-module(assignment1).
% Uncompile following line to don't compile the tests
%-define(NOTEST, false).
% Run eunit:test(assignment1). to run the tests
-include_lib("eunit/include/eunit.hrl").
-export([doubleAll/1, evens/1, product/1, zip/2, zip_with/3]).

% Define the functions doubleAll, evens, and product using
% the higher-order functions:
% lists:map, lists:filter and lists:foldr

doubleAll(XList) -> lists:map(fun(X) -> 2*X end, XList).

doubleAll_test_() ->
[
    ?_assert(doubleAll([]) == []),
    ?_assert(doubleAll([1, 2, 3]) =:= [2, 4, 6])
].


evens(XList) -> lists:filter(fun(X) -> X rem 2 == 0 end, XList).

evens_test_() ->
[
    ?_assert(evens([]) == []),
    ?_assert(evens([1, 2, 3]) =:= [2]),
    ?_assert(evens([1, 2, 3, 4, 5, 6]) =:= [2, 4, 6])
].


product(XList) -> lists:foldr(fun(X, Y) -> X*Y end, 1, XList).

product_test_() ->
[
    ?_assert(product([1, 2, 3]) == 6)
].

% a) Define a function zip/2 that “zips together” pairs of elements
% from two lists like this:
% zip([1,3,5,7], [2,4]) = [ {1,2}, {3,4} ]
% where you can see that the elements from the longer list are lost.

% b) Define a function zip_with/3 that “zips together”
% pairs of elements from two lists using the function in the first argument,
% like this:
% zip_with(fun(X,Y) -> X+Y end, [1,3,5,7], [2,4]) = [ 3, 7 ]

% c) Re-define the function zip_with/3 using zip and lists:map.
% d) Re-define zip/2 using zip_with/3.

% zip function (a)
zip(X, Y) -> zip(X, Y, []).
zip([X|Xs], [Y|Ys], Acc) -> [{X, Y}] ++ zip(Xs, Ys, Acc);
zip([], _, Acc) -> Acc;
zip(_, [], Acc) -> Acc.

zip_test_() ->
[
    ?_assert(zip([1,2,3], [4,5,6]) =:= [{1,4}, {2,5}, {3,6}]),
    ?_assert(zip([1,2,3], [4,5]) =:= [{1,4}, {2,5}]),
    ?_assert(zip([1,2], [4,5,6]) =:= [{1,4}, {2,5}])
].

% zip_with function (b)
zip_with(Fun, X, Y) -> zip_with(Fun, X, Y, []).
zip_with(Fun, [X|Xs], [Y|Ys], Acc) -> [Fun(X, Y)] ++ zip_with(Fun, Xs, Ys, Acc);
zip_with(_, [], _, Acc) -> Acc;
zip_with(_, _, [], Acc) -> Acc.

zip_with_test_() ->
[
    ?_assert(zip_with(fun(X, Y) -> X*Y end, [1,2,3], [4,5,6]) =:= [4, 10, 18]),
    ?_assert(zip_with(fun(X, Y) -> X*Y end, [1,2,3], [4,5]) =:= [4, 10]),
    ?_assert(zip_with(fun(X, Y) -> X*Y end, [1,2], [4,5,6]) =:= [4, 10])
].

% zip_with2 function (c)
zip_with2(Fun, X, Y) -> lists:map(fun({XI,YI}) -> Fun(XI, YI) end, zip(X, Y)).

zip_with2_test_() ->
[
    ?_assert(zip_with2(fun(X, Y) -> X*Y end, [1,2,3], [4,5,6]) =:= [4, 10, 18]),
    ?_assert(zip_with2(fun(X, Y) -> X*Y end, [1,2,3], [4,5]) =:= [4, 10]),
    ?_assert(zip_with2(fun(X, Y) -> X*Y end, [1,2], [4,5,6]) =:= [4, 10])
].

% zip3 function (d)
zip3(X, Y) -> zip_with2(fun(XI, YI) -> {XI,YI} end, X, Y).

zip3_test_() ->
[
    ?_assert(zip3([1,2,3], [4,5,6]) =:= [{1,4}, {2,5}, {3,6}]),
    ?_assert(zip3([1,2,3], [4,5]) =:= [{1,4}, {2,5}]),
    ?_assert(zip3([1,2], [4,5,6]) =:= [{1,4}, {2,5}])
].