commitea/packages/core/src/scheduler/scheduler-capacity-v0.ts

/**
 * Capacity-aware scheduler (#8). Reuses the single-worker scheduler's topological
 * order + critical-path marking, then re-lays the work across lanes: an issue
 * runs on its assignee's lane (or the earliest-free lane when unassigned), its
 * duration scaled by that lane's speed (ideal person-days/workday). Makespan
 * shrinks toward the critical path as lanes are added. Deterministic; falls back
 * to the single serial worker when no capacity is configured. The lane layout is
 * factored so the Monte Carlo forecast reuses it per trial with sampled durations.
 */

import {
  type DependencyEdge,
  schedule,
  type SchedulableIssue,
  type SchedulePlan,
} from './scheduler-v0.js'

/** A scheduling lane: a person and their throughput (ideal person-days / workday). */
export interface Worker {
  person: string
  speed: number
}

/** The topological order + per-issue relations the layout needs (from the base plan). */
export interface LaneInputs {
  order: number[]
  blockedBy: Map<number, number[]>
  assignee: Map<number, string | null>
}

/** Ensure a lane exists for every assignee; unconfigured assignees get the mean speed. */
export function resolveLanes(workers: Worker[], assignees: (string | null)[]): Worker[] {
  const lanes = [...workers]
  const known = new Set(lanes.map((w) => w.person))
  const meanSpeed = lanes.length ? lanes.reduce((s, w) => s + w.speed, 0) / lanes.length : 1
  for (const a of assignees) {
    if (a && !known.has(a)) {
      lanes.push({ person: a, speed: meanSpeed })
      known.add(a)
    }
  }
  return lanes
}

function pickWorker(lanes: Worker[], freeAt: Map<string, number>, assignee: string | null | undefined): Worker {
  if (assignee) {
    const own = lanes.find((w) => w.person === assignee)
    if (own) return own
  }
  let best = lanes[0]
  for (const w of lanes) if (freeAt.get(w.person)! < freeAt.get(best.person)!) best = w
  return best
}

/**
 * Lay a topologically-ordered set out across lanes. `duration(n)` supplies each
 * issue's duration for this layout (estimate, or a sampled value in a MC trial).
 * Returns each issue's finish day + the lane it ran on. Order guarantees a
 * dependency is always laid out before its dependents.
 */
export function layoutOnLanes(
  inputs: LaneInputs,
  lanes: Worker[],
  duration: (n: number) => number,
): { finishAt: Map<number, number>; startAt: Map<number, number>; laneOf: Map<number, string> } {
  const freeAt = new Map(lanes.map((w) => [w.person, 0]))
  const finishAt = new Map<number, number>()
  const startAt = new Map<number, number>()
  const laneOf = new Map<number, string>()
  for (const n of inputs.order) {
    const worker = pickWorker(lanes, freeAt, inputs.assignee.get(n))
    const blockers = inputs.blockedBy.get(n) ?? []
    const depFinish = blockers.length ? Math.max(...blockers.map((d) => finishAt.get(d) ?? 0)) : 0
    const start = Math.max(freeAt.get(worker.person)!, depFinish)
    const end = start + duration(n) / worker.speed
    startAt.set(n, start)
    finishAt.set(n, end)
    laneOf.set(n, worker.person)
    freeAt.set(worker.person, end)
  }
  return { finishAt, startAt, laneOf }
}

/**
 * Capacity-aware plan. An empty `workers` means no capacity is configured → the
 * single-worker plan verbatim.
 */
export function scheduleWithCapacity(
  issues: SchedulableIssue[],
  edges: DependencyEdge[],
  workers: Worker[],
): SchedulePlan {
  if (workers.length === 0) return schedule(issues, edges)
  const base = schedule(issues, edges)
  if (base.cycle || base.items.length === 0) return base

  const inputs: LaneInputs = {
    order: base.items.map((it) => it.number),
    blockedBy: new Map(base.items.map((it) => [it.number, it.blockedBy])),
    assignee: new Map(issues.map((i) => [i.number, i.assignee ?? null])),
  }
  const lanes = resolveLanes(workers, [...inputs.assignee.values()])
  const durationOf = new Map(base.items.map((it) => [it.number, it.durationDays]))
  const { finishAt, startAt, laneOf } = layoutOnLanes(inputs, lanes, (n) => durationOf.get(n)!)

  const items = base.items.map((it) => ({
    ...it,
    startDay: startAt.get(it.number)!,
    endDay: finishAt.get(it.number)!,
    durationDays: finishAt.get(it.number)! - startAt.get(it.number)!,
    worker: laneOf.get(it.number),
  }))
  return { items, cycle: null }
}

/** Makespan (last finish) of a plan — the project's landing day. */
export function makespan(plan: SchedulePlan): number {
  return plan.items.reduce((m, it) => Math.max(m, it.endDay), 0)
}