Add Indexer trait implementation built with rangemap

[ekroon]

Description

After implementation, something like this should work:

StringSpark::<BuildRangemapIndexer>::default().spark(data)

Links

• https://github.com/ekroon/sparklines/blob/799994da38da0eb4d847bd8dcb8931963310e8ff/src/indexer.rs#LL3-L5C2
• rangemap

[ekroon]

When implementing this, it breaks the defaulted version.

struct StringSpark<T=BuildAlgorithmicIndexer> { ... } 
StringSpark::default().spark(data); // doesn't compile because -> type annotations needed for `StringSpark<T>`

This is a known issue / design implementation in Rust, where StringSpark::default() does type inference without fallback to default types. There are work-arounds:

<StringSpark>::default().spark(data); 
let s: StringSpark = StringSpark::default();
s.spark(data);

The difference is that in Type location the Rust compiler will use fill in type parameters, BUT in a Path expression everything will be inferred.

let _ = StringSpark::default(); // StringSpark in path expression only, so defaults to StringSpark<_> which cannot be solved

// Following is the same, SpringSpark is also in Type location. 
let _: StringSpark = StringSpark::default();
let _: StringSpark<BuildAlgorithmicIndexer> = StringSpark::<_>::default();

// This doesn't work because _ for T disables the default and uses inference
let _: StringSpark<_> = StringSpark::default();

// Using <SpringSpark> in a path expression will interpret SpringSpark within the brackets as a Type
let _ = <StringSpark>::default(); // this works, as T will be replaced with default
let _ = <StringSpark<_>>::default(); // Doesn't work as the inference replaces the default again.

Links

• Tracking Issue for RFC 213: Default Type Parameter Fallback rust-lang/rust#27336 (comment)
• Cannot infer type with a default const parameter rust-lang/rust#95486