improvement: plugin interface v2

[sidkmenon]

significantly simplify the plugin interface. split into two types, now just defined by WASM:

1. The classifier plugin implements the generated GuestClassifier trait; it determines if and how an entire file is (1) collapsed and (2) skips structural diffing:

use diffr_plugin_sdk::prelude::*;

struct MyClassifier;
impl ClassifierGuest for MyClassifier {
    type Classifier = Self;
}
impl GuestClassifier for MyClassifier {
    fn new(options: String) -> Result<Self, String> { Ok(Self) }
    fn classify(&self, file: FileEntry) -> Result<Classification, String> {
        Ok(Classification { tags: vec![Tag::Custom("schema".into())], hidden: None })
    }
}
export_classifier!(MyClassifier);

2. A shape plugin implements the generated GuestPlugin trait; it will be reminiscent of golang's astutil.Apply, for those familiar. Shape plugins determine the presentation of individual AST folds in an open file (e.g. collapsing + replacing w pseudocode, or them grouping together, and so on):

use diffr_plugin_sdk::prelude::*;

struct MyPlugin;
impl Guest for MyPlugin {
    type Plugin = Self;
}
impl GuestPlugin for MyPlugin {
    fn new(options: String) -> Result<Self, String> { Ok(Self) }
    // cursor is a host-provided interface over the matched ast tree(s); phase indicates preorder/postorder.
    async fn visit(&self, cursor: &Cursor, phase: Visit) -> Result<bool, String> {
        // Inspect this node and edit it through the host cursor.
        Ok(true)
    }
}
export_shape!(MyPlugin);

it all ties together like this:

sequenceDiagram
    participant C as UI / API consumer
    participant D as diffr
    participant G as git
    participant K as Classifier plugin
    participant E as Diff engine
    participant S as Shape plugins

    D->>G: List the comparison's changed files
    G-->>D: Paths, blobs, statuses
    loop Each changed file
        D->>K: classify(file)
        K-->>D: Tags, and whether to hide the file
    end
    D-->>C: start record: the tagged manifest
    par Each file, on --jobs workers
        D->>G: Read both sides
        D->>E: Diff
        alt Hidden or generated
            E->>E: Line diff
        else
            E->>E: Parse with tree-sitter, diff ASTs (difftastic)
        end
        E-->>D: Aligned regions and folds
        loop Each shape plugin, in plugins.order
            D->>S: visit(cursor, pre | post), once per region
            S->>D: cursor edits: cut, join, link, collapse, label
        end
        D-->>C: file record, as soon as it is done
    end
    D-->>C: complete record

Loading

[sidkmenon]

upside: there is a HUGE performance improvemnet in here as well; for large diffs:

i.e. abt 30x improvement in time-to-first-diff and 5x improvement in total time taken :)