erlang homework 1

shape.erl

-module(shape).
-export([area/1, perimeter/1, bits/1]).

area({rectangle,{_, _}, W, H}) when (0<W) and (0<H) -> W*H;
area({circle,{_, _}, R}) when (R>0) -> R*R*math:pi();
area({triangle,A, B, C}) when (A>0) and (B>0) and (C>0) -> heron(A, B, C).
heron(A,B,C) ->
  S = (A + B + C)/2,
  math:sqrt(S*(S-A)*(S-B)*(S-C)).

perimeter({rectangle,{_, _}, W, H}) when (0<W) and (0<H) -> 2*(W+H);
perimeter({circle,{_, _}, R}) when (R>0) -> 2*R*math:pi();
perimeter({triangle,A, B, C}) when (A>0) and (B>0) and (C>0) -> A+B+C.

bits(N) when N >= 0 -> bits(N, 0).
bits(0, ACC) -> ACC;
bits(N, ACC) ->
  SHIFTED_VALUE = N div 2,
  INCREMENTED_IF_ONE = ACC + N rem 2,
  bits(SHIFTED_VALUE, INCREMENTED_IF_ONE).