import gleam/list
import gleam/dict.{type Dict}
import gleam/result

// A list is used as the map value as an empty list has the smallest
// representation in Erlang's binary format
@target(erlang)
type Token =
  List(Nil)

@target(erlang)
const token = []

@target(javascript)
type Token =
  Nil

@target(javascript)
const token = Nil

/// A set is a collection of unique members of the same type.
///
/// It is implemented using the `gleam/map` module, so inserts and lookups have
/// logarithmic time complexity.
///
pub opaque type Set(member) {
  Set(map: Dict(member, Token))
}

/// Creates a new empty set.
///
pub fn new() -> Set(member) {
  Set(dict.new())
}

/// Gets the number of members in a set.
///
/// This function runs in constant time.
///
/// ## Examples
///
/// ```gleam
/// new()
/// |> insert(1)
/// |> insert(2)
/// |> size
/// // -> 2
/// ```
///
pub fn size(set: Set(member)) -> Int {
  dict.size(set.map)
}

/// Inserts an member into the set.
///
/// This function runs in logarithmic time.
///
/// ## Examples
///
/// ```gleam
/// new()
/// |> insert(1)
/// |> insert(2)
/// |> size
/// // -> 2
/// ```
///
pub fn insert(into set: Set(member), this member: member) -> Set(member) {
  Set(map: dict.insert(set.map, member, token))
}

/// Checks whether a set contains a given member.
///
/// This function runs in logarithmic time.
///
/// ## Examples
///
/// ```gleam
/// new()
/// |> insert(2)
/// |> contains(2)
/// // -> True
/// ```
///
/// ```gleam
/// new()
/// |> insert(2)
/// |> contains(1)
/// // -> False
/// ```
///
pub fn contains(in set: Set(member), this member: member) -> Bool {
  set.map
  |> dict.get(member)
  |> result.is_ok
}

/// Removes a member from a set. If the set does not contain the member then
/// the set is returned unchanged.
///
/// This function runs in logarithmic time.
///
/// ## Examples
///
/// ```gleam
/// new()
/// |> insert(2)
/// |> delete(2)
/// |> contains(1)
/// // -> False
/// ```
///
pub fn delete(from set: Set(member), this member: member) -> Set(member) {
  Set(map: dict.delete(set.map, member))
}

/// Converts the set into a list of the contained members.
///
/// The list has no specific ordering, any unintentional ordering may change in
/// future versions of Gleam or Erlang.
///
/// This function runs in linear time.
///
/// ## Examples
///
/// ```gleam
/// new() |> insert(2) |> to_list
/// // -> [2]
/// ```
///
pub fn to_list(set: Set(member)) -> List(member) {
  dict.keys(set.map)
}

/// Creates a new set of the members in a given list.
///
/// This function runs in loglinear time.
///
/// ## Examples
///
/// ```gleam
/// import gleam/list
/// [1, 1, 2, 4, 3, 2] |> from_list |> to_list |> list.sort
/// // -> [1, 3, 3, 4]
/// ```
///
pub fn from_list(members: List(member)) -> Set(member) {
  let map =
    list.fold(over: members, from: dict.new(), with: fn(m, k) {
      dict.insert(m, k, token)
    })
  Set(map)
}

/// Combines all entries into a single value by calling a given function on each
/// one.
///
/// Sets are not ordered so the values are not returned in any specific order.
/// Do not write code that relies on the order entries are used by this
/// function as it may change in later versions of Gleam or Erlang.
///
/// # Examples
///
/// ```gleam
/// from_list([1, 3, 9])
/// |> fold(0, fn(accumulator, member) { accumulator + member })
/// // -> 13
/// ```
///
pub fn fold(
  over set: Set(member),
  from initial: acc,
  with reducer: fn(acc, member) -> acc,
) -> acc {
  dict.fold(over: set.map, from: initial, with: fn(a, k, _) { reducer(a, k) })
}

/// Creates a new set from an existing set, minus any members that a given
/// function returns `False` for.
///
/// This function runs in loglinear time.
///
/// ## Examples
///
/// ```gleam
/// import gleam/int
/// from_list([1, 4, 6, 3, 675, 44, 67])
/// |> filter(for: int.is_even)
/// |> to_list
/// // -> [4, 6, 44]
/// ```
///
pub fn filter(
  in set: Set(member),
  keeping predicate: fn(member) -> Bool,
) -> Set(member) {
  Set(dict.filter(in: set.map, keeping: fn(m, _) { predicate(m) }))
}

pub fn drop(
  from set: Set(member),
  drop disallowed: List(member),
) -> Set(member) {
  list.fold(over: disallowed, from: set, with: delete)
}

/// Creates a new map from a given map, only including any members which are in
/// a given list.
///
/// This function runs in loglinear time.
///
/// ## Examples
///
/// ```gleam
/// from_list([1, 2, 3])
/// |> take([1, 3, 5])
/// |> to_list
/// // -> [1, 3]
/// ```
///
pub fn take(
  from set: Set(member),
  keeping desired: List(member),
) -> Set(member) {
  Set(dict.take(from: set.map, keeping: desired))
}

fn order(
  first: Set(member),
  second: Set(member),
) -> #(Set(member), Set(member)) {
  case dict.size(first.map) > dict.size(second.map) {
    True -> #(first, second)
    False -> #(second, first)
  }
}

/// Creates a new set that contains all members of both given sets.
///
/// This function runs in loglinear time.
///
/// ## Examples
///
/// ```gleam
/// union(from_list([1, 2]), from_list([2, 3])) |> to_list
/// // -> [1, 2, 3]
/// ```
///
pub fn union(of first: Set(member), and second: Set(member)) -> Set(member) {
  let #(larger, smaller) = order(first, second)
  fold(over: smaller, from: larger, with: insert)
}

/// Creates a new set that contains members that are present in both given sets.
///
/// This function runs in loglinear time.
///
/// ## Examples
///
/// ```gleam
/// intersection(from_list([1, 2]), from_list([2, 3])) |> to_list
/// // -> [2]
/// ```
///
pub fn intersection(
  of first: Set(member),
  and second: Set(member),
) -> Set(member) {
  let #(larger, smaller) = order(first, second)
  take(from: larger, keeping: to_list(smaller))
}

/// Creates a new set that contains members that are present in the first set
/// but not the second.
///
/// ## Examples
///
/// ```gleam
/// difference(from_list([1, 2]), from_list([2, 3, 4])) |> to_list
/// // -> [1]
/// ```
///
pub fn difference(
  from first: Set(member),
  minus second: Set(member),
) -> Set(member) {
  drop(from: first, drop: to_list(second))
}