CRO Algorithm

[francastagna]

PR Summary

We implemented the Chemical Reaction Optimization (CRO) algorithm and added comprehensive tests.

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Pseudocode:

Source: https://www.researchgate.net/publication/327166354_An_Empirical_Evaluation_of_Evolutionary_Algorithms_for_Unit_Test_Suite_Generation

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Main Changes

• New Algorithm (CroAlgorithm):

  • Implements CRO over whole-test-suite individuals (WtsEvalIndividual), modeling each suite as a molecule with potential energy (−fitness) and kinetic energy.
  • Maintains a global energy container (buffer) and enforces strict energy conservation at each step; compensates for external fitness drifts by adjusting the container.
  • Selects reaction path per step:
    • Uni-molecular path: either on-wall ineffective collision or decomposition (threshold on numCollisions).
    • Inter-molecular path (probability croMolecularCollisionRate): either synthesis (both molecules eligible by KE threshold) or inter-molecular ineffective collision.
  • Uses existing GA operators for variation: mutate and xover are reused; potential is defined as -calculateCombinedFitness().
  • Initializes molecules from GA population with croInitialKineticEnergy; tracks collisions per molecule to gate decomposition.

• Reactor (CroReactor):

  • Encapsulates reactions and energy accounting with EnergyContext(container); provides:
    • onWallIneffectiveCollision: mutate copy; accept if energy surplus ≥ 0; split surplus between KE and container using retained-fraction ∈ [croKineticEnergyLossRate, 1].
    • decomposition: create two mutated offspring; if deficit, attempt borrow from container (random fractional product of container); split net KE between offspring.
    • intermolecularIneffectiveCollision: mutate both; accept if net surplus ≥ 0; split KE between the two.
    • synthesis: crossover two offspring, pick the fitter fused; accept if net surplus ≥ 0; assign KE=surplus, reset collisions; otherwise increment parents’ collisions.
  • Helper: computeEnergySurplus(totalBeforePotential, totalBeforeKinetic, totalAfterPotential).

• Molecule model (CroMolecule):

  • New data class holding suite, kineticEnergy, numCollisions.

• Config (EMConfig):

  • Added CRO to EMConfig.Algorithm.
  • Added CRO parameters (defaults in parentheses):
    • croMolecularCollisionRate (0.2): probability of binary reactions (0–1).
    • croKineticEnergyLossRate (0.2): lower bound of KE retained on on-wall (0–1).
    • croInitialKineticEnergy (1000.0): initial KE per molecule (≥ 0).
    • croDecompositionThreshold (500): min collisions before allowing decomposition (≥ 0).
    • croSynthesisThreshold (10.0): synthesis allowed if KE ≤ threshold (≥ 0).

• Integration (Main.kt):

  • Wires EMConfig.Algorithm.CRO to CroAlgorithm<RestIndividual> in algorithm selection.

• Docs (docs/options.md):

  • Added CRO options with descriptions and constraints in the options table.

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Tests

• Algorithm-level tests added (CroAlgorithmTest.kt):

  • Finds optimum on OneMax (testCroAlgorithmFindsOptimum).
  • Uni-molecular path:
    • Decomposition accepted → parent replaced by two offspring, collisions reset, KE ≥ 0.
    • Decomposition rejected (cannot borrow) → no replacement; parent collisions increment.
    • On-wall accepted → single molecule updated; collisions increment; KE/container updated.
    • On-wall rejected (negative net) → no changes.
  • Inter-molecular path:
    • Synthesis accepted → size decreases to 1; fused KE set; collisions reset.
    • Synthesis rejected (negative net) → size unchanged; both parents’ collisions increment.
    • Ineffective collision accepted → both replaced; collisions increment; KE split and ≥ 0.
    • Ineffective collision rejected → no changes.

• Reactor unit tests added (CroReactorTest.kt):

  • computeEnergySurplus returns positive/zero/negative as expected.
  • Decomposition: borrow fails when container is zero (reject + parent collisions++); borrow succeeds with large container (accept + container decreases + KE distributed).
  • On-wall: negative net rejects with no changes; positive net updates KE/container and conserves energy.
  • Inter-molecular ineffective collision: negative net rejects; positive net splits KE and increments collisions, conserving energy.
  • Synthesis: negative net rejects and increments parent collisions; positive net returns fused offspring with KE=net and collisions reset.