defmodule InPlace.LinkedList do
  @moduledoc """
  [Singly linked list](https://en.wikipedia.org/wiki/Linked_list#Singly_linked_list)
  The data entries are stored as integers.
  They will be interpreted (as references) by the calling application.
  Note: indices are 1-based.
  Options:
    - `mode` :: :singly_linked | :doubly_linked
    - `circular` :: boolean()
      optional, `false` by default;
    - `undo` :: boolean()
      If true (`false` by default), the removed elements can be restored, one at time, in reverse order
       (see `restore/1`). This can be used as a mechanism for backtracking
    - `:mapper_fun`  # maps data entries to application data
      optional, &Function.identity/1 by default
  """
  alias InPlace.{Array, Stack}

  ## List terminator
  @terminator 0
  @singly_linked_mode :singly_linked
  @doubly_linked_mode :doubly_linked

  def new(size, opts \\ []) when is_integer(size) and size > 0 do
    opts = Keyword.merge(default_opts(), opts)
    mode = Keyword.get(opts, :mode)
    circular? = Keyword.get(opts, :circular)
    undo? = Keyword.get(opts, :undo) && circular? && mode == @doubly_linked_mode

    if mode not in [@singly_linked_mode, @doubly_linked_mode] do
      throw({:error, {:unknown_mode, mode}})
    end

    %{
      capacity: size,
      ## mode (:singly_linked or :doubly_linked)
      mode: mode,
      ## circular?
      circular: circular?,
      ## Option to "undo" removals; see restore/1
      undo: undo?,
      ## holds the pointer to the first element of the list
      handle: init_handle(mode),
      # pointers (links) to the next element
      next: init_links(size),
      # keeps unused pointers
      free: init_free(size),
      # references to the data
      refs: :atomics.new(size, signed: true),
      # mapper `reference -> data`
      mapper_fun: Keyword.get(opts, :mapper_fun)
    }
    |> then(fn state ->
      if mode == @doubly_linked_mode do
        Map.put(state, :prev, init_links(size))
      else
        state
      end
    end)
    |> then(fn state ->
      if undo? do
        Map.put(state, :removed, Stack.new(size))
      else
        state
      end
    end)

  end

  def get(list, idx) when is_integer(idx) and idx > 0 do
    case get_pointer(list, idx) do
      nil -> nil
      pointer -> data(list, pointer)
    end
  end

  def get_pointer(list, idx) when is_integer(idx) and idx > 0  do
    get_pointer(list, head(list), 0, idx)
  end

  defp get_pointer(_list, pointer, step, idx) when step == idx - 1 do
    pointer
  end

  defp get_pointer(list, pointer, step, idx) do
    get_pointer(list, next(list, pointer), step + 1, idx)
  end

  def add_first(list, data) when is_integer(data) do
    ## Store data in 'free' element of the list
    allocated = allocate(list)
    set_data(list, allocated, data)
    head = head(list)
    set_next(list, allocated, head)
    add_first_mode(list, head, allocated)
    ## Allocated element becomes the new head
    set_head(list, allocated)
  end

  defp add_first_mode(%{mode: @singly_linked_mode} = _list, _head, _allocated) do
    :ok
  end

  defp add_first_mode(%{mode: @doubly_linked_mode} = list, head, allocated) do
    set_previous(list, allocated, @terminator)

    if head(list) == @terminator do
      set_tail(list, allocated)
    else
      set_previous(list, head, allocated)
    end
  end

  def insert(%{mode: mode} = list, idx, data)
      when is_integer(data) and is_integer(idx) and idx > 0 do
    cond do
      idx > size(list) ->
        {:error, {:no_index, idx}}

      empty?(list) ->
        add_first(list, data)

      true ->
        idx_pointer = get_pointer(list, idx)
        next_pointer = next_pointer(list, idx_pointer)
        allocated = allocate(list)
        set_data(list, allocated, data)
        set_next(list, allocated, next_pointer)
        set_next(list, idx_pointer, allocated)
        ## Build `prev` links
        ## allocated.prev <- idx_pointer
        if mode == @doubly_linked_mode do
          set_previous(list, allocated, idx_pointer)
          ## allocated.next.prev <- allocated
          if next_pointer == @terminator do
            ## insertion at the end - update the tail
            set_tail(list, allocated)
          else
            ## insertion in between - point the `prev` link of next pointer
            ## to newly allocated element
            set_previous(list, next_pointer, allocated)
          end
        end
    end
  end

  def add_last(list, data) when is_integer(data) do
    if empty?(list) do
      add_first(list, data)
    else
      insert(list, size(list), data)
    end
  end

  def delete(list, idx) when is_integer(idx) and idx > 0 do
    if idx > size(list) do
      {:error, {:no_index, idx}}
    else
      # Removing first element of the list
      if idx == 1 do
        current_head = head(list)
        delete_pointer(list, current_head)

        {:removed, current_head}
      else
        pointer = get_pointer(list, idx - 1)
        pointer_to_delete = next_pointer(list, pointer)
        pointer_next = next_pointer(list, pointer_to_delete)
        set_next(list, pointer, pointer_next)
        delete_pointer(list, pointer_to_delete)

        {:removed, pointer_to_delete}
      end
    end
  end

  def delete_pointer(_list, @terminator) do
    :ok
  end

  def delete_pointer(%{mode: mode} = list, pointer) do
    forget_or_hide(list, pointer)

    cond do
      pointer == head(list) ->
        next_head = next_pointer(list, pointer)
        set_head(list, next_head)

        if mode == @doubly_linked_mode && next_head != @terminator do
          set_previous(list, next_head, @terminator)
        end

      mode == @doubly_linked_mode ->
        next_pointer = next_pointer(list, pointer)
        if pointer == tail(list) do
            ## Last element
          new_tail = prev(list, pointer)
          set_tail(list, new_tail)
        end
        prev_pointer = prev(list, pointer)
        set_next(list, prev_pointer, next_pointer)
        set_previous(list, next_pointer, prev_pointer)

      true ->
        :ok
    end
  end

  def to_list(list) do
    reduce(list, []) |> Enum.reverse()
  end

  def reduce(list, initial_value, reducer \\ nil) do
    iterate(list, action: (reducer || default_reducer(list)), initial_value: initial_value)
  end

  defp default_reducer(list) do
    (fn p, acc ->
        [data(list, p) | acc]
    end)
  end

  def iterate(list, opts \\ [])

  def iterate(list, opts) do
    start = Keyword.get(opts, :start, head(list))
    action = Keyword.get(opts, :action, default_reducer(list))
    stop_on = Keyword.get(opts, :stop_on, fn next ->
      last_pointer = list.circular && start || @terminator
      next == last_pointer
    end)

    ## If action is of arity 1, it's a "side-effect" function.
    ## The argument is a pointer.
    ## For instance, it might conditionally delete some entries,
    ## etc.
    ## The result of the action call would be ignored.
    ## If action is of arity 2, it's a "reducer" function
    ## The arguments are : pointer and accumulated value
    ## If {:halt, term()} is returned, the iteration is halted with term()
    ## If {:cont, term()} or term() is returned, the iteration continues.
    ##
    cond do
      is_function(action, 1) ->
        iterate_impl(list, start, stop_on, action)
      is_function(action, 2) ->
        iterate_impl(list, start, stop_on, action, Keyword.get(opts, :initial_value))
      true ->
        throw({:error, :action_invalid_arity})
    end
  end

  ## Iteration with side-effects
  defp iterate_impl(_list, @terminator, _stop_on, action) when is_function(action, 1) do
    :ok
  end

  defp iterate_impl(list, current_pointer, stop_on, action) when is_function(action, 1) do
    action.(current_pointer)

    next_p = next(list, current_pointer)
    if stop_on.(next_p) do
      :ok
    else
      iterate_impl(list, next_p, stop_on, action)
    end

  end

  ## "Reducer" iteration
  defp iterate_impl(_list, @terminator, _stop_on, action, acc) when is_function(action, 2) do
    acc
  end

  defp iterate_impl(list, current_pointer, stop_on, action, acc) when is_function(action, 2) do
    case action.(current_pointer, acc) do
      {:halt, new_acc} ->
        new_acc
      result ->
        new_acc = case result do
            {:cont, r} ->
              r
            r -> r
          end
          next_p = next(list, current_pointer)

          if stop_on.(next_p) do
              new_acc
          else
            iterate_impl(list, next_p, stop_on, action, new_acc)
          end
        end

  end

  def empty?(list) do
    head(list) == @terminator
  end

  def size(%{undo: undo?, capacity: capacity, free: free} = list) do
    capacity - Stack.size(free) - (undo? && Stack.size(list.removed) || 0)
  end

  def default_opts() do
    [
      mode: @doubly_linked_mode,
      circular: false,
      undo: false,
      mapper_fun: &Function.identity/1
    ]
  end

  ## Helpers
  ##
  ## Initialize handle.
  ## First element - pointer to the head
  ## Second element (for :doubly_linked) is a tail
  defp init_handle(mode) do
    size =
      if mode == @singly_linked_mode do
        1
      else
        2
      end

    Array.new(size, @terminator)
  end

  ## Allocate links (pointers to the next element)
  defp init_links(size) do
    :atomics.new(size, signed: false)
  end

  ## The stack for tracking 'free' indices
  ## They can be reused after the element is removed from linked list
  ## (unless `undo=true`, which disables reuse).
  defp init_free(size) when is_integer(size) do
    ref = Stack.new(size)
    Enum.each(1..size, fn idx -> Stack.push(ref, idx) end)
    ref
  end

  def head(%{handle: handle} = _list) do
    Array.get(handle, 1)
  end

  def tail(%{mode: @singly_linked_mode} = _list) do
    nil
  end

  def tail(%{handle: handle, mode: @doubly_linked_mode}) do
    Array.get(handle, 2)
  end

  def set_next(_list, @terminator, _next_pointer) do
    :ok
  end

  def set_next(%{next: pointers} = _list, pointer, next_pointer) do
    Array.put(pointers, pointer, next_pointer)
  end

  def set_previous(_list, @terminator, _next_pointer) do
    :ok
  end

  def set_previous(%{mode: @doubly_linked_mode, prev: pointers} = _list, pointer, prev_pointer) do
    Array.put(pointers, pointer, prev_pointer)
  end

  def set_previous(_list, _pointer, _prev_pointer) do
    :ok
  end

  def set_data(%{refs: refs} = _list, pointer, data_ref) do
    Array.put(refs, pointer, data_ref)
  end

  def set_head(%{handle: handle} = list, pointer) do
    Array.put(handle, 1, pointer)

    if pointer == @terminator || size(list) == 1 do
      set_tail(list, pointer)
    end
    :ok
  end

  def set_tail(%{handle: handle, mode: @doubly_linked_mode} = _list, pointer) do
    Array.put(handle, 2, pointer)
  end

  def set_tail(_list, _pointer) do
    :ok
  end

  def data(%{refs: refs, mapper_fun: mapper} = _list, pointer) do
    mapper.(Array.get(refs, pointer))
  end

  def next(%{circular: circular?} = list, pointer) do
    case next_pointer(list, pointer) do
      @terminator when circular? ->
        head(list)
      next_pointer ->
        next_pointer
      end
  end

  def prev(%{circular: circular?} = list, pointer) do
    case prev_pointer(list, pointer) do
      @terminator when circular? ->
        tail(list)
      next_pointer ->
        next_pointer
      end
  end


  ## The `next_pointer/2` and `prev_pointer/2` functions ignore `circular` option,
  ## and act based on position of list terminator.
  ## We will use it when updating the list,
  ## so we can treat internal structure uniformly.
  ## The navigation over circular lists is implemented by next/2 and prev/2.
  ##
  defp next_pointer(%{next: pointers} = _list, pointer) do
    Array.get(pointers, pointer)
  end

  defp prev_pointer(%{prev: pointers} =  _list, pointer) do
    Array.get(pointers, pointer)
  end

  defp prev_pointer(_singly_linked, _pointer) do
    nil
  end

  defp allocate(%{free: free} = _list) do
    Stack.pop(free) || throw(:list_over_capacity)
  end

  ## If `undo` is enabled, record the pointer to removed element.
  ## Could be restored later for circular doubly linked list
  ## See `restore/1
  ##
  defp forget_or_hide(%{undo: undo?} = list, pointer) when is_integer(pointer) and pointer > 0 do
    if undo? do
      hide_pointer(list, pointer)
    else
      forget_pointer(list, pointer)
    end
  end

  defp forget_pointer(%{free: free} = _list, pointer) do
    Stack.push(free, pointer)
  end

  defp hide_pointer(%{removed: removed} = _list, pointer) do
   Stack.push(removed, pointer)
  end

  def num_free_pointers(%{free: free} = _list) do
    Stack.size(free)
  end

  def restore(%{undo: true, removed: removed} = list) do
    case Stack.pop(removed) do
      nil -> false
      restored_pointer ->
        restore_pointer(list, restored_pointer)
      end
  end

  def restore(_list) do
    false
  end

  defp restore_pointer(list, pointer) do
    next_pointer = next_pointer(list, pointer)
    ## Special case: next_pointer for restored pointer
    ## is a current head
    if next_pointer == head(list) do
      ## We replace head with the restored pointer
      set_head(list, pointer)
    else
      prev_pointer = prev(list, pointer)
      set_next(list, prev_pointer, pointer)
    end
    set_previous(list, next_pointer, pointer)
  end

end