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Codebase Overview

Directory Map

src/
├── action/        Action entity, ActionState (NEW/FINISHED/FAILED), UsageInterval (pro-rating),
│                  TemporaryAction (runtime-only, never persisted), mux/ (ActionMuxInterface)
├── bill/          Bill entity — aggregated invoice line item; BillState, BillRequisite
├── charge/        Charge entity, ChargeModifier interface, modifier classes and addons;
│                  ChargeState (NEW/FINISHED); derivative/ (ChargeDerivative copy-with pattern)
├── customer/      Customer entity — billable party with seller hierarchy
├── event/         Domain events (e.g., InstallmentWasCharged)
├── Exception/     Shared exceptions (CannotReassignException, etc.)
├── formula/       FormulaEngine — parses DSL strings into ChargeModifier objects
├── helpers/       Utility classes
├── Money/         Money value object, MultipliedMoney for sub-cent prices
├── order/         Order (action collection), Calculator (main billing pipeline),
│                  Billing (top-level orchestrator: calculate + aggregate + persist),
│                  Collector (normalizes Order/Action/Action[] input)
├── plan/          Plan entity — tariff containing a collection of Prices
├── price/         Price implementations (SinglePrice, EnumPrice, RatePrice, ProgressivePrice)
├── product/       Higher-level product/tariff registry: TariffTypeDefinition, BillingRegistry,
│                  behavior system, invoice representations, quantity formatters
├── sale/          Sale entity — subscription binding Customer → Target → Plan
├── statement/     Statement — billing snapshot for a period (balance, total, bills, plans)
├── target/        Target entity — object being billed
├── tools/         Aggregator (Charge → Bill grouping), Generalizer (Charge → Bill mapping)
├── type/          Type entity — classification of actions and charges
└── usage/         Usage tracking classes

Key Patterns

  • Interface + Implementation: core entities define interfaces (e.g., PriceInterface, BillInterface)
  • Value Objects: Money, Quantity, Target, Type are immutable value objects
  • Trait Composition: HasMoney, HasQuantity, SettableChargeModifierTrait, fluent With* traits for modifier addons
  • Factory: ModifierFactory for formula context, static ::fromAction() / ::fromActions() on Order
  • Strategy: Price types implement different calculation strategies via calculateSum() / calculateUsage()
  • Immutability Guards: CannotReassignException prevents mutation of billing-critical fields

Testing

  • Behat features in tests/behat/ — living documentation for the formula DSL (FixedDiscount, GrowingDiscount, Installment, MonthlyCap, Combination)
  • PHPUnit tests in tests/unit/ — unit tests for prices, formulas, modifiers, and entities

Further Reading