
  None
}

fn map_error(result: Result(a, FsError)) -> Result(a, FsError) {
  use err <- result.map_error(result)
  case err {
    Eacces | Enotdir | Enoent -> err
    _ -> EUnknown(string.inspect(err))
  }
}

fn readdir(path: String) -> Result(List(String), FsError) {
  do_readdir(path)
  |> map_error
  |> result.map(list.sort(_, by: string.compare))
}

@external(erlang, "polly_ffi", "readdir")
@external(javascript, "./polly_ffi.mjs", "readdir")
fn do_readdir(path: String) -> Result(List(String), FsError)

fn lstat(path: String) -> Result(Stat, FsError) {
  map_error(do_lstat(path))
}

@external(erlang, "polly_ffi", "lstat")
@external(javascript, "./polly_ffi.mjs", "lstat")
fn do_lstat(path: String) -> Result(Stat, FsError)