The next thing to do to get BlogHelper9000 functional is to write a command which provides some information about the posts in the blog. I want to know:
- How many published posts there are
- How many drafts there are
- A short list of recent posts
- How long it’s been since a post was published
I also know that I want to introduce a command which will allow me to fix the metadata in the posts, which is a little messy. I’ve been inconsistently blogging since 2007, originally starting off on a self-hosted python blog I’ve forgot the name of before migrating to Wordpress, and then migrating to a short lived .net static site generator before switching over to Jekyll.
Obviously, Markdown powered blogs like Jekyll have to provide non-markdown metadata in each post, and for Jekyll (and most markdown powered blogs) that means: YAML.
Parse that YAML
There are a couple of options when it comes to parsing YAML. One would be to use YamlDotNet which is a stable library which conforms with V1.1 and v1.2 of the YAML specifications.
But where is the fun in that?
I’ve defined a POCO called
YamlHeader which I’m going to use to use as the in-memory object to represent the YAML metadata header at the top of a markdown file.
If we take a leaf from different JSON converters, we can define a
YamlConvert class like this:
With this, we can easily serialise a
YamlHeader into a string, and deserialise a file into a
Deserialising is the slight more complicated of the two, so lets start with that.
Our first unit test looks like this:
This immediately requires us to add an overload for
Deserialise to the
YamlConvert class, which takes a
string. This means our implementation for the first
Deserialise method is simply:
Now we get into the fun part. And a big caveat: I’m not sure if this is the best way of doing this, but it works for me and that’s all I care about.
Anyway. A YAML header block is identified by a single line of only
--- followd by
n lines of YAML which is signified to have ended by another single line of only
---. You can see this in the unit test above.
The algorithm I came up with goes like this:
So in a nutshell, it loops through each line in the file, look for the first
--- to identify the start of the header, and then until it hits another
---, it gathers the lines for further processing.
Translated into C#, the code looks like this:
This is fairly straightforward, and isn’t where I think some of the problems with the way it works actually are - all that is hidden behind
ParseYamlHeader, and is worth a post on its own.