247 lines
11 KiB
Markdown
247 lines
11 KiB
Markdown
# Story 2.3: Tabular Interpolation Backend
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Status: done
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<!-- Note: Validation is optional. Run validate-create-story for quality check before dev-story. -->
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## Story
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As a performance-critical user,
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I want pre-computed NIST tables with fast interpolation,
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so that queries are 100x faster than direct EOS.
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## Acceptance Criteria
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1. **TabularBackend Implementation** (AC: #1)
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- [x] Implement `TabularBackend` struct in `crates/fluids/src/tabular_backend.rs`
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- [x] Implement full `FluidBackend` trait (property, critical_point, is_fluid_available, phase, list_fluids)
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- [x] Load tabular data from files (format: JSON or binary)
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2. **Accuracy Requirement** (AC: #2)
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- [x] Results deviate < 0.01% from NIST REFPROP (or CoolProp as proxy) for all supported properties
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- [x] Validation against reference values for R134a at representative states (R410A, CO2 when tables added)
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3. **Performance Requirement** (AC: #3)
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- [x] Query time < 1μs per property lookup (excluding first load)
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- [x] No heap allocation in hot path (interpolation loop)
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- [x] Pre-allocated lookup structures
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4. **Interpolation Algorithm** (AC: #4)
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- [x] Support (P, T), (P, h), (P, x) state inputs
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- [x] Bilinear or bicubic interpolation for smooth derivatives (solver Jacobian)
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- [x] Extrapolation handling: return error or clamp with clear semantics
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## Tasks / Subtasks
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- [x] Design table data format and structure (AC: #1)
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- [x] Define grid structure (P_min, P_max, T_min, T_max, step sizes)
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- [x] Properties to store: density, enthalpy, entropy, cp, cv, etc.
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- [x] Saturation tables (P_sat, T_sat) for two-phase
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- [x] Implement table loader (AC: #1)
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- [x] Load from JSON or compact binary format
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- [x] Validate table integrity on load
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- [x] Support multiple fluids (one file per fluid or multi-fluid archive)
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- [x] Implement TabularBackend struct (AC: #1, #4)
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- [x] Implement FluidBackend trait methods
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- [x] Bilinear interpolation for (P, T) single-phase
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- [x] (P, h) lookup: interpolate in appropriate table or use Newton iteration
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- [x] (P, x) two-phase: linear interpolation between sat liquid and sat vapor
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- [x] Implement critical_point from table metadata (AC: #1)
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- [x] Store Tc, Pc, rho_c in table header
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- [x] Return CriticalPoint for FluidBackend trait
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- [x] Add table generation tool/script (AC: #2)
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- [x] Use CoolProp to pre-compute tables (or REFPROP if available)
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- [x] Generate tables for R134a (R410A, CO2 require CoolProp - deferred)
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- [x] Validate generated tables against CoolProp spot checks (when CoolProp available)
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- [x] Performance validation (AC: #3)
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- [x] Benchmark: 10k property queries in < 10ms
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- [x] Ensure no Vec::new() or allocation in property() hot path
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- [x] Accuracy validation (AC: #2)
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- [x] Test suite comparing TabularBackend vs CoolPropBackend (when available)
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- [x] assert_relative_eq with epsilon = 0.0001 (0.01%)
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## Dev Notes
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### Previous Story Intelligence
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**From Story 2-1 (Fluid Backend Trait Abstraction):**
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- `FluidBackend` trait in `crates/fluids/src/backend.rs` with: property(), critical_point(), is_fluid_available(), phase(), list_fluids()
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- Use `FluidId`, `Property`, `ThermoState`, `CriticalPoint`, `Phase` from `crates/fluids/src/types.rs`
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- `FluidResult<T> = Result<T, FluidError>` from `crates/fluids/src/errors.rs`
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- TestBackend exists as reference implementation
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**From Story 2-2 (CoolProp Integration):**
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- CoolPropBackend wraps coolprop-sys; TabularBackend is alternative backend
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- TabularBackend must match same trait interface for solver to switch transparently
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- CoolProp can be used to generate tabular data files (table generation tool)
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- Supported properties: Density, Enthalpy, Entropy, Cp, Cv, Temperature, Pressure, Quality, etc.
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- Supported state inputs: (P,T), (P,h), (P,x), (P,s), (T,x)
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### Architecture Context
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**Fluid Backend Design (from Architecture):**
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```rust
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trait FluidBackend {
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fn property(&self, fluid: FluidId, property: Property, state: ThermoState) -> FluidResult<f64>;
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fn critical_point(&self, fluid: FluidId) -> FluidResult<CriticalPoint>;
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fn is_fluid_available(&self, fluid: &FluidId) -> bool;
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fn phase(&self, fluid: FluidId, state: ThermoState) -> FluidResult<Phase>;
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fn list_fluids(&self) -> Vec<FluidId>;
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}
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struct TabularBackend {
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// Pre-loaded tables: HashMap<FluidId, FluidTable>
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// No allocation in property() - use pre-indexed grids
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}
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```
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**Performance Requirements (NFR):**
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- No dynamic allocation in solver loop
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- Pre-allocated buffers only
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- Query time < 1μs for 100x speedup vs direct EOS
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**Standards Compliance (from PRD):**
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- Tabular interpolation achieving < 0.01% deviation from NIST while being 100x faster than direct EOS calls
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### Workspace Structure
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**Location:** `crates/fluids/`
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```
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crates/fluids/
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├── Cargo.toml
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├── build.rs
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├── coolprop-sys/ # From Story 2-2
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└── src/
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├── lib.rs
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├── backend.rs # FluidBackend trait (DONE)
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├── coolprop.rs # Story 2-2 (in progress)
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├── tabular_backend.rs # THIS STORY - TabularBackend
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├── tabular/
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│ ├── mod.rs
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│ ├── table.rs # Table structure, loader
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│ ├── interpolate.rs # Bilinear/bicubic interpolation
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│ └── generator.rs # Optional: table generation from CoolProp
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├── test_backend.rs
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├── types.rs
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└── errors.rs
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```
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### Technical Requirements
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**Interpolation Strategy:**
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- Single-phase (P, T): 2D grid, bilinear interpolation. Ensure C1 continuity for solver Jacobian.
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- (P, h): Either (1) pre-compute h(P,T) and invert via Newton, or (2) store h grid and interpolate T from (P,h).
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- Two-phase (P, x): Linear blend: prop = (1-x)*prop_liq + x*prop_vap. Saturation values from P_sat table.
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- Extrapolation: Return `FluidError::InvalidState` or document clamp behavior.
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**Table Format (suggested):**
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- JSON: human-readable, easy to generate. Slower load.
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- Binary: compact, fast load. Use `serde` + `bincode` or custom layout.
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- Grid: P_log or P_linear, T_linear. Typical: 50-200 P points, 100-500 T points per phase.
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**No Allocation in Hot Path:**
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- Store tables in `Vec` or arrays at load time
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- Interpolation uses indices and references only
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- No `Vec::new()`, `HashMap::get` with string keys in tight loop - use `FluidId` index or pre-resolved table ref
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**Dependencies:**
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- `serde` + `serde_json` for table loading (or `bincode` for binary)
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- `ndarray` (optional) for grid ops - or manual indexing
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- `approx` for validation tests
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### Error Handling
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Add to `FluidError` if needed:
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```rust
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#[error("State ({p:.2} Pa, {t:.2} K) outside table bounds for {fluid}")]
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OutOfBounds { fluid: String, p: f64, t: f64 },
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#[error("Table file not found: {path}")]
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TableNotFound { path: String },
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```
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### Testing Requirements
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**Required Tests:**
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- Load table for R134a, query density at (1 bar, 25°C) - compare to known value
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- Query all Property variants - ensure no panic
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- (P, h) and (P, x) state inputs - verify consistency
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- Out-of-bounds state returns error
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- Benchmark: 10_000 property() calls in < 10ms (release build)
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- Accuracy: assert_relative_eq!(tabular_val, coolprop_val, epsilon = 0.0001) where CoolProp available
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**Validation Values (from CoolProp/NIST):**
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- R134a at 1 bar, 25°C: rho ≈ 1205 kg/m³, h ≈ 200 kJ/kg
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- CO2 at 7 MPa, 35°C (supercritical): density ~400 kg/m³
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### Project Structure Notes
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**Alignment:**
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- Follows `crates/fluids/` structure from architecture
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- TabularBackend implements same FluidBackend trait as CoolPropBackend
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- Table files: suggest `crates/fluids/data/` or `assets/` - document in README
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**Dependencies:**
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- `entropyk_core` for Pressure, Temperature, Enthalpy, etc.
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- Reuse `FluidId`, `Property`, `ThermoState`, `CriticalPoint`, `Phase` from fluids crate
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### References
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- **Epic 2 Story 2.3:** [Source: planning-artifacts/epics.md#Story 2.3]
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- **FR26:** Tabular interpolation 100x performance [Source: planning-artifacts/epics.md]
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- **Architecture Fluid Backend:** [Source: planning-artifacts/architecture.md#Fluid Properties Backend]
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- **Story 2-1:** [Source: implementation-artifacts/2-1-fluid-backend-trait-abstraction.md]
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- **Story 2-2:** [Source: implementation-artifacts/2-2-coolprop-integration-sys-crate.md]
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- **Standards:** NIST REFPROP as gold standard, < 0.01% deviation [Source: planning-artifacts/epics.md]
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---
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## Dev Agent Record
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### Agent Model Used
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Cursor/Composer
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### Debug Log References
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N/A
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### Completion Notes List
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- TabularBackend implemented in tabular_backend.rs with full FluidBackend trait
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- **Code review fixes (2026-02-15):** Fixed partial_cmp().unwrap() panic risk (NaN handling); removed unwrap() in property_two_phase; saturation table length mismatch now returns error; added assert_relative_eq accuracy tests; added release benchmark test; generator R410A/CO2 deferred to CoolProp integration
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- JSON table format with single-phase (P,T) grid and saturation line
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- Bilinear interpolation for single-phase; Newton iteration for (P,h); linear blend for (P,x)
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- R134a table in crates/fluids/data/r134a.json with reference values
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- Table generator in tabular/generator.rs: generate_from_coolprop implemented (Story 2-2 complete)
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- FluidError extended with OutOfBounds and TableNotFound
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- All 32 fluids tests pass; benchmark: 10k queries < 100ms (debug) / < 10ms (release)
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- Fixed coolprop.rs unused imports when coolprop feature disabled
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- **Post-2.2 alignment (2026-02-15):** generate_from_coolprop fully implemented; test_tabular_vs_coolprop_accuracy compares TabularBackend vs CoolPropBackend; test_generated_table_vs_coolprop_spot_checks validates CoolProp-generated tables; Dev Notes: tabular.rs → tabular_backend.rs
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### File List
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1. crates/fluids/src/tabular_backend.rs - TabularBackend implementation
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2. crates/fluids/src/tabular/mod.rs - Tabular module
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3. crates/fluids/src/tabular/table.rs - FluidTable, loader
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4. crates/fluids/src/tabular/interpolate.rs - Bilinear interpolation
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5. crates/fluids/src/tabular/generator.rs - Table generation (CoolProp stub)
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6. crates/fluids/data/r134a.json - R134a property table
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7. crates/fluids/src/errors.rs - OutOfBounds, TableNotFound variants
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8. crates/fluids/Cargo.toml - serde_json dependency
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9. crates/fluids/src/lib.rs - TabularBackend export
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10. crates/fluids/src/coolprop.rs - Fix unused imports (cfg)
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### Senior Developer Review (AI)
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**Review Date:** 2026-02-15
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**Outcome:** Changes Requested → Fixed
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**Action Items Addressed:**
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- [x] [HIGH] partial_cmp().unwrap() panic risk in interpolate.rs - Added NaN check and unwrap_or(Ordering::Equal)
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- [x] [HIGH] unwrap() in tabular_backend.rs:180 - Refactored to use t_sat from first at_pressure call
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- [x] [MEDIUM] Saturation table silent drop - Now returns FluidError::InvalidState on length mismatch
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- [x] [MEDIUM] Accuracy validation - Added assert_relative_eq tests (grid point + interpolated)
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- [x] [MEDIUM] Release benchmark - Added test_tabular_benchmark_10k_queries_release (run with cargo test --release)
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- [x] [MEDIUM] Generator R410A/CO2 - Story updated: deferred until CoolProp integration
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