-module(gleam@iterator).
-compile(no_auto_import).

-export([unfold/2, repeatedly/1, repeat/1, from_list/1, fold/3, run/1, to_list/1, step/1, take/2, drop/2, map/2, append/2, flatten/1, flat_map/2, filter/2, cycle/1, range/2, find/2]).
-export_type([action/1, iterator/1, step/2]).

-type action(ATI) :: stop | {continue, ATI, fun(() -> action(ATI))}.

-opaque iterator(ATJ) :: {iterator, fun(() -> action(ATJ))}.

-type step(ATK, ATL) :: {next, ATK, ATL} | done.

-spec do_unfold(ATO, fun((ATO) -> step(ATP, ATO))) -> fun(() -> action(ATP)).
do_unfold(Initial, F) ->
    fun() -> case F(Initial) of
            {next, X, Acc} ->
                {continue, X, do_unfold(Acc, F)};

            done ->
                stop
        end end.

-spec unfold(ATT, fun((ATT) -> step(ATU, ATT))) -> iterator(ATU).
unfold(Initial, F) ->
    {iterator, do_unfold(Initial, F)}.

-spec repeatedly(fun(() -> ATY)) -> iterator(ATY).
repeatedly(F) ->
    unfold(nil, fun(_) -> {next, F(), nil} end).

-spec repeat(AUA) -> iterator(AUA).
repeat(X) ->
    repeatedly(fun() -> X end).

-spec from_list(list(AUC)) -> iterator(AUC).
from_list(List) ->
    Yield = fun(Acc) -> case Acc of
            [] ->
                done;

            [Head | Tail] ->
                {next, Head, Tail}
        end end,
    unfold(List, Yield).

-spec do_fold(fun(() -> action(AUF)), AUH, fun((AUF, AUH) -> AUH)) -> AUH.
do_fold(Continuation, Initial, F) ->
    case Continuation() of
        {continue, Element, Iterator} ->
            do_fold(Iterator, F(Element, Initial), F);

        stop ->
            Initial
    end.

-spec fold(iterator(AUI), AUK, fun((AUI, AUK) -> AUK)) -> AUK.
fold(Iterator, Initial, F) ->
    do_fold(erlang:element(2, Iterator), Initial, F).

-spec run(iterator(any())) -> nil.
run(Iterator) ->
    fold(Iterator, nil, fun(_, _) -> nil end).

-spec to_list(iterator(AUN)) -> list(AUN).
to_list(Iterator) ->
    gleam@list:reverse(fold(Iterator, [], fun(E, Acc) -> [E | Acc] end)).

-spec step(iterator(AUQ)) -> step(AUQ, iterator(AUQ)).
step(Iterator) ->
    case (erlang:element(2, Iterator))() of
        stop ->
            done;

        {continue, E, A} ->
            {next, E, {iterator, A}}
    end.

-spec do_take(fun(() -> action(AUV)), integer(), list(AUV)) -> list(AUV).
do_take(Continuation, Desired, Acc) ->
    case Desired > 0 of
        true ->
            case Continuation() of
                {continue, Element, Iterator} ->
                    do_take(Iterator, Desired - 1, [Element | Acc]);

                stop ->
                    gleam@list:reverse(Acc)
            end;

        false ->
            gleam@list:reverse(Acc)
    end.

-spec take(iterator(AUZ), integer()) -> list(AUZ).
take(Iterator, Desired) ->
    do_take(erlang:element(2, Iterator), Desired, []).

-spec do_drop(fun(() -> action(AVC)), integer()) -> fun(() -> action(AVC)).
do_drop(Continuation, Desired) ->
    case Desired > 0 of
        true ->
            case Continuation() of
                {continue, _, Iterator} ->
                    do_drop(Iterator, Desired - 1);

                stop ->
                    fun() -> stop end
            end;

        false ->
            Continuation
    end.

-spec drop(iterator(AVF), integer()) -> iterator(AVF).
drop(Iterator, Desired) ->
    {iterator, do_drop(erlang:element(2, Iterator), Desired)}.

-spec do_map(fun(() -> action(AVI)), fun((AVI) -> AVK)) -> fun(() -> action(AVK)).
do_map(Continuation, F) ->
    fun() -> case Continuation() of
            {continue, E, Continuation@1} ->
                {continue, F(E), do_map(Continuation@1, F)};

            stop ->
                stop
        end end.

-spec map(iterator(AVM), fun((AVM) -> AVO)) -> iterator(AVO).
map(Iterator, F) ->
    {iterator, do_map(erlang:element(2, Iterator), F)}.

-spec do_append(fun(() -> action(AVQ)), fun(() -> action(AVQ))) -> fun(() -> action(AVQ)).
do_append(First, Second) ->
    fun() -> case First() of
            {continue, E, First@1} ->
                {continue, E, do_append(First@1, Second)};

            stop ->
                Second()
        end end.

-spec append(iterator(AVU), iterator(AVU)) -> iterator(AVU).
append(First, Second) ->
    {iterator, do_append(erlang:element(2, First), erlang:element(2, Second))}.

-spec do_flatten(fun(() -> action(iterator(AVY)))) -> fun(() -> action(AVY)).
do_flatten(Continuation) ->
    fun() -> case Continuation() of
            {continue, E, Continuation@1} ->
                (do_append(erlang:element(2, E), do_flatten(Continuation@1)))();

            stop ->
                stop
        end end.

-spec flatten(iterator(iterator(AWC))) -> iterator(AWC).
flatten(Iterator) ->
    {iterator, do_flatten(erlang:element(2, Iterator))}.

-spec flat_map(iterator(AWG), fun((AWG) -> iterator(AWI))) -> iterator(AWI).
flat_map(Iterator, F) ->
    flatten(map(Iterator, F)).

-spec do_filter(fun(() -> action(AWL)), fun((AWL) -> boolean())) -> fun(() -> action(AWL)).
do_filter(Continuation, Predicate) ->
    fun() -> case Continuation() of
            {continue, E, Iterator} ->
                case Predicate(E) of
                    true ->
                        {continue, E, do_filter(Iterator, Predicate)};

                    false ->
                        (do_filter(Iterator, Predicate))()
                end;

            stop ->
                stop
        end end.

-spec filter(iterator(AWO), fun((AWO) -> boolean())) -> iterator(AWO).
filter(Iterator, Predicate) ->
    {iterator, do_filter(erlang:element(2, Iterator), Predicate)}.

-spec do_cycle(fun(() -> action(AWR)), fun(() -> action(AWR))) -> fun(() -> action(AWR)).
do_cycle(Next, Reset) ->
    fun() -> case Next() of
            {continue, E, Iterator} ->
                {continue, E, do_cycle(Iterator, Reset)};

            stop ->
                (do_cycle(Reset, Reset))()
        end end.

-spec cycle(iterator(AWV)) -> iterator(AWV).
cycle(Iterator) ->
    {iterator,
     do_cycle(erlang:element(2, Iterator), erlang:element(2, Iterator))}.

-spec do_range(integer(), integer(), integer()) -> fun(() -> action(integer())).
do_range(Current, Limit, Inc) ->
    case Current =:= Limit of
        true ->
            fun() -> stop end;

        false ->
            fun() ->
                {continue, Current, do_range(Current + Inc, Limit, Inc)}
            end
    end.

-spec range(integer(), integer()) -> iterator(integer()).
range(Start, Stop) ->
    {iterator, do_range(Start, Stop, case Start < Stop of
             true ->
                 1;

             false ->
                 -1
         end)}.

-spec find(iterator(AXD), fun((AXD) -> boolean())) -> {ok, AXD} | {error, nil}.
find(Haystack, Is_desired) ->
    case (erlang:element(2, Haystack))() of
        {continue, Element, Continuation} ->
            case Is_desired(Element) of
                true ->
                    {ok, Element};

                false ->
                    find({iterator, Continuation}, Is_desired)
            end;

        stop ->
            {error, nil}
    end.