describe-doc: clarify default length of abbreviation

From: =?UTF-8?q?Anders=20H=C3=B6ckersten?= <[email protected]>

Clarify the default length used for the abbreviated form used for commits in git describe.

The behavior was modified in Git 2.11.0, but the documentation was not updated to clarify the new behavior.

---
describe-doc: clarify default length of abbreviation

Clarify the default length used for the abbreviated form used for commits in git describe.

The behavior was modified in Git 2.11.0, but the documentation was not updated to clarify the new behavior.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1026%2Fahockersten%2Fdescribe-doc-abbreviation-clarification-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1026/ahockersten/describe-doc-abbreviation-clarification-v1
Pull-Request: https://github.com/git/git/pull/1026

 Documentation/git-describe.txt | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

index a88f6ae2c6e7..f3f41cfac496 100644
@@ -63,10 +63,11 @@ OPTIONS
 	Automatically implies --tags.
 
 --abbrev=<n>::
-	Instead of using the default 7 hexadecimal digits as the
-	abbreviated object name, use <n> digits, or as many digits
-	as needed to form a unique object name.  An <n> of 0
-	will suppress long format, only showing the closest tag.
+	Instead of using the default number of hexadecimal digits (which
+	will vary according to the size of the repository with a default of 
+	7) of the abbreviated object name, use <n> digits, or as many digits
+	as needed to form a unique object name.  An <n> of 0 will suppress 
+	long format, only showing the closest tag.
 
 --candidates=<n>::
 	Instead of considering only the 10 most recent tags as

@@ -139,8 +140,11 @@ at the end.
 
 The number of additional commits is the number
 of commits which would be displayed by "git log v1.0.4..parent".
-The hash suffix is "-g" + unambiguous abbreviation for the tip commit
-of parent (which was `2414721b194453f058079d897d13c4e377f92dc6`).
+The hash suffix is "-g" + an unambigous abbreviation for the tip commit
+of parent (which was `2414721b194453f058079d897d13c4e377f92dc6`). The 
+length of the abbreviation scales as the repository grows, using the 
+approximate number of objects in the repository and a bit of math 
+around the birthday paradox, and defaults to a minimum of 7.
 The "g" prefix stands for "git" and is used to allow describing the version of
 a software depending on the SCM the software is managed with. This is useful
 in an environment where people may use different SCMs.

base-commit: 48bf2fa8bad054d66bd79c6ba903c89c704201f7

gitgitgadget

"Anders Höckersten via GitGitGadget" <[email protected]>

.git/rebase-apply/patch:32: trailing whitespace.
will vary according to the size of the repository with a default of
.git/rebase-apply/patch:34: trailing whitespace.
as needed to form a unique object name. An <n> of 0 will suppress
.git/rebase-apply/patch:46: trailing whitespace.
of parent (which was 2414721b194453f058079d897d13c4e377f92dc6). The .git/rebase-apply/patch:47: trailing whitespace.
length of the abbreviation scales as the repository grows, using the .git/rebase-apply/patch:48: trailing whitespace.
approximate number of objects in the repository and a bit of math
warning: 5 lines applied after fixing whitespace errors.
Applying: describe-doc: clarify default length of abbreviation

Will fix them up while applying, but please be careful the next time. A good way to double check is to do

    $ git format-patch -o my.patch origin..
    $ git checkout --detach origin
    $ git am --whitespace=warn my.patch

that is, (1) create a patch series out of the branch you have worked on, designed to be applied on top of the origin, (2) detach HEAD at the origin, and (3) apply the patch. Once you are done, you can go back to the original branch with

    $ git checkout -

The patch text looks good.

Thanks.

I agree. I will modify this to: "(which will vary according to the number of objects in the repository with a default of 7)" unless you have a different suggestion.

The explanation was mostly copied from the 2.11.0 release notes, but mentioning the birtday paradox is unnecessary. I suggest changing this sentence to:
"The length of the abbreviation scales as the repository grows using the approximate number of objects in the repository, and by default the minimum length is 7."

Best regards,
Anders

Heh. In my priate review, I said that I very much liked the way the new description was phrased with "a bit of math around the birthday paradox". Now I know why I liked that phrasing---it turns out to be my own ;-)

I don't mind with or without mention of the birthday math. Thanks for working on this.

It's a probability fact that goes against common sense. In a romm with 23 people you are 50% likely to find two people with the same birthday.

https://en.wikipedia.org/wiki/Birthday_problem

Felipe Contreras

i've had to explain the logic behind this to people who really have
a tough time understanding this, and it's a concept that applies in a lot of places (surprisingly).

what trips people up is thinking they need to calculate the
probability that two or more people have the same birthday.

no.

what you want to do is calculate the ongoing probability that each
person's birthday is *different* from all the earlier ones. as in:

* prob that 2nd person has a different bday is 364/365
* prob that 3rd person has a different bday is 363/365
* prob that 4th person has a different bday is 362/365

and so on, and as you multiply those together, it's right at 23 people that the chance that that person has a different bday from all earlier ones drops below 50%.

what's neat is that this way of looking at things applies in a lot
of places.

rday

Indeed. Very very few people actually understand probability. Any intuition you have is almost always wrong. Even professional probabilists get probability wrong consistently.

I've found it's safer and easier to not trust my intuition, write code, and that way get the probability (also called Monte Carlo method).

I have a git repository with tricky simulations and I actually had written one for the birthday paradox, but I had not pushed it. Now I have [1].

The actual code is just two lines:

birthdays = Array.new($n) { rand(365.25) }
birthdays.any? { |e| birthdays.count(e) > 1 }

Yet our brain somehow has trouble figuring out the approximate result of that computation.

Cheers.

[1] https://github.com/felipec/simulation/blob/master/examples/birthday

Felipe Contreras

Actually, changed my mind again. I like the phrasing and it's in the "examples" section so a bit of verbosity doesn't hurt. Will submit a new patch with the other changes mentioned + whitespace fixes momentarily.

/A

From: =?UTF-8?q?Anders=20H=C3=B6ckersten?= <[email protected]>

Clarify the default length used for the abbreviated form used for commits in git describe.

The behavior was modified in Git 2.11.0, but the documentation was not updated to clarify the new behavior.

---
describe-doc: clarify default length of abbreviation

Clarify the default length used for the abbreviated form used for commits in git describe.

The behavior was modified in Git 2.11.0, but the documentation was not updated to clarify the new behavior.

Changes since v1:

* tweaked the wording in --abbrev parameter to better match implementation, as suggested by Bagas Sanjaya
* trailing whitespace removed

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1026%2Fahockersten%2Fdescribe-doc-abbreviation-clarification-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1026/ahockersten/describe-doc-abbreviation-clarification-v2
Pull-Request: https://github.com/git/git/pull/1026

Range-diff vs v1:

1: a3aa09ab6fca ! 1: 4cf527801901 describe-doc: clarify default length of abbreviation
@@ Documentation/git-describe.txt: OPTIONS
- Instead of using the default 7 hexadecimal digits as the
- abbreviated object name, use <n> digits, or as many digits
- as needed to form a unique object name. An <n> of 0
-- will suppress long format, only showing the closest tag.
+ Instead of using the default number of hexadecimal digits (which
-+ will vary according to the size of the repository with a default of
-+ 7) of the abbreviated object name, use <n> digits, or as many digits
-+ as needed to form a unique object name. An <n> of 0 will suppress
-+ long format, only showing the closest tag.
++ will vary according to the number of objects in the repository with
++ a default of 7) of the abbreviated object name, use <n> digits, or
++ as many digits as needed to form a unique object name. An <n> of 0
+ will suppress long format, only showing the closest tag.

--candidates=<n>::
- Instead of considering only the 10 most recent tags as
@@ Documentation/git-describe.txt: at the end.

The number of additional commits is the number
@@ Documentation/git-describe.txt: at the end.
-The hash suffix is "-g" + unambiguous abbreviation for the tip commit -of parent (which was 2414721b194453f058079d897d13c4e377f92dc6). +The hash suffix is "-g" + an unambigous abbreviation for the tip commit
-+of parent (which was 2414721b194453f058079d897d13c4e377f92dc6). The -+length of the abbreviation scales as the repository grows, using the -+approximate number of objects in the repository and a bit of math ++of parent (which was 2414721b194453f058079d897d13c4e377f92dc6). The ++length of the abbreviation scales as the repository grows, using the ++approximate number of objects in the repository and a bit of math
+around the birthday paradox, and defaults to a minimum of 7.
The "g" prefix stands for "git" and is used to allow describing the version of a software depending on the SCM the software is managed with. This is useful

 Documentation/git-describe.txt | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

index a88f6ae2c6e7..c6a79c2a0f29 100644
@@ -63,9 +63,10 @@ OPTIONS
 	Automatically implies --tags.
 
 --abbrev=<n>::
-	Instead of using the default 7 hexadecimal digits as the
-	abbreviated object name, use <n> digits, or as many digits
-	as needed to form a unique object name.  An <n> of 0
+	Instead of using the default number of hexadecimal digits (which
+	will vary according to the number of objects in the repository with
+	a default of 7) of the abbreviated object name, use <n> digits, or
+	as many digits as needed to form a unique object name. An <n> of 0
 	will suppress long format, only showing the closest tag.
 
 --candidates=<n>::

@@ -139,8 +140,11 @@ at the end.
 
 The number of additional commits is the number
 of commits which would be displayed by "git log v1.0.4..parent".
-The hash suffix is "-g" + unambiguous abbreviation for the tip commit
-of parent (which was `2414721b194453f058079d897d13c4e377f92dc6`).
+The hash suffix is "-g" + an unambigous abbreviation for the tip commit
+of parent (which was `2414721b194453f058079d897d13c4e377f92dc6`). The
+length of the abbreviation scales as the repository grows, using the
+approximate number of objects in the repository and a bit of math
+around the birthday paradox, and defaults to a minimum of 7.
 The "g" prefix stands for "git" and is used to allow describing the version of
 a software depending on the SCM the software is managed with. This is useful
 in an environment where people may use different SCMs.

base-commit: 48bf2fa8bad054d66bd79c6ba903c89c704201f7

gitgitgadget

I'm a bit late to the party, but am interested in our abbreviation code. I found these updates to be sensible, and that you appropriately adjusted from v1 according to the feedback.

Thanks,
-Stolee