Fix physics Force relation

[leynos]

Summary

• define Force as an input relation instead of a stream in physics.dl
• document per-tick Force usage in physics design docs and BDD guide
• bind default mass using var syntax to satisfy ddlog compiler

Testing

• make fmt
• make markdownlint
• make lint
• make test
• make build-support-run (fails: Unknown variable: mass)

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https://chatgpt.com/codex/tasks/task_e_6857e13457508322a1be6f184c10029d

Summary by Sourcery

Convert Force from an ephemeral stream to a per-tick input relation, fix default mass binding to satisfy the DDlog compiler, and update documentation accordingly.

Bug Fixes:

• Bind default mass using DDlog’s var syntax to resolve the "Unknown variable: mass" compilation error.

Enhancements:

• Define Force as an input relation overwritten each frame instead of an input stream in the physics ruleset.

Documentation:

• Revise BDD guide and physics engine design docs to describe Force as a per-tick input relation and clarify its usage.

Summary by CodeRabbit

• Documentation
  • Updated documentation to clarify that the Force input is now a per-tick relation, overwritten each frame, rather than a stream.
  • Improved explanation and examples of mass retrieval using named fields for better clarity in pattern matching.

[sourcery-ai]

Reviewer's Guide

This PR refactors the Force concept into a per-tick input relation (instead of a stream), updates documentation to clarify its host-engine semantics, and replaces the Mass pattern with a field-based binding plus var syntax to satisfy the DDlog compiler.

Entity relationship diagram for Force and Mass relations in physics engine

erDiagram
    Entity {
        EntityID id
    }
    Force {
        EntityID entity
        GCoord fx
        GCoord fy
        GCoord fz
    }
    Mass {
        EntityID entity
        GCoord kg
    }
    AppliedAcceleration {
        EntityID entity
        GCoord ax
        GCoord ay
        GCoord az
    }

    Entity ||--o{ Force : "applies"
    Entity ||--o{ Mass : "has"
    Force ||--o{ AppliedAcceleration : "contributes to"
    Mass ||--o{ AppliedAcceleration : "contributes to"

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Class diagram for updated Force and Mass usage in physics module

classDiagram
    class Force {
        +EntityID entity
        +GCoord fx
        +GCoord fy
        +GCoord fz
    }
    class Mass {
        +EntityID entity
        +GCoord kg
    }
    class AppliedAcceleration {
        +EntityID entity
        +GCoord ax
        +GCoord ay
        +GCoord az
    }
    Force <.. AppliedAcceleration : input
    Mass <.. AppliedAcceleration : input

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File-Level Changes

Change	Details	Files
Redefine Force as a per-tick input relation instead of a stream	Replace “input stream Force” with “input relation Force” Update section header from “New Ephemeral Input Stream” to “New Per-Tick Input Relation”	src/ddlog/physics.dl docs/bdd-strategies-for-differential-datalog-rulesets.md docs/lille-physics-engine-design.md
Clarify Force lifecycle in documentation	Insert comment noting the host engine overwrites Force every frame Adjust explanatory comments on acceleration computation	docs/bdd-strategies-for-differential-datalog-rulesets.md docs/lille-physics-engine-design.md
Bind default mass using var syntax and field destructuring	Replace positional Mass(e,mass) with Mass{.entity=e, .kg=mass} Use “or var mass = DEFAULT_MASS” for default binding Retain mass > 0.0 guard	src/ddlog/physics.dl docs/bdd-strategies-for-differential-datalog-rulesets.md docs/lille-physics-engine-design.md

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[coderabbitai]

Walkthrough

The changes update documentation and code to redefine the Force input from an ephemeral input stream to a per-tick input relation in the physics DDlog model. Additionally, the pattern matching for accessing mass in the AppliedAcceleration rule is revised from positional to record-style, with clearer fallback handling for default mass.

Changes

File(s)	Change Summary
docs/bdd-strategies-for-differential-datalog-rulesets.md, docs/lille-physics-engine-design.md	Updated documentation to describe Force as an input relation (not stream) and clarified mass pattern matching.
src/ddlog/physics.dl	Changed Force from input stream to input relation; updated AppliedAcceleration to use record-style mass matching and clearer fallback.

Sequence Diagram(s)

sequenceDiagram
    participant HostEngine
    participant DDlogPhysics

    HostEngine->>DDlogPhysics: Overwrite Force relation each tick
    DDlogPhysics->>DDlogPhysics: For each entity, retrieve Mass (record-style pattern)
    alt Mass found
        DDlogPhysics->>DDlogPhysics: Use entity's mass
    else Mass not found
        DDlogPhysics->>DDlogPhysics: Use default mass
    end
    DDlogPhysics->>DDlogPhysics: Compute AppliedAcceleration = Force / Mass

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Possibly related PRs

• Fix physics Force relation #90: Directly modifies the same Force declaration and mass pattern matching in src/ddlog/physics.dl.
• Fix applied acceleration and add mass #34: Introduces the Mass relation and DEFAULT_MASS, and alters acceleration calculation logic in the same physics DDlog model.

Poem

A hop, a skip, a tick anew,
Forces now per frame accrue.
Mass is matched with fields in view,
No streams to chase, just records true.
Physics leaps with rules refined—
A rabbit’s code, by logic signed!
🐇✨

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📜 Recent review details

Configuration used: CodeRabbit UI
Review profile: ASSERTIVE
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between c0644b0 and 5a2773b.

📒 Files selected for processing (3)

• docs/bdd-strategies-for-differential-datalog-rulesets.md (2 hunks)
• docs/lille-physics-engine-design.md (2 hunks)
• src/ddlog/physics.dl (1 hunks)

🔇 Additional comments (6)

src/ddlog/physics.dl (2)

7-9: Excellent improvement to Force relation semantics.

The change from input stream to input relation with the clarifying comment makes the intended usage much clearer. This aligns with the per-tick nature of forces in physics simulations where the host application provides all forces acting on entities for each frame.

---

16-16: Good modernisation of pattern matching syntax.

The change from positional pattern matching Mass(e, mass) to record-style Mass{.entity = e, .kg = mass} improves code readability by making field names explicit. The use of var mass = default_mass() instead of direct assignment also follows DDlog's variable declaration conventions.

docs/lille-physics-engine-design.md (2)

173-177: Documentation properly updated to reflect Force relation changes.

The documentation correctly describes the Force relation as a per-tick input that gets overwritten each frame by the host engine. This aligns perfectly with the implementation changes in src/ddlog/physics.dl and clarifies the intended usage pattern.

---

204-204: Pattern matching syntax consistently updated.

The documentation properly reflects the updated pattern matching syntax from the implementation, maintaining consistency between code and documentation. The record-style matching Mass{.entity = e, .kg = mass} and var mass = DEFAULT_MASS syntax is correctly documented.

docs/bdd-strategies-for-differential-datalog-rulesets.md (2)

168-172: BDD documentation updated to reflect Force relation semantics.

The documentation correctly describes the Force relation as a per-tick input that gets overwritten each frame, maintaining consistency with both the implementation and other documentation files. This ensures developers understand the proper usage pattern across all documentation.

---

199-199: Pattern matching syntax consistently documented.

The BDD documentation properly reflects the updated pattern matching syntax, ensuring consistency across all documentation files. The record-style matching and variable declaration syntax is correctly documented.

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[sourcery-ai]

Hey @leynos - I've reviewed your changes and they look great!

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