order spans by their start index, return start index to the UI (#259)

Addresses allenai/playground-issues-repo#156

This PR ensures that spans are sorted by their `start_index` (left)
before being returned to the UI. This allows the UI to iterate through
the spans one at a time, only adding highlights for the first span.

It also returns the `start_index` to the UI. That's not essential, but
may be nice in the future or while debugging!

Author: mtblanton

src/attribution/attribution_service.py

@@ -123,6 +123,7 @@ class GetAttributionRequest(APIInterface):
 @dataclass
 class ResponseAttributionSpan:
     text: str
+    start_index: int
     documents: list[int] = field(default_factory=list)
 
 
@@ -138,9 +139,11 @@ def from_flattened_span(cls, span: FlattenedSpan) -> Self:
                 ResponseAttributionSpan(
                     text=nested_span.text,
                     documents=[document.document_index for document in nested_span.documents],
+                    start_index=nested_span.left,
                 )
                 for nested_span in span.nested_spans
             ],
+            start_index=span.left,
         )
 
 
@@ -191,9 +194,7 @@ def get_attribution(
         )
     except UnexpectedStatus as e:
         msg = f"Something went wrong when calling the infini-gram API: {e.status_code} {e.content.decode()}"
-        raise exceptions.BadGateway(
-            msg
-        )
+        raise exceptions.BadGateway(msg) from e
 
     if isinstance(attribution_response, HTTPValidationError):
         # validation error handling
@@ -273,12 +274,14 @@ def get_attribution(
 
     return {
         "index": index,
+        # The UI uses this sort order to show the documents in desc relevance
         "documents": sorted(
             mapped_documents.values(),
             key=lambda document: document.relevance_score,
             reverse=True,
         ),
-        "spans": list(mapped_spans.values()),
+        # The UI uses this sort order to highlight spans in order
+        "spans": sorted(mapped_spans.values(), key=lambda span: span.start_index),
     }