-module(efficiency_guide).
-compile([export_all,nowarn_export_all).
%% DO NOT
naive_reverse([H|T]) ->
naive_reverse(T)++[H];
naive_reverse([]) ->
[].
%% OK
naive_but_ok_reverse([H|T], Acc) ->
naive_but_ok_reverse(T, [H]++Acc);
naive_but_ok_reverse([], Acc) ->
Acc.
%% DO
vanilla_reverse([H|T], Acc) ->
vanilla_reverse(T, [H|Acc]);
vanilla_reverse([], Acc) ->
Acc.
multiple_setelement(T0) ->
T1 = setelement(9, T0, bar),
T2 = setelement(7, T1, foobar),
setelement(5, T2, new_value).
my_list_to_binary(List) ->
my_list_to_binary(List, <<>>).
my_list_to_binary([H|T], Acc) ->
my_list_to_binary(T, <<Acc/binary,H>>);
my_list_to_binary([], Acc) ->
Acc.
my_old_list_to_binary(List) ->
my_old_list_to_binary(List, []).
my_old_list_to_binary([H|T], Acc) ->
my_old_list_to_binary(T, [Acc,H]);
my_old_list_to_binary([], Acc) ->
list_to_binary(Acc).
my_binary_to_list(<<H,T/binary>>) ->
[H|my_binary_to_list(T)];
my_binary_to_list(<<>>) -> [].
my_complicated_binary_to_list(Bin) ->
my_complicated_binary_to_list(Bin, 0).
my_complicated_binary_to_list(Bin, Skip) ->
case Bin of
<<_:Skip/binary,Byte,_/binary>> ->
[Byte|my_complicated_binary_to_list(Bin, Skip+1)];
<<_:Skip/binary>> ->
[]
end.
after_zero(<<0,T/binary>>) ->
T;
after_zero(<<_,T/binary>>) ->
after_zero(T);
after_zero(<<>>) ->
<<>>.
all_but_zeroes_to_list(Buffer, Acc, 0) ->
{lists:reverse(Acc),Buffer};
all_but_zeroes_to_list(<<0,T/binary>>, Acc, Remaining) ->
all_but_zeroes_to_list(T, Acc, Remaining-1);
all_but_zeroes_to_list(<<Byte,T/binary>>, Acc, Remaining) ->
all_but_zeroes_to_list(T, [Byte|Acc], Remaining-1).
count1(<<_,T/binary>>, Count) -> count1(T, Count+1);
count1(<<>>, Count) -> Count.
count2(<<H,T/binary>>, Count) -> count2(T, Count+1);
count2(<<>>, Count) -> Count.
count3(<<_H,T/binary>>, Count) -> count3(T, Count+1);
count3(<<>>, Count) -> Count.
fib(N) ->
fib(N, 0, 1, []).
fib(0, _Current, _Next, Fibs) ->
lists:reverse(Fibs);
fib(N, Current, Next, Fibs) ->
fib(N - 1, Next, Current + Next, [Current|Fibs]).
recursive_fib(N) ->
recursive_fib(N, 0, 1).
recursive_fib(0, _Current, _Next) ->
[];
recursive_fib(N, Current, Next) ->
[Current|recursive_fib(N - 1, Next, Current + Next)].
bad_fib(N) ->
bad_fib(N, 0, 1, []).
bad_fib(0, _Current, _Next, Fibs) ->
Fibs;
bad_fib(N, Current, Next, Fibs) ->
bad_fib(N - 1, Next, Current + Next, Fibs ++ [Current]).
tail_recursive_fib(N) ->
tail_recursive_fib(N, 0, 1, []).
tail_recursive_fib(0, _Current, _Next, Fibs) ->
lists:reverse(Fibs);
tail_recursive_fib(N, Current, Next, Fibs) ->
tail_recursive_fib(N - 1, Next, Current + Next, [Current|Fibs]).
append([H|T], Tail) ->
[H|append(T, Tail)];
append([], Tail) ->
Tail.
kilo_byte() ->
kilo_byte(10, [42]).
kilo_byte(0, Acc) ->
Acc;
kilo_byte(N, Acc) ->
kilo_byte(N-1, [Acc|Acc]).
recursive_sum([H|T]) ->
H+recursive_sum(T);
recursive_sum([]) -> 0.
sum(L) -> sum(L, 0).
sum([H|T], Sum) -> sum(T, Sum + H);
sum([], Sum) -> Sum.
days_in_month(M) ->
element(M, {31,28,31,30,31,30,31,31,30,31,30,31}).
atom_map1(one) -> 1;
atom_map1(two) -> 2;
atom_map1(three) -> 3;
atom_map1(Int) when is_integer(Int) -> Int;
atom_map1(four) -> 4;
atom_map1(five) -> 5;
atom_map1(six) -> 6.
atom_map2(one) -> 1;
atom_map2(two) -> 2;
atom_map2(three) -> 3;
atom_map2(four) -> 4;
atom_map2(five) -> 5;
atom_map2(six) -> 6;
atom_map2(Int) when is_integer(Int) -> Int.
atom_map3(Int) when is_integer(Int) -> Int;
atom_map3(one) -> 1;
atom_map3(two) -> 2;
atom_map3(three) -> 3;
atom_map3(four) -> 4;
atom_map3(five) -> 5;
atom_map3(six) -> 6.
map_pairs1(_Map, [], Ys) ->
Ys;
map_pairs1(_Map, Xs, [] ) ->
Xs;
map_pairs1(Map, [X|Xs], [Y|Ys]) ->
[Map(X, Y)|map_pairs1(Map, Xs, Ys)].
map_pairs2(_Map, [], Ys) ->
Ys;
map_pairs2(_Map, [_|_]=Xs, [] ) ->
Xs;
map_pairs2(Map, [X|Xs], [Y|Ys]) ->
[Map(X, Y)|map_pairs2(Map, Xs, Ys)].
explicit_map_pairs(Map, Xs0, Ys0) ->
case Xs0 of
[X|Xs] ->
case Ys0 of
[Y|Ys] ->
[Map(X, Y)|explicit_map_pairs(Map, Xs, Ys)];
[] ->
Xs0
end;
[] ->
Ys0
end.
