Calculate patch – the number of commits since the last change in major.minor version.

Create a file in your repository, usually called VERSION, and put major.minor version in it.

echo 1.0 >VERSION

Then use the action in your workflow.

on: push
jobs:
  release:
    runs-on: ubuntu-latest
    steps:
      - uses: avakar/derive-version@v1
        id: version
      - run: |
        mkdir _build
        cd _build
        cmake .. -DVERSION="${{ steps.version.outputs.version }}"
      - uses: avakar/tag-and-release@v1
        with:
          tag_name: ${{ steps.version.outputs.version }}

• file: the name of the version file; the contents of the file will be prepended to the calculated patch number. Defaults to VERSION
• commit: the commit for which the version should be derived, defaults to HEAD
• deepen-by: if the repo is too shallow for the calculation, deepen it by the specified number of commits, defaults to 100

• patch: the number of commits since the last change in the version file
• version: the same as patch, but prepended by the contents of the version file and a dot, e.g if the version file contains 1.0 and patch is 42, this output will be set to 1.0.42

Formally, this action calculates the length of the longest path starting at the specified commit C (usually HEAD) in a subgraph generated from the commit graph by removing commits in which the contents of the version file differs from the contents of the version file in C.

Let's break this down.

If the version file is called VERSION, which is recommended and the default value for the file parameter, then commits in which you change the contents of VERSION will have the patch number of 0. The next commit will have patch number of 1, the next one 2, and so on until you modify VERSION again. The patch number then resets back to 0.

The full version is ${VERSION}.${patch}, if you put major.minor in the VERSION file, the resulting version can be used as a semantic version of the commit.

Here's how versions might look like in a linear graph (parents are to the left).

0.1.0---0.1.1---0.1.2---0.9.0---0.9.1---1.0.0---1.0.1

And here's are some examples of a branching history.

0.1.0---0.1.1-----------------------0.1.4
      \                          /
       \                        /
        --0.1.1---0.1.2---0.1.3-

0.1.0---0.1.1---0.1.2---0.1.3-------0.1.4
      \                          /
       \                        /
        --0.1.1---0.1.2---0.1.3-

0.1.0---0.1.1---0.2.0---0.2.1-------0.2.2
      \                          /
       \                        /
        --0.1.1---0.1.2---0.1.3-

Notice that although multiple commits can have the same version, versions of commits on each linear path strictly decrease. Therefore, pushing to a branch will always increase its version.