my.erl

perf_tests() ->
    true = perf(6),
    true = perf(28),
    false = perf(7),
    false = perf(15),
    false = perf(0),
    all_tests_passed.

perf(0) -> false;
perf(N) -> 
    Divisor = 1,
    DivSum = 0,
    perf(N, Divisor, DivSum).

perf(N, N, DivSum) ->
    DivSum =:= N;
perf(N, Divisor, DivSum) when (N rem Divisor =:= 0) -> 
    perf(N, Divisor+1, DivSum+Divisor);
perf(N, Divisor, DivSum) ->
    perf(N, Divisor+1, DivSum).


%---stop checking divisors when you reach N/2:

perf(0) -> false;
perf(N) -> 
    Divisor = 1,
    DivSum = 0,
    MaxDivisor = (N div 2) + 1,
    perf(N, MaxDivisor, Divisor, DivSum).

perf(N, MaxDivisor, MaxDivisor, DivSum) ->
    DivSum =:= N;
perf(N, MaxDivisor, Divisor, DivSum) when (N rem Divisor =:= 0) -> 
    perf(N, MaxDivisor, Divisor+1, DivSum+Divisor);
perf(N, MaxDivisor, Divisor, DivSum) ->
    perf(N, MaxDivisor, Divisor+1, DivSum).

%-----------------

fib_tests() ->
    0 = fib(0),
    1 = fib(1),
    1 = fib(2),
    2 = fib(3),
    3 = fib(4),
    5 = fib(5),
    all_tests_passed.

fib(0) -> 0; 
fib(1) -> 1;
fib(N) ->
    N1 = 1,     % N1 => N-1
    N2 = 0,     % N2 => N-2
    fib_acc(N, N1, N2).

fib_acc(2, N1, N2) ->
    N1 + N2;
fib_acc(N, N1, N2) ->
    fib_acc(N-1, N1+N2, N1).

%-----------------

fmax_tests() ->
    10 = fmax(fun(X) -> X*2 end, 5),
    2  = fmax(fun(X) -> X+2 end, 0),
    all_tests_passed.

fmax(Func, N) -> 
    Max = 0,
    fmax(Func, N, Max).

fmax(Func, 0, Max) ->
    max(Func(0), Max);
fmax(Func, N, Max)  -> 
    fmax(Func, N-1, max(Func(N), Max) ).

%------------------

fsum_tests() ->
    12 = fsum(fun(X) -> X*2 end, 3),
    14 = fsum(fun(X) -> X+2 end, 3),
    all_tests_passed.

fsum(Func, N) -> 
    Acc = 0,
    fsum(Func, N, Acc).

fsum(Func, 0, Acc) ->
    Acc + Func(0);
fsum(Func, N, Acc) ->
    fsum(Func, N-1, Acc + Func(N) ).