[PATCH 07 of 10 V2] sparse-revlog: add a `_trimchunk` function in C

[Yuya Nishihara]

On Thu, 15 Nov 2018 11:38:45 +0100, Boris Feld wrote:
> # HG changeset patch
> # User Boris Feld <boris.feld at octobus.net>
> # Date 1541785523 -3600
> #      Fri Nov 09 18:45:23 2018 +0100
> # Node ID f76fc4ed86fd2072c861f57a3c1c3e892648fc92
> # Parent  d8b70c216567aebbd5aeabf0cbee2ce192212e0e
> # EXP-Topic sparse-perf
> # Available At https://bitbucket.org/octobus/mercurial-devel/
> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r f76fc4ed86fd
> sparse-revlog: add a `_trimchunk` function in C
> 
> We are about to implement a native version of `slicechunktodensity`. For
> clarity, we introduce the helper functions first.
> 
> This function is a native implementation of the python function `_trimchunk`
> in `mercurial/revlogutils/deltas.py`.
> 
> diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
> --- a/mercurial/cext/revlog.c
> +++ b/mercurial/cext/revlog.c
> @@ -1031,6 +1031,25 @@ static inline long index_segment_span(in
>  	        index_get_length(self, end_rev));
>  }
>  
> +/* returns revs[startidx:endidx] without empty trailing revs */
> +static PyObject *_trimchunk(indexObject *self, PyObject *revs, long startidx,
> +                            long endidx)

Perhaps start/endidx should be Py_ssize_t.

> +{
> +	while (endidx > 1 && endidx > startidx) {

Nit: I slightly prefer using "for" statement, but I'm fine with "while"
to be aligned with the pure Python implementation.

> +		PyObject *rev = PyList_GET_ITEM(revs, endidx - 1);
> +		Py_ssize_t r = PyInt_AsLong(rev);
> +		if (r == -1 && PyErr_Occurred()) {
> +			return NULL;
> +		}

Can't we use a native revs list instead of PyObject one?

> +		if (index_get_length(self, r - 1) != 0) {

What would happen if r == 0?

FWIW, the pure-Python implementation does length(revs[endidx -1]), not
length(revs[endidx - 1] - 1).

> +			break;
> +		}
> +		endidx -= 1;
> +	}
> +	PyObject *chunk = PyList_GetSlice(revs, startidx, endidx);
> +	return chunk;

Variable can't be declared here in C89, and Windows compiler is really strict
about that. Just return the chunk without naming it.