This mostly reverts 926ec960 which incorrectly removed the remainder operator while fixing a buffer under-run due to a negative index.

Added a regression test that would have caught this mistake. Thanks to Andras Lasso for creating the test.

Restored the remainder operator, but make sure to never give it a negative number, used a different technique to wrap there.

Co-authored-by: Andras Lasso <lasso@queensu.ca>