What’s the difference between the prime version and the previous version?

The previous version is, broadly speaking, the version that came before the current one (current the one Nyx is in the process of releasing).

The prime version is a concept introduced by Nyx for collapsed versioning (also see this F.A.Q.), where the previous version alone is not enough in order to generate a new version number. The prime version is only used for this kind of versioning and you can see an example here with a broad explanation.

The distinction between previous and prime version is specific to Nyx and does not apply out of this scope.

The prime version is the version tag of the first reachable parent commit that doesn’t use extra identifiers (so using only core identifiers). When evaluating the parent commit of a merge commit only the first parent (the commit that received the merge) is followed. The prime and previous versions may or may not match.

For example 1.2.3 may be a prime version but 1.2.3-alpha.1 may not as it uses additional identifiers.

For an example let’s compare these two commit histories side by side. On the left side you have a regular commit history using standard versioning only, while on the right side you have an alpha pre-release branch (on the right-side lane) using the collapsed versioning. These two examples are widely illustrated here.

Regular	Collapsed

Prime versions are those on the commits highlighted with orange circles while the path from any commit in the pre-release branch to (the commit tagged with) its prime version is highlighted in orange.

A couple of things to note:

• prime versions are only in the main line branch as they must not have extra identifiers
• the path from any pre-release commit to its prime version commit is interrupted whenever the pre-release branch is closed or deleted (like at commits c8 and c12)
• a simple merge into the pre-release branch doesn’t change the path to the prime version commit as you can see by commit m18, which still has 1.1.0 as its prime version (from commit m13), not 1.3.0 (from commit m17), because m18’s first parent is c16 (which leads back to m13), not m17

If you want to know more about how the prime version is used when computing collapsed versions see here.