Add backend abstraction for sketch implementations

[GnaneshGnani]

Summary

Introduces the backend abstraction layer (ImplMode) that enables switching between legacy hand-written sketch implementations and future sketchlib-rust backends. This PR adds the configuration infrastructure without yet integrating sketchlib-rust.

Changes

New Files

asap-common/sketch-core/src/config.rs

• Defines ImplMode enum with Legacy and Sketchlib variants
• Provides OnceLock-based global configuration for three sketch types:
  • Count-Min Sketch
  • KLL (quantile estimation)
  • Count-Min-With-Heap (top-k tracking)
• configure() function to set backends once at process startup
• Helper functions: use_sketchlib_for_count_min(), use_sketchlib_for_kll(), use_sketchlib_for_count_min_with_heap()
• force_legacy_mode_for_tests() helper for test environments

asap-query-engine/tests/test_both_backends.rs

• Integration test that automatically runs lib tests with both Legacy and Sketchlib backends
• When running cargo test -p query_engine_rust, tests run with Legacy by default
• This test spawns a second run with --features sketchlib-tests to exercise Sketchlib mode
• Prevents regression by ensuring both backends work correctly

asap-common/sketch-core/src/bin/sketchlib_fidelity.rs

• Skeleton fidelity testing binary with CLI argument parsing
• Helper functions for metrics: Pearson correlation, MAPE, RMSE, rank fraction
• RNG implementation for reproducible benchmarks
• Will be expanded in subsequent PRs as sketch types are integrated

asap-common/sketch-core/report.md

• Documentation for running fidelity tests
• Instructions for backend selection via CLI flags
• Placeholder for benchmark results (to be added in subsequent PRs)

Modified Files

asap-common/sketch-core/src/lib.rs

• Added pub mod config;
• Added test constructor to force legacy mode during tests (ensures existing tests continue to pass)

asap-common/sketch-core/Cargo.toml

• Added clap for CLI parsing in the fidelity binary
• Added ctor dev dependency for test initialization

asap-query-engine/src/main.rs

• Added three new CLI arguments:
  • --sketch-cms-impl: Backend for Count-Min Sketch (default: sketchlib)
  • --sketch-kll-impl: Backend for KLL Sketch (default: sketchlib)
  • --sketch-cmwh-impl: Backend for Count-Min-With-Heap (default: sketchlib)
• Calls config::configure() at startup before any sketch operations

asap-query-engine/src/lib.rs

• Added test constructor that configures backends based on sketchlib-tests feature
• Without feature: uses Legacy backend
• With feature: uses Sketchlib backend

asap-query-engine/Cargo.toml

• Added sketchlib-tests feature flag
• Added ctor dev dependency for test initialization

Technical Approach

• OnceLock Pattern: Thread-safe, one-time configuration prevents race conditions
• Feature-Based Testing: sketchlib-tests feature allows testing both backends in a single cargo test invocation
• No Runtime Overhead: Backend selection happens once at startup, zero overhead during operation
• Fail-Fast: Configuration errors detected immediately at startup

Testing

# Build succeeds
cargo build

# Tests pass with legacy backend (default)
cargo test -p sketch-core
cargo test -p query_engine_rust

# Fidelity binary runs (no benchmarks yet)
cargo run -p sketch-core --bin sketchlib_fidelity

[milindsrivastava1997]

CI is failing. Apart from this, and the one comment, LGTM