// file:    toBinaryMSBF.rex
// author:  Robert Keller
// purpose: to convert a number to a list of its binary bits, 
//          most-significant bit first

toBinaryMSBF(0) => [0];
toBinaryMSBF(N) => toBinaryMSBFaux(N, []);

toBinaryMSBFaux(0, Acc) => Acc;
toBinaryMSBFaux(N, Acc) => toBinaryMSBFaux(N/2, [N%2 | Acc]);

// Rationale: The number 0 is handled specially, since we don't want
// 0 to be converted to [] (no bits).

// Acc is an accumulator argument.  The bits of the number are determined
// least to most, using N%2.  As they are, they are cons'ed to the accumulator
// N/2 is truncating integer arithmetic, which effectively shifts the binary
// representation one place to the right, losing the MSB.
// toBinaryMSBFaux is tail recursive

test() = map(toBinaryMSBF, range(0, 16));