[PATCH v3 0/5] Improve abbreviation disambituation

Thanks for the feedback on my previous versions, and for patience with my inexperience on the mailing list. I tried to respond to all feedback, but decided to not apply one suggestion:

* Removed the "sort -R" from p0008-abbrev.sh, since it is a GNU-
  specific flag. The perf test now considers objects in "discovery"
  order, which improved performance all versions of the test as
  expected. New perf numbers are provided.

* get_hex_char_from_oid(): renamed `i` to `pos`. I did not unify the
  code in this method with that in hex.c:sha1_to_hex_r due to the
  extra branch in get_hex_char_from_oid and wanting to inline the
  method. If we want to make this method available for other callers,
  then I would be happy to submit a separate patch for this change
  after the current patch is accepted.

* Removed unecessary includes.

* Use uint32_t instead of unsigned int in test-list-objects.c

* Rearranged arguments in test-list-objects and fixed the n == 0 bug.

---

When displaying object ids, we frequently want to see an abbreviation for easier typing. That abbreviation must be unambiguous among all object ids.

The current implementation of find_unique_abbrev() performs a loop checking if each abbreviation length is unambiguous until finding one that works. This causes multiple round-trips to the disk when starting with the default abbreviation length (usually 7) but needing up to 12 characters for an unambiguous short-sha. For very large repos, this effect is pronounced and causes issues with several commands, from obvious consumers `status` and `log` to less obvious commands such as `fetch` and `push`.

This patch improves performance by iterating over objects matching the short abbreviation only once, inspecting each object id, and reporting the minimum length of an unambiguous abbreviation.

A helper program `test-list-objects` outputs a sampling of object ids, which we reorder using `sort -R` before using them as input to a performance test.

A performance helper `test-abbrev` and performance test `p0008-abbrev.sh` are added to demonstrate performance improvements in this area.

I include performance test numbers in the commit messages for each change, but I also include the difference between the baseline and the final change here:

p0008.1: find_unique_abbrev() for existing objects --------------------------------------------------

For 10 repeated tests, each checking 100,000 known objects, we find the following results when running in a Linux VM:

|       | Pack  | Packed  | Loose  | Base   | New    |        |
| Repo  | Files | Objects | Objects| Time   | Time   | Rel%   |
|-------|-------|---------|--------|--------|--------|--------|
| Git   |     1 |  230078 |      0 | 0.09 s | 0.05 s | -44.4% |
| Git   |     5 |  230162 |      0 | 0.11 s | 0.08 s | -27.3% |
| Git   |     4 |  154310 |  75852 | 0.09 s | 0.06 s | -33.3% |
| Linux |     1 | 5606645 |      0 | 0.13 s | 0.05 s | -61.5% |
| Linux |    24 | 5606645 |      0 | 1.13 s | 0.88 s | -22.1% |
| Linux |    23 | 5283204 | 323441 | 1.08 s | 0.80 s | -25.9% |
| VSTS  |     1 | 4355923 |      0 | 0.12 s | 0.05 s | -58.3% |
| VSTS  |    32 | 4355923 |      0 | 1.02 s | 0.95 s | - 6.9% |
| VSTS  |    31 | 4276829 |  79094 | 2.25 s | 1.93 s | -14.2% |

For the Windows repo running in Windows Subsystem for Linux:

    Pack Files: 50
Packed Objects: 22,385,898
 Loose Objects: 492
     Base Time: 5.69 s
      New Time: 4.09 s
         Rel %: -28.1%

p0008.2: find_unique_abbrev() for missing objects -------------------------------------------------

For 10 repeated tests, each checking 100,000 missing objects, we find the following results when running in a Linux VM:

|       | Pack  | Packed  | Loose  | Base   | New    |        |
| Repo  | Files | Objects | Objects| Time   | Time   | Rel%   |
|-------|-------|---------|--------|--------|--------|--------|
| Git   |     1 |  230078 |      0 | 0.66 s | 0.07 s | -89.4% |
| Git   |     5 |  230162 |      0 | 0.90 s | 0.12 s | -86.7% |
| Git   |     4 |  154310 |  75852 | 0.79 s | 0.09 s | -88.6% |
| Linux |     1 | 5606645 |      0 | 0.48 s | 0.04 s | -91.7% |
| Linux |    24 | 5606645 |      0 | 4.41 s | 0.85 s | -80.7% |
| Linux |    23 | 5283204 | 323441 | 4.11 s | 0.78 s | -81.0% |
| VSTS  |     1 | 4355923 |      0 | 0.46 s | 0.04 s | -91.3% |
| VSTS  |    32 | 4355923 |      0 | 5.40 s | 0.98 s | -81.9% |
| VSTS  |    31 | 4276829 |  79094 | 5.88 s | 1.04 s | -82.3% |

For the Windows repo running in Windows Subsystem for Linux:

    Pack Files: 50
Packed Objects: 22,385,898
 Loose Objects: 492
     Base Time: 38.9 s
      New Time:  2.7 s
         Rel %: -93.1%

Derrick Stolee (5):
  test-list-objects: List a subset of object ids
  p0008-abbrev.sh: Test find_unique_abbrev() perf
  sha1_name: Unroll len loop in find_unique_abbrev_r
  sha1_name: Parse less while finding common prefix
  sha1_name: Minimize OID comparisons during disambiguation
 Makefile                     |   2 +
 sha1_name.c                  | 129 ++++++++++++++++++++++++++++++++++++++-----
 t/helper/.gitignore          |   2 +
 t/helper/test-abbrev.c       |  18 ++++++
 t/helper/test-list-objects.c |  87 +++++++++++++++++++++++++++++
 t/perf/p0008-abbrev.sh       |  22 ++++++++
 6 files changed, 245 insertions(+), 15 deletions(-)
 create mode 100644 t/helper/test-abbrev.c
 create mode 100644 t/helper/test-list-objects.c
 create mode 100755 t/perf/p0008-abbrev.sh