I have tested my program, and the complex_2 test says that the output should be {a=1, b=0, d=1}, and my output is {a=1, b=0, c=1, d=1}, and that is absolutely imposible, because if there's no letter c it wouldn't had added it to the map, so it has to be a letter c on the test. Which it would be nice to see what tests the program does, to prove them in the IDE and find errors.
Sorry for my english, i'm not native and i don't know speak very well.

i don't understand why coins(2, 2) should return -1, if the biggest combination that those numbers can't get is 1, becouse they can't reach the number one with an integer combination.

Please, how can you put a foreach with a "break"(return) inside, just use a while