Fornalder ("Bygone Age") is a small utility that can be used to ingest commit data from collections of git repositories and visualize it in various ways.

It was used to generate the graphs in this blog post. It's made to work with data going back a long time (a decade or two); with shorter time spans the output will probably look quite bad.

Clone the project repository and make sure you have the Rust prerequisites installed, then:

$ cargo build

You need a fairly recent version of Gnuplot to generate plots. Make sure it is installed.

Clone the repositories of interest to a local directory, then ingest them. This can be run multiple times to add to or update the database:

$ target/debug/fornalder --meta projects/project-meta.json \
                         ingest db.sqlite repo-1.git repo-2.git ...

When the database has been created, generate one or more plots, e.g:

$ target/debug/fornalder --meta projects/project-meta.json \
                         plot db.sqlite \
                         --cohort firstyear \
                         --interval year \
                         --unit authors \
                         graph.png

If something looks odd in the result, you can also explore the database directly.

$ sqlite3 db.sqlite
sqlite> .tables
authors      raw_commits
sqlite> .schema authors
CREATE TABLE authors(
  author_name TEXT,
  first_time,
  first_year,
  last_time,
  last_year,
  active_time,
  n_commits,
  n_changes
);
sqlite> SELECT author_name, first_year FROM authors ORDER BY first_year;
[...]

Guide to arguments:

--meta <meta>
    Optional. Project metadata to use. See projects/ for examples.

--cohort < domain | firstyear | prefix | repo | suffix >
    Optional. How to split the data into cohorts.

--interval < year | month >
    Optional. Time interval of each histogram bin.

--unit < authors | changes | commits >
    Optional. What's being measured -- active authors, number of lines
    changed, or commit count.

--from year
    Optional. First year to plot.

--to year
    Optional. Last year to plot.

We support ingestion from both bare and non-bare repositories:

git clone https://git.example.com
git clone --bare https://git.example.com

Another option that can massively reduce data transfer is --filter=blob:none, which will only clone commit metadata, not the files themselves.

git clone --bare --filter=blob:none https://git.example.com

Note that this is not supported by all git servers (cloning may fail). Using this mode also prevents the use of the --unit changes option (counting the number of changed lines), or in general inspecting --stat output from the commit database.