"""Layer 2: structured field comparison.

Compares the *set* of items (job postings, press releases, filings,
products - whatever the source's documents represent) between two
snapshots' `structured_summary["urls"]`/`["titles"]`. This is what catches
"a new job posting appeared" or "a press release was removed" without
needing a bespoke parser per source type - collection_service already
records the full current item set on every run, so this is a plain set
diff between two runs.
"""

from __future__ import annotations

from dataclasses import dataclass, field


@dataclass(frozen=True)
class StructuredDiff:
    added: list[str] = field(default_factory=list)
    removed: list[str] = field(default_factory=list)

    @property
    def has_changes(self) -> bool:
        return bool(self.added or self.removed)


def diff_item_sets(previous_urls: list[str], current_urls: list[str]) -> StructuredDiff:
    previous_set = set(previous_urls)
    current_set = set(current_urls)
    return StructuredDiff(
        added=sorted(current_set - previous_set),
        removed=sorted(previous_set - current_set),
    )