(The below text is licensed with CC0, which means that if you want to use or translate it, that is OK by me.)
Ok, I geeked out, and this is probably more information than you need. But it completely answers the question. Sorry. ☺
Locally, I'm at this commit:
$ git show
commit d6cd1e2bd19e03a81132a23b2025920577f84e37
Author: jnthn <[email protected]>
Date: Sun Apr 15 16:35:03 2012 +0200
When I added FIRST/NEXT/LAST, it was idiomatic but not quite so fast. This makes it faster. Another little bit of masak++'s program.
So that's the sha1 I want to reproduce. d6cd1e2bd19e03a81132a23b2025920577f84e37
When I started my investigations, I thought it was something like these things that went into a commit:
$ git --no-replace-objects cat-file commit HEAD
tree 9bedf67800b2923982bdf60c89c57ce6fd2d9a1c
parent de1eaf515ebea46dedea7b3ae0e5ebe3e1818971
author jnthn <[email protected]> 1334500503 +0200
committer jnthn <[email protected]> 1334500545 +0200
When I added FIRST/NEXT/LAST, it was idiomatic but not quite so fast. This makes it faster. Another little bit of masak++'s program.
That is
- The source tree of the commit (which unravels to all the subtrees and blobs)
- The parent commit sha1
- The author info
- The committer info (right, those are different!)
- The commit message
But it turns out there is also a NUL-terminated header that gets appended to this, containing the word "commit", and the length in bytes of all of the above information:
$ printf "commit %s\0" $(git --no-replace-objects cat-file commit HEAD | wc -c)
commit 327
(No, you can't see the NUL byte.)
Put this header and the rest of the information together:
$ (printf "commit %s\0" $(git --no-replace-objects cat-file commit HEAD | wc -c); git cat-file commit HEAD)
commit 327tree 9bedf67800b2923982bdf60c89c57ce6fd2d9a1c
parent de1eaf515ebea46dedea7b3ae0e5ebe3e1818971
author jnthn <[email protected]> 1334500503 +0200
committer jnthn <[email protected]> 1334500545 +0200
When I added FIRST/NEXT/LAST, it was idiomatic but not quite so fast. This makes it faster. Another little bit of masak++'s program.
...and what you get hashes to the right sha1!
$ (printf "commit %s\0" $(git --no-replace-objects cat-file commit HEAD | wc -c); git cat-file commit HEAD) | sha1sum
d6cd1e2bd19e03a81132a23b2025920577f84e37 -
@masak i went with my own meta versioning system i simple store the additional git compatible information as additional meta i go for content unification and then SHA-512 also doing the same for Large assets as i can version blocks. I do not store initial files i store content Blobs of a fixed size 20MB Blocks as this looks like a magic number at present for performance. And as this does not depend on one block per file it reduces space needs a lot.
I do it a bit like dockers overlay file implementations which do also content hashing on block level but less deterministic and predictable. But when you use Docker + BTRFS you come near to my feature set.