mingyli/hg-rs

An implementation of Mercurial

hg-rs

An implementation of Mercurial in pure Rust.

Example

Add files to be tracked

$ hg-rs init
$ echo 'fn main() { println!("Hello!"); }' > hello.rs
$ hg-rs add hello.rs
$ hg-rs commit -m "Add hello.rs"
$ hg-rs log
changeset: 0:f987a9e38cfcafa5f3149b94149a00fd9857547a
user:      mingyli34@gmail.com
date:      2020-06-22 00:20:57.545640 UTC
summary:   Add hello.rs

Modify files

$ echo 'fn main() { println!("Hello world!"); }' > hello.rs
$ hg-rs status
M hello.rs
$ hg-rs commit -m "Say Hello World instead"
$ hg-rs log
changeset: 1:943d117b5effc7e93d47ca92853a251b2dd8ad03
user:      mingyli34@gmail.com
date:      2020-06-22 00:22:14.086339 UTC
summary:   Say Hello World instead

changeset: 0:f987a9e38cfcafa5f3149b94149a00fd9857547a
user:      mingyli34@gmail.com
date:      2020-06-22 00:20:57.545640 UTC
summary:   Add hello.rs

Inspect file history

$ hg-rs debugdata 0 hello.rs
fn main() { println!("Hello!"); }
$ hg-rs debugdata 1 hello.rs
fn main() { println!("Hello world!"); }

Roadmap

• single file
  • write to rev log
  • dump revisions from rev log
  • inspect indices
• multiple files
  • manage manifest with rev log
  • dirstate to view tracked and untracked files
• collaboration
  • clone
  • merges
  • remote clone
• optimizations
  • deltas
  • compression
• nice to haves
  • run from nested directories
  • formatted output
• writing
  • write a series on implementing Mercurial

Learnings

Mercurial uses revlogs to track revisions of files. A revlog for a particular file consists of an index file containing metadata about its revisions and a data file containing the contents of that file over time. hg-rs's indices are identical to those in hg:

$ hexyl .hg-rs/store/data/hello.rs.i
┌────────┬─────────────────────────┬─────────────────────────┬────────┬────────┐
│00000000│ 00 00 00 00 00 00 00 00 ┊ 22 00 00 00 00 00 00 00 │00000000┊"0000000│
│00000010│ 00 00 00 00 00 00 00 00 ┊ ff ff ff ff ff ff ff ff │00000000┊××××××××│
│00000020│ 43 3c 98 ad 5f 23 dc 68 ┊ 43 07 eb d8 36 25 17 7f │C<××_#×h┊C•××6%••│
│00000030│ 80 92 c4 9a 00 00 00 00 ┊ 00 00 00 00 00 00 00 00 │××××0000┊00000000│
│00000040│ 22 00 00 00 00 00 00 00 ┊ 28 00 00 00 00 00 00 00 │"0000000┊(0000000│
│00000050│ 01 00 00 00 00 00 00 00 ┊ 00 00 00 00 ff ff ff ff │•0000000┊0000××××│
│00000060│ e3 fb 82 85 44 95 a9 db ┊ c6 80 c9 9e 1b f4 f4 d7 │××××D×××┊×××ו×××│
│00000070│ eb cb b2 6d 00 00 00 00 ┊ 00 00 00 00 00 00 00 00 │×××m0000┊00000000│
└────────┴─────────────────────────┴─────────────────────────┴────────┴────────┘

The index file is composed of many 64 byte records, each of which represents a particular revision of the file. A record points to a section of the data file (.hg-rs/store/data/hello.rs.d) which contains the bytes necessary to reconstruct the contents of the file at that revision. The data section can be either a snapshot of the entire file or a delta from a previous revision. A nice thing about revlogs is that since the index file is composed of fixed-sized structs, we can bound the number of disk seeks required to view a given revision.

Compatibility with Mercurial

hg-rs is not compatible with existing Mercurial repositories.