/**
 * LiangShen lever logic — framework-free, no DOM, compiled by both programs.
 *
 * The lever is a homepage control beside the model selector: pulling it down
 * turns the current session's preset into LiangShen mode, pushing it up
 * restores the preset the user was on before. The facts it needs are the
 * current session's preset, whether that session may still change preset at
 * all, and the preset roster; the decisions are pure so they can be tested
 * without the browser runtime.
 *
 * A preset is fixed once a session starts (the host refuses the switch with
 * `agent-preset/locked`), so the lever is only meaningful while the session is
 * blank — which is exactly the new-session screen it renders on.
 */

/** The preset id this lever turns on; also the preset directory name. */
export const LIANGSHEN_PRESET_ID = 'liangshen'

/** What the lever currently reports. */
export type LeverState =
  /** LiangShen mode is the current preset. */
  | 'on'
  /** Another preset is current and the session may still switch. */
  | 'off'
  /** The session has started; the preset can no longer change. */
  | 'locked'
  /** The deployment does not supply the LiangShen preset. */
  | 'missing'

/** The facts one lever decision reads. */
export interface LeverFacts {
  /** Whether the current session may still change preset (blank). */
  blank: boolean
  /** The preset the current session runs, when the session reports one. */
  agentPreset?: string
  /** Ids every usable roster row supplies, in roster order. */
  available: readonly string[]
  /** The deployment default preset id, when the roster names one. */
  fallback?: string
  /** The preset the user ran before the last pull-down, remembered by the client. */
  previous?: string
}

/** Resolve what the lever shows for one set of facts. */
export function leverState(facts: LeverFacts): LeverState {
  if (!facts.available.includes(LIANGSHEN_PRESET_ID)) return 'missing'
  if (!facts.blank) return 'locked'
  return facts.agentPreset === LIANGSHEN_PRESET_ID ? 'on' : 'off'
}

/**
 * The preset a push-up restores: the preset the user was on before pulling the
 * lever, else the deployment default, else the first usable roster row that is
 * not the LiangShen preset. A candidate is skipped when it is the LiangShen
 * preset itself (restoring it would be a no-op) or when the roster no longer
 * supplies it. That last resort is what keeps the push direction alive when the
 * deployment default IS LiangShen mode: `fallback` is skipped then, and after a
 * reload `previous` is gone, so without it the gesture would have no target.
 */
export function restoreTarget(facts: LeverFacts): string | undefined {
  for (const candidate of [facts.previous, facts.fallback]) {
    if (candidate === undefined || candidate === LIANGSHEN_PRESET_ID) continue
    if (facts.available.includes(candidate)) return candidate
  }
  return facts.available.find(candidate => candidate !== LIANGSHEN_PRESET_ID)
}

/** Whether the lever can act at all in its current state. */
export function isActionable(state: LeverState): boolean {
  return state === 'on' || state === 'off'
}