Git Hash Digits
Do you ever have discussions with your colleagues about how many Git hash digits should be used to reference Git commits within your Git repository?
The probability of a commit hash collision, of course, depends on the number of digits you track and the number of commits you expect. (Note: I’m talking about colliding on the first few digits you track, not the entire SHA1 hash of 40 hex digits).
I used to use 5 digits, but now I use 6. Some people caution that I should be using 7. So I decided to crank the data. I used the standard formula for choosing r items from n items without replacement. I’d display it here, but I don’t know how to display math formulas on web pages.
If you’re expecting a small project at around 500 commits, the diagram below illustrates the probability of a commit collision using 4-, 5-, 6-, and 7-hex digit hash references.
You can see that even for fewer than 100 commits, the 4-hex-digit scheme has a significant probability of a collision. You may be OK with 5 hex-digits for less than 100.
If you’re expecting closer to 1,000 commits, you’ll definitely want more than 5 hex digits. And even for 6 digits the likelihood of a collision is not trivial.
We can see the 6- and 7-digit data better if we remove the 5-digit data.
We can see the 6-digit labels approaching a 3% chance of at least one collision before reaching 1,000 commits. A 7-digit label still has less than a quarter of a percent.
These graphs are great fun. I’ll leave you with a final one that shows the probability of at least one collision for up to 10,000 commits!
Even for 7-digits the collision likelihood approaches 18%.