export type TaskState =
  | "IDLE"
  | "SNAPSHOT"
  | "DIAGNOSE"
  | "PLAN"
  | "WAIT_APPROVAL"
  | "IMPLEMENT"
  | "VERIFY"
  | "REPORT"
  | "DONE"
  | "BLOCKED";

export type WorkstreamCategory =
  | "debug-build"
  | "installer"
  | "audit"
  | "local-fix"
  | "ci"
  | "deploy"
  | "backend-port"
  | "coding"
  | "unknown";

export interface TaskStateSnapshot {
  state: TaskState;
  currentWorkstream: WorkstreamCategory;
  approvedWorkstreams: WorkstreamCategory[];
  lastReason?: string;
}

export interface DriftDecision {
  drift: boolean;
  blocked: boolean;
  reason: string;
  nextState: TaskState;
}

const ALLOWED_TRANSITIONS: Record<TaskState, TaskState[]> = {
  IDLE: ["SNAPSHOT", "BLOCKED"],
  SNAPSHOT: ["DIAGNOSE", "BLOCKED"],
  DIAGNOSE: ["PLAN", "IMPLEMENT", "BLOCKED"],
  PLAN: ["WAIT_APPROVAL", "IMPLEMENT", "BLOCKED"],
  WAIT_APPROVAL: ["IMPLEMENT", "BLOCKED", "DONE"],
  IMPLEMENT: ["VERIFY", "BLOCKED"],
  VERIFY: ["REPORT", "IMPLEMENT", "BLOCKED"],
  REPORT: ["DONE", "BLOCKED"],
  DONE: ["IDLE", "BLOCKED"],
  BLOCKED: ["SNAPSHOT", "DONE"],
};

export function createTaskState(currentWorkstream: WorkstreamCategory = "unknown"): TaskStateSnapshot {
  return {
    state: "IDLE",
    currentWorkstream,
    approvedWorkstreams: [currentWorkstream],
  };
}

export function transitionTaskState(snapshot: TaskStateSnapshot, nextState: TaskState, reason?: string): TaskStateSnapshot {
  if (!ALLOWED_TRANSITIONS[snapshot.state].includes(nextState)) {
    return {
      ...snapshot,
      state: "BLOCKED",
      lastReason: `Invalid state transition: ${snapshot.state} -> ${nextState}`,
    };
  }

  return {
    ...snapshot,
    state: nextState,
    lastReason: reason || snapshot.lastReason,
  };
}

export function approveWorkstream(snapshot: TaskStateSnapshot, workstream: WorkstreamCategory): TaskStateSnapshot {
  if (snapshot.approvedWorkstreams.includes(workstream)) return snapshot;
  return {
    ...snapshot,
    approvedWorkstreams: [...snapshot.approvedWorkstreams, workstream],
  };
}

export function detectDrift(snapshot: TaskStateSnapshot, proposed: WorkstreamCategory): DriftDecision {
  if (proposed === snapshot.currentWorkstream) {
    return {
      drift: false,
      blocked: false,
      reason: "Same workstream.",
      nextState: snapshot.state,
    };
  }

  if (snapshot.approvedWorkstreams.includes(proposed)) {
    return {
      drift: false,
      blocked: false,
      reason: "Workstream explicitly approved.",
      nextState: snapshot.state,
    };
  }

  return {
    drift: true,
    blocked: true,
    reason: `Task drift detected: current workstream '${snapshot.currentWorkstream}' vs proposed '${proposed}'. Approval required before switching.`,
    nextState: "BLOCKED",
  };
}