# [PATCH] Require 0 context lines in git-blame algorithm
2 messages from 2016-05-27 to 2016-05-27. Participants: David Kastrup, Junio C Hamano.
Thread: https://gitlist.dev/t/42468
## David Kastrup, 2016-05-27 14:16
Subject: [PATCH] Require 0 context lines in git-blame algorithm
Message-ID: <1464358592-5409-1-git-send-email-dak@gnu.org>
URL: https://gitlist.dev/e/1464358592-5409-1-git-send-email-dak%40gnu.org
```
Previously, the core part of git blame -M required 1 context line.
There is no rationale to be found in the code (one guess would be that
the old blame algorithm was unable to deal with immediately adjacent
regions), and it causes artifacts like discussed in the thread
---
builtin/blame.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/builtin/blame.c b/builtin/blame.c
index 21f42b0..a3f6874 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -134,7 +134,7 @@ struct progress_info {
int blamed_lines;
};
-static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b, long ctxlen,
+static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b,
xdl_emit_hunk_consume_func_t hunk_func, void *cb_data)
{
xpparam_t xpp = {0};
@@ -142,7 +142,6 @@ static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b, long ctxlen,
xdemitcb_t ecb = {NULL};
xpp.flags = xdl_opts;
- xecfg.ctxlen = ctxlen;
xecfg.hunk_func = hunk_func;
ecb.priv = cb_data;
return xdi_diff(file_a, file_b, &xpp, &xecfg, &ecb);
@@ -980,7 +979,7 @@ static void pass_blame_to_parent(struct scoreboard *sb,
fill_origin_blob(&sb->revs->diffopt, target, &file_o);
num_get_patch++;
- if (diff_hunks(&file_p, &file_o, 0, blame_chunk_cb, &d))
+ if (diff_hunks(&file_p, &file_o, blame_chunk_cb, &d))
die("unable to generate diff (%s -> %s)",
oid_to_hex(&parent->commit->object.oid),
oid_to_hex(&target->commit->object.oid));
@@ -1129,7 +1128,7 @@ static void find_copy_in_blob(struct scoreboard *sb,
* file_p partially may match that image.
*/
memset(split, 0, sizeof(struct blame_entry [3]));
- if (diff_hunks(file_p, &file_o, 1, handle_split_cb, &d))
+ if (diff_hunks(file_p, &file_o, handle_split_cb, &d))
die("unable to generate diff (%s)",
oid_to_hex(&parent->commit->object.oid));
/* remainder, if any, all match the preimage */
--
2.7.4
```
## Junio C Hamano, 2016-05-27 20:59
Subject: Re: [PATCH] Require 0 context lines in git-blame algorithm
Message-ID:
URL: https://gitlist.dev/e/xmqqtwhjp694.fsf%40gitster.mtv.corp.google.com
In-Reply-To: <1464358592-5409-1-git-send-email-dak@gnu.org>
```
David Kastrup writes:
> Previously, the core part of git blame -M required 1 context line.
> There is no rationale to be found in the code (one guess would be that
> the old blame algorithm was unable to deal with immediately adjacent
> regions), and it causes artifacts like discussed in the thread
>
The only thing that remotely hints why we thought a non-zero context
was a good idea was this:
http://thread.gmane.org/gmane.comp.version-control.git/28336/focus=28580
in which I said:
| we may need to use a handful surrounding context lines for
| better identification of copy source by the "ciff" algorithm but
| that is a minor implementation detail.
But I do not think the amount of context affects the quality of the
match. So it could be that it was completely a misguided attempt
since the very beginning, cee7f245 (git-pickaxe: blame rewritten.,
2006-10-19), which allowed the caller to specify context when
calling compare_buffer(), the function that corresponds to
diff_hunks() in today's code.
> ---
> builtin/blame.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
I totally forgot about the discussion around $gmane/255289; thanks
for bringing this back again.
As usual, we'd need your sign-off to use this patch.
Thanks.
> diff --git a/builtin/blame.c b/builtin/blame.c
> index 21f42b0..a3f6874 100644
> --- a/builtin/blame.c
> +++ b/builtin/blame.c
> @@ -134,7 +134,7 @@ struct progress_info {
> int blamed_lines;
> };
>
> -static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b, long ctxlen,
> +static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b,
> xdl_emit_hunk_consume_func_t hunk_func, void *cb_data)
> {
> xpparam_t xpp = {0};
> @@ -142,7 +142,6 @@ static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b, long ctxlen,
> xdemitcb_t ecb = {NULL};
>
> xpp.flags = xdl_opts;
> - xecfg.ctxlen = ctxlen;
> xecfg.hunk_func = hunk_func;
> ecb.priv = cb_data;
> return xdi_diff(file_a, file_b, &xpp, &xecfg, &ecb);
> @@ -980,7 +979,7 @@ static void pass_blame_to_parent(struct scoreboard *sb,
> fill_origin_blob(&sb->revs->diffopt, target, &file_o);
> num_get_patch++;
>
> - if (diff_hunks(&file_p, &file_o, 0, blame_chunk_cb, &d))
> + if (diff_hunks(&file_p, &file_o, blame_chunk_cb, &d))
> die("unable to generate diff (%s -> %s)",
> oid_to_hex(&parent->commit->object.oid),
> oid_to_hex(&target->commit->object.oid));
> @@ -1129,7 +1128,7 @@ static void find_copy_in_blob(struct scoreboard *sb,
> * file_p partially may match that image.
> */
> memset(split, 0, sizeof(struct blame_entry [3]));
> - if (diff_hunks(file_p, &file_o, 1, handle_split_cb, &d))
> + if (diff_hunks(file_p, &file_o, handle_split_cb, &d))
> die("unable to generate diff (%s)",
> oid_to_hex(&parent->commit->object.oid));
> /* remainder, if any, all match the preimage */
```