Functional Programming in Erlang - First Assignment

assignment1.erl

-module(assignment1).
-export([bits/1, area/1, perimeter/1, enclose/1, bits_rec/1]).

% Shapes

% How the shapes are defined:
% circle: {circle, {x, y}, r}
% rectangle: {rectangle, {x1, y1},{x2, y2}}
% triangle: {triangle, {x1, y1}, A, B, C}
area({circle, {_X, _Y}, R}) ->
    math:pi() * R * R;
area({rectangle, {X1, Y1},{X2, Y2}}) ->
    (X2 - X1) * (Y2 - Y1);
area({triangle, _P, A, B, C}) ->
    S = (A + B + C) / 2,
    math:sqrt(S * (S - A) * (S - B) * (S - C)).

perimeter({circle, {_X, _Y}, R}) ->
    2 * math:pi() * R;
perimeter({rectangle, {X1, Y1}, {X2, Y2}}) ->
    2 * (X2 - X1) + 2 * (Y2 * Y1);
perimeter({triangle, _P, A, B, C}) ->
    A + B + C.

enclose({circle, {X, Y}, R}) ->
    {rectangle, {X - R, Y - R}, {X + R, Y + R}};
enclose({rectangle, P1, P2}) ->
    {rectangle, P1, P2};
enclose({triangle, {X, Y}, A, B, C}) ->
    Area = area({triangle, {X, Y}, A, B, C}),
    Base = max(max(A, B), max(B, C)),
    Height = (Area * 2) / Base,
    {rectangle, {X, Y}, {X + Base, Y + Height}}.


% Summing the bits

bits_rec(0) ->
    0;
bits_rec(N) -> 
    (N rem 2) + bits_rec(N div 2).

% Better (because faster) tail-recursive variant
bits(0, M) ->
    M;
bits(N, M) ->
    bits(N div 2, M + N rem 2).

bits(N) ->
    bits(N, 0).