median single.erl

-module(median).
-export([from_file/1,from_list/1]).

from_file(File) ->
    io:format("~w\n",[from_list(parse_file:to_list(File))]),
    init:stop().

from_list(List) ->
    nth_order_stat(round(length(List)/2), List).

nth_order_stat(Target_order_stat, List) ->
    Min = lists:min(List),
    Max = lists:max(List),
    if 
        (Min == Max) or (Target_order_stat == 1) ->
            Min;
        Target_order_stat == length(List) ->
            Max;
        true ->
            partition_on_pivot(Target_order_stat, List) 
    end.

partition_on_pivot(Target_order_stat, List) ->
    Pivot = hd(List),
    Lt_pivot_predicate = fun(X) -> X < Pivot end,
    Num_less_than = count_times_predicate_true(Lt_pivot_predicate, List),
    Pivot_order_stat = Num_less_than + 1,

    if 
        Pivot_order_stat == Target_order_stat ->
            Pivot;

        Pivot_order_stat == 1 ->
            nth_order_stat(Target_order_stat, rotate(List));

        Pivot_order_stat < Target_order_stat ->
            Lt = [ X || X <- List, X >=Pivot ],
            Rotated = rotate(Lt),
            Adjusted_target_order_stat = Target_order_stat - Num_less_than, 
            nth_order_stat(Adjusted_target_order_stat, Rotated);

        true -> % Pivot_order_stat > Target_order_stat
            Gt = [ X || X <- List, X < Pivot ],
            nth_order_stat(Target_order_stat, Gt)
    end. 
  
count_times_predicate_true(Predicate, List) ->
    lists:foldl(fun(X,Acc) ->
			case Predicate(X) of
			    true ->
				Acc+1;
			    false ->
				Acc
			end
		end,
		0, List).

rotate([H|T]) -> 
    T ++ [H].