git index: how does it work?

[Shaun Cutts <shaun@cuttshome.net>]

Hello,

I am wondering if someone could explain and/or point me to an explanation of how
the git index works.

For instance, suppose I have a tracked file: "foo.c"

1) [I modify "foo.c"]
2) git add foo.c
3) [modify again]
4) git commit -m "blah blah"

Since I don't include the "-a" switch, the version I added on step 2 is
committed. But how does the index keep track of these changes? Does the index
file actually contain the hunks of "foo.c" that have been modified? Or is there
a "temporary" blob created, which the index points to? 

In either case, is there some interface to access these hunks and/or get a
reference to the blob?

Thanks,

-- Shaun

PS I'm considering writing an extension to git where the "diff" understands the
semantics of certain types of files: hunks wouldn't just be textual blobs but
would try to represent a minimal change from one version to the next based on an
edit distance, so that, e.g. changing the location of a function would be
represented by a "move" edit, rather than two text changes.

I have been building a prototype as a wrapper around git, intervening to store
extra information, etc before passing commands on to git. Blobs, commits, etc
are nice abstractions I can leave as is, but the index seems sort of foggy to
me. Any advice appreciated!