perf(vscode_parser): use MappingProxyType for immutable summary dicts (#848)

[microsasa]

Closes #848

Problem

get_vscode_summary() called _copy_summary() on every cache hit, constructing 4 new dict objects (copying ~1,600 key-value pairs for a user with 400 date-bucketed entries). This generated unnecessary GC pressure in the interactive refresh loop.

Solution

Replace the four mutable dict[str, int] fields in VSCodeLogSummary with Mapping[str, int] backed by types.MappingProxyType:

• VSCodeLogSummary: Field types changed from dict[str, int] to Mapping[str, int]
• _finalize_summary: Wraps accumulator dicts in MappingProxyType at construction time
• _copy_summary: Removed entirely — no longer needed
• get_vscode_summary: Cache hit returns the summary object directly (O(1), zero allocation); cache store no longer copies

Performance

Eliminates 4 dict allocations + deep copies per get_vscode_summary() call in steady state. Each cache-hit call is now O(1) with zero allocation.

Tests

• Cache-hit identity: two consecutive get_vscode_summary() calls return the same object (is identity)
• Immutability: attempting summary.requests_by_date["x"] = 1 raises TypeError (all 4 dict fields)
• build_vscode_summary also produces immutable MappingProxyType fields
• Updated existing test_cached_return_is_mutation_safe and test_finalized_summary_uses_mapping_proxy
• All 1,224 tests pass, pyright strict clean

Generated by Issue Implementer · ● 13.5M · ◷

[Copilot]

Pull request overview

This PR improves the steady-state performance of VS Code Copilot Chat log summarization by eliminating repeated deep copies on get_vscode_summary() cache hits, making cached summaries safe to share by ensuring the per-dimension counters are immutable.

Changes:

• Change VSCodeLogSummary counter fields from dict[str, int] to Mapping[str, int], and back them with types.MappingProxyType.
• Remove _copy_summary() and return cached summaries directly on cache hits (O(1), zero allocation).
• Update and add tests to assert cache-hit identity and immutability of summary mapping fields.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/copilot_usage/vscode_parser.py	Wrap summary dict fields in MappingProxyType and remove defensive copying on cache hits.
tests/copilot_usage/test_vscode_parser.py	Update tests to validate mapping immutability and cache-hit identity semantics.

[github-actions]

Commit pushed: bfdf1da

Generated by Review Responder

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[github-actions]

Commit pushed: a9d63f3

Generated by Review Responder

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

src/copilot_usage/vscode_parser.py:509

• _finalize_summary() currently does dict(acc.<field>) for each counter, but VSCodeLogSummary.__post_init__ then snapshots again via MappingProxyType(dict(val)). When val is already a plain dict, this results in an extra full copy/allocation of each mapping (4 redundant dict copies) on every summary construction (cache miss / initial build). Consider passing the accumulator mappings directly (e.g., the defaultdicts) into VSCodeLogSummary(...) and letting __post_init__ perform the single snapshot/copy, which preserves the immutability/aliasing guarantees while reducing allocations.

    return VSCodeLogSummary(
        total_requests=acc.total_requests,
        total_duration_ms=acc.total_duration_ms,
        requests_by_model=dict(acc.requests_by_model),
        duration_by_model=dict(acc.duration_by_model),
        requests_by_category=dict(acc.requests_by_category),
        requests_by_date=dict(acc.requests_by_date),
        first_timestamp=acc.first_timestamp,

[github-actions]

Quality Gate: APPROVE — Medium-impact performance refactoring with comprehensive test coverage. Auto-approving for merge.

What was evaluated:

• VSCodeLogSummary dict fields changed from dict[str, int] to Mapping[str, int] backed by MappingProxyType — enforces immutability at construction time via __post_init__
• _copy_summary removed; cache hits now return the same object (O(1), zero allocation)
• All downstream consumers (vscode_report.py) use read-only Mapping operations — no breakage
• 8 new/updated tests cover: cache-hit identity, immutability for all 4 fields, constructor wrapping, defaults
• Fully compliant with CODING_GUIDELINES.md (strict typing, Final, frozen dataclass with slots, default_factory=lambda:)