Re: [PATCH v3 5/5] gitdiffcore doc: mention new preliminary step for rename detection

"Elijah Newren via GitGitGadget" [off-list ref] writes:

I'd rather use docs/extensions.md instead of src/extension-checks.c;
it would be more realistic for .md to be similar to .txt than .c.

With a raised bar for this step, the equation changes a bit, no?  

    So, for example, if a deleted docs/ext.txt and an added
    docs/config/ext.txt are similar enough, they will be marked as a
    rename and prevent an added docs/ext.md that may be even similar
    to the deleted docs/ext.txt from being considered as the rename
    destination in the later step.  For this reason, the preliminary
    "match same filename" step uses a bit higher threshold to mark a
    file pair as a rename and stop considering other candidates for
    better matches.

or something?

+At most, one comparison is done per file in this preliminary pass; so
+if there are several extensions.txt files throughout the directory
+hierarchy that were added and deleted, this preliminary step will be
+skipped for those files.

Other than that, the whole series looked sensible to my cursory
read.

Thanks.

On Wed, Feb 10, 2021 at 8:41 AM Junio C Hamano [off-list ref] wrote:

"Elijah Newren via GitGitGadget" [off-list ref] writes:

quoted

From: Elijah Newren <redacted>

The last few patches have introduced a new preliminary step when rename
detection is on but both break detection and copy detection are off.
Document this new step.  While we're at it, add a testcase that checks
the new behavior as well.

Signed-off-by: Elijah Newren <redacted>
---
 Documentation/gitdiffcore.txt | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/gitdiffcore.txt b/Documentation/gitdiffcore.txt
index c970d9fe438a..36ebe364d874 100644
--- a/Documentation/gitdiffcore.txt
+++ b/Documentation/gitdiffcore.txt

@@ -168,6 +168,23 @@ a similarity score different from the default of 50% by giving a
 number after the "-M" or "-C" option (e.g. "-M8" to tell it to use
 8/10 = 80%).

+Note that when rename detection is on but both copy and break
+detection are off, rename detection adds a preliminary step that first
+checks if files are moved across directories while keeping their
+filename the same.  If there is a file added to a directory whose
+contents is sufficiently similar to a file with the same name that got
+deleted from a different directory, it will mark them as renames and
+exclude them from the later quadratic step (the one that pairwise
+compares all unmatched files to find the "best" matches, determined by
+the highest content similarity).  So, for example, if
+docs/extensions.txt and docs/config/extensions.txt have similar
+content, then they will be marked as a rename even if it turns out
+that docs/extensions.txt was more similar to src/extension-checks.c.

I'd rather use docs/extensions.md instead of src/extension-checks.c;
it would be more realistic for .md to be similar to .txt than .c.

With a raised bar for this step, the equation changes a bit, no?

    So, for example, if a deleted docs/ext.txt and an added
    docs/config/ext.txt are similar enough, they will be marked as a
    rename and prevent an added docs/ext.md that may be even similar
    to the deleted docs/ext.txt from being considered as the rename
    destination in the later step.  For this reason, the preliminary
    "match same filename" step uses a bit higher threshold to mark a
    file pair as a rename and stop considering other candidates for
    better matches.

or something?

Good points; I've updated the docs locally to reflect your
suggestions, I'll wait a bit for any other feedback and then send out
a new round with this update.

quoted

+At most, one comparison is done per file in this preliminary pass; so
+if there are several extensions.txt files throughout the directory
+hierarchy that were added and deleted, this preliminary step will be
+skipped for those files.

Other than that, the whole series looked sensible to my cursory
read.

Thanks.