Question

I’ve been looking at Subtext’s Lucene.Net implementation as a guide to do something similar with our websites. When Subtext index or search for a given post, it runs the ID through NumericUtils.IntToPrefixCoded.
According to the Lucene docs, it does some shifting, but doesn’t lose precision.
So, what’s the point? What does it do, and why?

Answer 1

You need to look at the class documentation, which explains it in more detail:

To quickly execute range queries in Apache Lucene, a range is divided
recursively into multiple intervals for searching: The center of the
range is searched only with the lowest possible precision in the trie,
while the boundaries are matched more exactly. This reduces the number
of terms dramatically.

This class generates terms to achieve this: First the numerical
integer values need to be converted to strings. For that integer
values (32 bit or 64 bit) are made unsigned and the bits are converted
to ASCII chars with each 7 bit. The resulting string is sortable like
the original integer value. Each value is also prefixed (in the first
char) by the shift value (number of bits removed) used
during encoding.

As I understand, intToPrefixCoded method does exactly that: takes int value, shifts it and returns a sortable String as explained above.