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Diffstat (limited to 'system/doc/efficiency_guide/efficiency_guide.erl')
| -rw-r--r-- | system/doc/efficiency_guide/efficiency_guide.erl | 214 |
1 files changed, 214 insertions, 0 deletions
diff --git a/system/doc/efficiency_guide/efficiency_guide.erl b/system/doc/efficiency_guide/efficiency_guide.erl new file mode 100644 index 0000000000..e982bdae65 --- /dev/null +++ b/system/doc/efficiency_guide/efficiency_guide.erl @@ -0,0 +1,214 @@ +-module(efficiency_guide). +-compile(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). + +non_opt_eq([H|T1], <<H,T2/binary>>) -> + non_opt_eq(T1, T2); +non_opt_eq([_|_], <<_,_/binary>>) -> + false; +non_opt_eq([], <<>>) -> + true. + +opt_eq(<<H,T1/binary>>, [H|T2]) -> + opt_eq(T1, T2); +opt_eq(<<_,_/binary>>, [_|_]) -> + false; +opt_eq(<<>>, []) -> + true. + +match_head(List, <<_:10,Data/binary>>) -> + match_body(List, Data). + +match_body([0|_], <<H,_/binary>>) -> + done; +match_body([H|T1], <<H,T2/binary>>) -> + {T1,T2}. + +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. + + |