  We consider the problem of constructing prefix-free codes in which a
designated symbol, a space, can only appear at the end of codewords. We provide
a linear-time algorithm to construct almost-optimal codes with this property,
meaning that their average length differs from the minimum possible by at most
one. We obtain our results by uncovering a relation between our class of codes
and the class of one-to-one codes. Additionally, we derive upper and lower
bounds to the average length of optimal prefix-free codes with a space in terms
of the source entropy.
