// invariant: next LOOKAHEAD characters of bufc are available for direct indexing.
// note that bufc_refill isn't the end of the indexable range but LOOKAHEAD before that.
// when we refill the buffer, we have to carry forward the last LOOKAHEAD-1 characters to the
// beginning of the new buffer.
// if you're chaining mmapped file segments, that means you need an interstitial non-mmap buffer
// to join the beginning and end of the two segments.
// if we reach the true end of the buffer, pad with 0s so lookahead doesn't index out of bounds.

#define nextc() (bufc != bufc_refill ? c = *bufc++ : nextc_slow())

// can also advance by larger number of characters in one step by an inequality check.

// note this avoids the refill-sentinel of the previous solution. once you're doing lookahead,
// there's not much point (my thinking with the refill sentinel is that you're reading *bufc to set
// c, so you might as well use it rather than doing a pointer compare vs something else you'd have
// to load first, but that's no longer possible once the sentinel value is at a different point from
// the read value).