for my blog

my_lists.erl

-module my_lists.
%% seq(Min, Max) -> [Min,Min+1, ..., Max]
%% seq(Min, Max, Incr) -> [Min,Min+Incr, ..., Max]
%%  returns the sequence Min..Max
%%  Min <= Max and Min and Max must be integers

-export [seq/2, seq/3].

-spec seq(From, To) -> Seq when
      From :: integer(),
      To :: integer(),
      Seq :: [integer()].

seq(First, Last)
    when is_integer(First), is_integer(Last), First-1 =< Last ->
    seq_loop(Last-First+1, Last, []).

seq_loop(N, X, L) when N >= 4 ->
     seq_loop(N-4, X-4, [X-3,X-2,X-1,X|L]);
seq_loop(N, X, L) when N >= 2 ->
     seq_loop(N-2, X-2, [X-1,X|L]);
seq_loop(1, X, L) ->
     [X|L];
seq_loop(0, _, L) ->
     L.

-spec seq(From, To, Incr) -> Seq when
      From :: integer(),
      To :: integer(),
      Incr :: integer(),
      Seq :: [integer()].

seq(First, Last, Inc)
    when is_integer(First), is_integer(Last), is_integer(Inc) ->
    if
        Inc > 0, First - Inc =< Last;
        Inc < 0, First - Inc >= Last ->
            N = (Last - First + Inc) div Inc,
            seq_loop(N, Inc*(N-1)+First, Inc, []);
        Inc =:= 0, First =:= Last ->
            seq_loop(1, First, Inc, [])
    end.

seq_loop(N, X, D, L) when N >= 4 ->
     Y = X-D, Z = Y-D, W = Z-D,
     seq_loop(N-4, W-D, D, [W,Z,Y,X|L]);
seq_loop(N, X, D, L) when N >= 2 ->
     Y = X-D,
     seq_loop(N-2, Y-D, D, [Y,X|L]);
seq_loop(1, X, _, L) ->
     [X|L];
seq_loop(0, _, _, L) ->
     L.