Elided page numbers in registers

[Duncan Hothersall <dh@capdm.com>]

Another question on registers (just getting into indexing!).

I have a book which numbers its pages by chapter, and so page numbers
are 1/1, 1/2 ... for the pages of chapter 1, and then 2/1, 2/2 ... for
the pages of chapter 2.

I have page ranges in my index, such as 1/1-1/3, or 2/24-2/26.

I would like these ranges to be elided - in other words, the first one
would read 1/1-3 (remove repeated chapter number), and the second would
read 2/24-6 (remove repeated chapter number and also the repeated 2 in
page number).

Question: is there an already existing system to do this? I guess no :-)

Followup: I would be willing to help to code this if it doesn't exist (I
have some experience with such systems) if someone can point me at the
right bit of code to look at (things seem to be changing around
texutils, newtexexec etc. so I don't want to start at the wrong place).

Thanks...

Duncan