Problem Statement

Ethiopian multiplication is a method of multiplying integers using only addition, doubling, and halving.

Method:

For example:   17 × 34 Halving the first column: Doubling the second column: Strike-out rows whose first cell is even: Sum the remaining numbers in the right-hand column: So 17 multiplied by 34, by the Ethiopian method is 578.

The task is to define three named functions/methods/procedures/subroutines:

Use these functions to create a function that does Ethiopian multiplication.

Let's start with the solution:

-module(ethopian).
-export([multiply/2]).

halve(N) ->
    N div 2.

double(N) ->
    N * 2.

even(N) ->
    (N rem 2) == 0.

multiply(LHS,RHS) when is_integer(Lhs) and Lhs > 0 and
			is_integer(Rhs) and Rhs > 0 ->
    multiply(LHS,RHS,0).

multiply(1,RHS,Acc) ->
    RHS+Acc;
multiply(LHS,RHS,Acc) ->
    case even(LHS) of
        true ->
            multiply(halve(LHS),double(RHS),Acc);
        false ->
            multiply(halve(LHS),double(RHS),Acc+RHS)
    end.