Custom Types and Rules-Based Ordering

2025-02-24

I've come up with a system for ordering typelines with custom types (and supporting custom types from the reducer side, but that is another post).

Using the power of Rust features, we can conditionally compile in regex and heck (which are technically unnecessary if we don't support custom types, as every type would be enumerated) so that the problem of Display for an arbitrary CardType still works in the face of custom types. It works by relying on a rules file (the default one is TOML, any serde-supported format is fine), which is converted into regexes and cached to produce and ordering for these types.

A rule looks like

[[creature]]
matchers = [
	{literal = "Medusa-Head"},
	{literal = "Medusa-Hand"},
	{regex = "^Medusa-\\w+$"}
]
precedence = 10

A rule (which applies to a card type: artifact, creature, land, etc.) has 2 parts:

• matchers which are a list of either literals (which specify an exact string to look for), or regexs (which specify a regex to look for when matching strings)
• precedence which is an unsigned integer (uses the platforms pointer size, but that's more of an implementation detail). The lower the value is, the earlier in the resulting type line it goes.

A rules file specifies an exact order using these meta rules:

• A type that is caught by a rule will always go before a type that isn't
• Two types that are assigned the same precedence are ordered using the default ordering rules (defined types then custom types in alphabetical order)
• If there are 2 rules that would match the same type:
  • The rule with lower precedence is the one that applies
    • If they have the same precedence, the first declared rule applies

By enabling feature regex (which will be automatically enabled with the usage of any custom type), this rules-based engine turns on and switches from a Rust-defined ordering algorithm to this rules-based one.