A preprocessor for mdBook which allows selected (sub-)chapter numbers to begin at 0 instead of 1.

Sometimes you want to have a comprehensive preface with nested subchapters or introductory subchapters in specific chapters in your book. The prefix chapter from mdbook is just not flexible enough to handle this. This preprocessor allows you to have possibly nested (sub-)chapter 0.

This preprocessor can be installed with:

cargo install mdbook-chapter-zero

Afterwards, add it to your book.toml:

[preprocessor.chapter-zero]

Finally, run mdbook build as usual.

This preprocessor accepts two configuration options, with their defaults shown below:

[preprocessor.chapter-zero]
levels = []
marker = "<!-- ch0 -->\n"

To apply 0 indexing to all (sub-)chapters at a specific level, use the levels option:

• levels: A list of chapter levels which should start at 0 globally. Defaults to [], which means no global changes to chapter numbering. Here are some examples:
  • If set to [0], then the top level chapters will be 0 indexed.
  • If set to [1], then the first level of subchapters of all chapters will be 0 indexed.
  • If set to [0, 1], then the top level chapters and the first level of subchapters of all chapters will be 0 indexed.

All (sub-)chapters affected by levels will ignore the marker.

To apply 0 indexing only to specific (sub-)chapters, use the marker option:

• marker: A string which signifies that the direct children of this chapter should be 0 indexed. Defaults to <!-- ch0 -->\n.

To apply 0 indexing to children of a specific (sub-)chapter, add the marker anywhere in the Markdown file of the parent chapter. It is better to have it at the top of the file for clarity though.

You can use any string as the marker, not only HTML comments, since the marker will be removed from the Markdown by the preprocessor.

When used together with processors like mdbook-numeq or mdbook-numthm, which can number items with section number as prefixes, make sure to ensure this preprocessor runs before the numbering preprocessors by including e.g.

[preprocessor.chapter-zero]
before = ["numeq", "numthm"]

to your book.toml.