git force-prefix alters the commit hash of the HEAD commit so that its prefix
is a specific value.
This program came about because I was in a debate with a coworker about whether it is possible to have any control of the git hash of a commit.
The commit hash for a commit is calculated by taking the SHA1 hash of a
specific set of bytes representing the commit. This can seen approximately
using git cat-file commit HEAD:
tree 35e2cf7b5ae7082675c96246740f5026ff6b796d
parent d06f00d1679a4a0fdf38bc4b95309514acd6011b
author Bryan Burgers <[email protected]> 1524851698 -0500
committer Bryan Burgers <[email protected]> 1524859906 -0500
Cleanup error handling
If we assume that we can't change any data in this structure, then my coworker was right: we don't have any control over the commit hash.
However, the author date and commit date are measure in a resolution of one second, and who really cares if you committed your code at 4:38:07 pm or 4:41:19 pm? Nobody, that's who.
This program exploits that fact and twiddles with the author and committer date until it finds some that make the commit hash match the desired prefix.
Build the program using cargo build --release.
Then copy target/release/git-force-prefix to somewhere in your $PATH.
The next time you want to force a commit hash prefix, first make the commit,
then run git force-prefix 012345 to get a commit with the first 6 characters
of the commit hash as 012345. The output will be a command that can be run to
change the commit hash.
Every commit in this repository starts with the prefix d06f00d because this
project dogfood's its own process.