recurrence.bend source

recurrence.bend on the hub · documented module

# Pure recurrence on arbitrary-precision Nat ticks: no wrapping or U32 conversion.import Basetype Missed is Data:  Skip{}  CatchUp{bound: Nat}type Cadence is Data:  FixedDelay{}  FixedRate{policy: Missed}type Plan is Data:  Planned{replay: List<&2, Nat>, next: Nat}def replay(p: Plan) -> List<&2, Nat>:  match p:    case Planned{ticks, next}:      ticksdef next(p: Plan) -> Nat:  match p:    case Planned{ticks, next}:      next# Generate only the bounded replay prefix, never the skipped remainder.def ticks(count: Nat, +previous: Nat, +period: Nat) -> List<&2, Nat>:  match count:    case 0n:      Nil{}    case 1n+n:      +deadline = Nat.add(previous, period)      Con{deadline, ticks(n, deadline, period)}def limit(policy: Missed, overdue: Nat) -> Nat:  match policy:    case Skip{}:      0n    case CatchUp{bound}:      Nat.min(bound, overdue)# A tick equal to completion is overdue. The already-completed previous tick# is excluded. A positive period keeps all arithmetic exact, without overflow.def rate(+previous: Nat, +completion: Nat, +period: Nat, policy: Missed) -> Plan:  +overdue = Nat.div(Nat.sub(completion, previous), period)  Planned{ticks(limit(policy, overdue), previous, period),    Nat.add(previous, Nat.mul(1n+overdue, period))}def positive(cadence: Cadence, +previous: Nat, +completion: Nat, +period: Nat) -> Plan:  match cadence:    case FixedDelay{}:      Planned{Nil{}, Nat.add(completion, period)}    case FixedRate{policy}:      rate(previous, completion, period, policy)# None is an explicit invalid-zero-period result, not a zero cadence.def plan(previous: Nat, completion: Nat, period: Nat, cadence: Cadence) -> Maybe<&2, Plan>:  match period:    case 0n:      None{}    case 1n+p:      Some{positive(cadence, previous, completion, 1n+p)}