numbers.erl

-module(numbers).
-export([fib/1, triangle_numbers/1, triangle_number/1, bsearch/2, pair_sum/2]).

fib(1)->
  [0];
fib(2)->
  [0,1];
fib(N)->
  PrevFibs = fib(N-1),
  lists:append(PrevFibs, [ lists:nth(length(PrevFibs) - 1, PrevFibs) + lists:last(PrevFibs) ] ).

triangle_numbers(1)->
  [1];
triangle_numbers(N)->
  PrevTriangles = triangle_numbers(N-1),
  lists:append(PrevTriangles, [ lists:last(PrevTriangles) + N]).

triangle_number(N)->
  (N + 1) * ( N / 2 ).

bsearch([], Target)->
  false;
bsearch(Nums, Target)->
  Pivot = (length(Nums) + 1) div 2,

  case lists:nth(Pivot, Nums) of
    Target ->
      Pivot;
    _ ->
      case lists:nth(Pivot, Nums) > Target of
        true ->
          bsearch(lists:sublist(Nums, length(Nums) - Pivot), Target);
        false ->
          Searched = bsearch(lists:nthtail(Pivot, Nums), Target),

          if
            Searched == false ->
              false;
            true ->
              Pivot + Searched
          end
      end
  end.

all_equal(Num1, Num2, Num3)->
  if
    Num1 == Num2 ->
      if
        Num2 == Num3 ->
          true;
        true ->
          false
      end;
    true ->
      false
  end.

pair_sum(Nums, Target)->
  pair_sum(Nums, Target, 1, 2).

pair_sum(Nums, Target, Start, Current)->
  NumsLength = length(Nums),
  if
    Start == NumsLength ->
      [];
    true ->
      if
        Current > NumsLength ->
          pair_sum(Nums, Target, Start + 1, Start + 2);
        true ->
          NextPairSums = pair_sum(Nums, Target, Start, Current + 1),
          PairSum = lists:nth(Start, Nums) + lists:nth(Current, Nums),
          if
            PairSum == Target ->
              lists:append([[Start, Current]], NextPairSums);
            true ->
              NextPairSums
          end
      end
  end.