-module(generators).
-author([{name,"Fred T-H"},{email, hidden}]).
-description("An example of generators implemented in erlang,
 allowing to generate the equivalent of lists with constant memory.").
-vsn(0.0).
-export([t_fib/1, y_fib/1, sum/1, fold/3, test/2, batch_test/3]).

% macros to make the creation of generators a tad more user-friendly.
-define(yield(X), receive {Pid, next} -> Pid ! {self(), X} end).
-define(last_yield(X), receive {Pid, next} -> Pid ! {self(), X, done} end).

%% Tail-recursive fibonacci function.
t_fib(1) -> [1];
t_fib(2) -> [1,1];
t_fib(L) -> t_fib(3,L,[1,1]).

t_fib(L,L,[N1,N2|_]=F) ->
    lists:reverse([N1+N2|F]);
t_fib(C, L, [N1,N2|_]=F) ->
    t_fib(C+1,L,[N1+N2|F]).

%% Generator-like tail-recursive fibonacci function.
y_fib(L) ->
    y_fib(1,L,1,0).

y_fib(L,L,N1,N2) ->
    ?last_yield(N1+N2);
y_fib(C, L, N1,N2) ->
    N = N1+N2,
    ?yield(N),
    y_fib(C+1,L,N2,N).

%% what the lists:sum version for generators could look like
sum({M,Gen,Args}) ->
    Pid = spawn(M, Gen, Args),
    sum(Pid,0).
sum(Pid,S) ->
    Pid ! {self(), next},
    receive
        {Pid, N} -> sum(Pid, S+N);
        {Pid, N, done} -> S+N
    end.

%% ... and lists:foldl/3 (no foldr possible with generators).
fold(F, Acc,{M,Gen,Args}) when is_function(F,2) ->
    Pid = spawn(M,Gen,Args),
    fold(F, Acc, Pid);
fold(F, Acc, Pid) ->
    Pid ! {self(), next},
    receive
        {Pid, N} -> fold(F, F(N,Acc), Pid);
        {Pid, N, done} -> F(N,Acc)
    end.

%% Comparing execution times of lists vs generators
test(fold,N) ->
    erlang:statistics(wall_clock),
    F = fun(X, Acc) ->
          if X rem 2 == 0 -> Acc+1;
             true -> Acc
          end
        end,
    ?MODULE:fold(F, 0, {?MODULE, y_fib, [N]}),
    io:format("generator ok~n"),
    {_, T1} = erlang:statistics(wall_clock),
    lists:foldl(F, 0, t_fib(N)),
    io:format("list ok~n"),
    {_, T2} = erlang:statistics(wall_clock),
    if T1>0 -> {T1,T2, T2/T1};
       T2 == T1, T1 == 0 -> {T1,T2,1};
       true -> {T1,T2, '?'}
    end;
test(sum, N) ->
    erlang:statistics(wall_clock),
    ?MODULE:sum({?MODULE, y_fib, [N]}),
    io:format("generator ok~n"),
    {_, T1} = erlang:statistics(wall_clock),
    lists:sum(t_fib(N)),
    io:format("list ok~n"),
    {_, T2} = erlang:statistics(wall_clock),
    if T1>0 -> {T1,T2, T2/T1};
       T2 == T1, T1 == 0 -> {T1,T2,1};
       true -> {T1,T2, '?'}
    end.

%% Testing lists vs generators many times.
batch_test(Type, N, Lim) ->
    batch_test(Type, N, 0, Lim, 0).
batch_test(_Type,_N,Lim,Lim,Acc) ->
    Acc/Lim;
batch_test(Type,N,Ct,Lim,Acc) ->
    {_,_,X} = test(Type,N),
    batch_test(Type,N,Ct+1,Lim,Acc+X).