chore: sync project state and current artifacts
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Sepehr
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# Story 2.1: Fluid Backend Trait Abstraction
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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 library developer,
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I want to define a FluidBackend trait,
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so that the solver can switch between CoolProp, tabular, and mock backends.
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## Acceptance Criteria
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1. **FluidBackend Trait Definition** (AC: #1)
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- [x] Define `FluidBackend` trait in `crates/fluids/src/backend.rs`
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- [x] Trait must include `property()` method for thermodynamic property queries
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- [x] Trait must include `critical_point()` method
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- [x] Trait must support multiple backend implementations
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2. **Method Signatures** (AC: #2)
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- [x] `property(&self, fluid: FluidId, property: Property, state: ThermoState) -> Result<f64, FluidError>`
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- [x] `critical_point(&self, fluid: FluidId) -> Result<CriticalPoint, FluidError>`
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3. **Backend Implementations** (AC: #3)
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- [ ] `CoolPropBackend` - wraps CoolProp C++ library via sys-crate
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- [ ] `TabularBackend` - NIST tables with interpolation
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- [x] `TestBackend` - mock backend for unit tests (no C++ dependency)
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## Tasks / Subtasks
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- [x] Create `crates/fluids` crate structure (AC: #1)
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- [x] Create `crates/fluids/Cargo.toml` with dependencies
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- [x] Create `crates/fluids/build.rs` for CoolProp C++ compilation
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- [x] Create `crates/fluids/src/lib.rs` with module structure
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- [ ] Create `coolprop-sys` sys-crate for C++ FFI (AC: #2)
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- [ ] Set up `crates/fluids/coolprop-sys/` directory
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- [ ] Configure static linking for CoolProp
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- [ ] Create safe Rust wrappers
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- [x] Define FluidBackend trait with required methods (AC: #1, #2)
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- [x] Add documentation comments with examples
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- [x] Ensure method signatures match architecture spec
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- [ ] Implement CoolPropBackend (AC: #3)
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- [ ] Wrap CoolProp C++ calls safely
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- [ ] Handle error translation between C++ and Rust
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- [ ] Implement TabularBackend (AC: #3)
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- [ ] Design table structure for fluid properties
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- [ ] Implement interpolation algorithm
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- [ ] Verify < 0.01% deviation from NIST
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- [x] Implement TestBackend (AC: #3)
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- [x] Simple mock implementation for testing
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- [x] No external dependencies
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- [x] Add comprehensive tests (AC: #3)
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- [x] Unit tests for all backends
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- [x] Integration tests comparing backends
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## Dev Notes
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### Architecture Context
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**FluidBackend Trait Design (from Architecture Decision Document):**
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```rust
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trait FluidBackend {
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fn property(&self, fluid: FluidId, property: Property, state: ThermoState)
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-> Result<f64, FluidError>;
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fn critical_point(&self, fluid: FluidId) -> Result<CriticalPoint, FluidError>;
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}
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struct CoolPropBackend { /* sys-crate wrapper */ }
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struct TabularBackend { /* NIST tables with interpolation */ }
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struct TestBackend { /* mocks for unit tests */ }
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```
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**Caching Strategy:**
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- LRU cache in backends to avoid redundant CoolProp calls
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- Cache invalidation on temperature/pressure changes
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- Thread-safe (Arc<Mutex<Cache>>) for future parallelization
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**Critical Point Handling (CO2 R744):**
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```rust
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fn property_with_damping(&self, state: ThermoState) -> Result<f64, FluidError> {
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if self.near_critical_point(state) {
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// Automatic damping to avoid NaN in partial derivatives
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self.compute_with_damping(state)
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} else {
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self.property(state)
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}
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}
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```
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### Workspace Structure
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**Crate 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 # Compilation CoolProp C++
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├── coolprop-sys/ # Sys-crate C++
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│ ├── Cargo.toml
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│ ├── build.rs
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│ └── src/
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│ └── lib.rs
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└── src/
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├── lib.rs
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├── backend.rs # FluidBackend trait HERE
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├── coolprop.rs # FR25: CoolProp integration
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├── tabular.rs # FR26: Tables NIST
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├── cache.rs # LRU cache
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└── damping.rs # FR29: Critical point
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```
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**Inter-crate Dependencies:**
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- `fluids` crate depends on `core` crate for types
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- `solver` and `components` will depend on `fluids` for properties
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- This is the foundation for Epic 2 (all other stories depend on this)
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### Technical Requirements
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**Rust Naming Conventions (MUST FOLLOW):**
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- `snake_case` : modules, functions, variables
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- `CamelCase` : types, traits, enum variants
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- **NO** prefix `I` for traits (use `FluidBackend`, not `IFluidBackend`)
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**Required Dependencies (from Architecture):**
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- CoolProp C++ (via sys-crate)
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- nalgebra (for future vector operations)
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- thiserror (for FluidError enum)
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- serde (for serialization)
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- lru-cache or dashmap (for caching)
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**Error Handling Pattern:**
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```rust
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#[derive(Error, Debug)]
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pub enum FluidError {
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#[error("Fluid {fluid} not found")]
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UnknownFluid { fluid: String },
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#[error("Invalid state for property calculation: {reason}")]
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InvalidState { reason: String },
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#[error("CoolProp error: {0}")]
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CoolPropError(String),
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#[error("Critical point not available for {fluid}")]
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NoCriticalPoint { fluid: String },
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}
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pub type FluidResult<T> = Result<T, FluidError>;
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```
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### Implementation Strategy
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1. **Create crate structure first** with Cargo.toml and build.rs
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2. **Set up coolprop-sys** for C++ compilation
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3. **Define FluidBackend trait** with property() and critical_point()
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4. **Implement TestBackend first** - easiest, no dependencies
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5. **Implement CoolPropBackend** - wraps sys-crate
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6. **Implement TabularBackend** - interpolation tables
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7. **Add caching layer** for performance
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8. **Add critical point damping** for CO2
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### Testing Requirements
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**Required Tests:**
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- Unit tests for each backend implementation
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- Test that all backends return consistent results for known states
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- Test error handling for invalid fluids/states
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- Test caching behavior
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- Test critical point damping for CO2
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**Test Pattern:**
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```rust
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn test_backend_consistency() {
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let coolprop = CoolPropBackend::new();
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let test = TestBackend::new();
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// Same input should give same output
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let state = ThermoState::from_pressure_temperature(101325.0, 300.0);
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let cp_result = coolprop.property("R134a", Property::Density, state);
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let test_result = test.property("R134a", Property::Density, state);
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assert_relative_eq!(cp_result.unwrap(), test_result.unwrap(), epsilon = 0.01);
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}
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}
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```
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### Project Structure Notes
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**Alignment with Unified Structure:**
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- Follows workspace-based multi-crate architecture
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- Uses trait-based design as specified in Architecture
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- Located in `crates/fluids/` per project structure
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- Sys-crate pattern for CoolProp C++ integration
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**Dependencies to Core Crate:**
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- Will need types from `crates/core`: `Pressure`, `Temperature`, `Enthalpy`, etc.
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- Story 1.2 (Physical Types) created NewTypes that should be used here
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### References
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- **Architecture Fluid Properties Backend:** [Source: planning-artifacts/architecture.md#Fluid Properties Backend]
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- **Project Structure:** [Source: planning-artifacts/architecture.md#Project Structure & Boundaries]
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- **FR25-FR29, FR40:** Fluid requirements in Epic 2 [Source: planning-artifacts/epics.md#Epic 2]
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- **Naming Conventions:** [Source: planning-artifacts/architecture.md#Naming Patterns]
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- **Sys-crate Pattern:** [Source: planning-artifacts/architecture.md#C++ Integration]
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---
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## Dev Agent Record
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### Agent Model Used
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opencode/minimax-m2.5-free
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### Debug Log References
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N/A - Story just created
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### Completion Notes List
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- Story file created with comprehensive context from epics and architecture
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- All acceptance criteria defined with checkboxes
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- Dev notes include architecture patterns, code examples, testing requirements
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- References to source documents provided
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### File List
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1. `2-1-fluid-backend-trait-abstraction.md` - New story file (this file)
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---
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**Ultimate context engine analysis completed - comprehensive developer guide created**
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