Macro sp_npos_elections::generate_solution_type

generate_solution_type!() { /* proc-macro */ }

Generates a struct to store the election result in a small/compact way. This can encode a structure which is the equivalent of a sp_npos_elections::Assignment<_>.

The following data types can be configured by the macro.

• The identifier of the voter. This can be any type that supports parity-scale-codec’s compact encoding.
• The identifier of the target. This can be any type that supports parity-scale-codec’s compact encoding.
• The accuracy of the ratios. This must be one of the PerThing types defined in sp-arithmetic.

Moreover, the maximum number of edges per voter (distribution per assignment) also need to be specified. Attempting to convert from/to an assignment with more distributions will fail.

For example, the following generates a public struct with name TestSolution with u16 voter type, u8 target type and Perbill accuracy with maximum of 4 edges per voter.

generate_solution_type!(pub struct TestSolution::<
    VoterIndex = u16,
    TargetIndex = u8,
    Accuracy = Perbill,
>(4));

The output of this macro will roughly look like:

struct TestSolution {
	voters1: vec![(u16 /* voter */, u8 /* target */)]
	voters2: vec![
		(u16 /* voter */, [u8 /* first target*/, Perbill /* proportion for first target */], u8 /* last target */)
	]
	voters3: vec![
		(u16 /* voter */,  [
			(u8 /* first target*/, Perbill /* proportion for first target */ ),
			(u8 /* second target */, Perbill /* proportion for second target*/)
		], u8 /* last target */)
	],
	voters4: ...,
}

impl NposSolution for TestSolution {};
impl Solution for TestSolution {};

The given struct provides function to convert from/to Assignment as part of [sp_npos_elections::Solution] trait:

• fn from_assignment<..>(..)
• fn into_assignment<..>(..)

Compact Encoding

The generated struct is by default deriving both Encode and Decode. This is okay but could lead to many 0s in the solution. If prefixed with #[compact], then a custom compact encoding for numbers will be used, similar to how parity-scale-codec’s Compact works.

generate_solution_type!(
    #[compact]
    pub struct TestSolutionCompact::<VoterIndex = u16, TargetIndex = u8, Accuracy = Perbill>(8)
);