/**
 * Spend display: the ledger keeps every cost estimate in the priced provider's
 * billing currency (CNY), and that stored value never moves. This module turns
 * one CNY figure into the currency the user chose to READ it in, so switching
 * the display is a rendering decision, not a data migration (issue #1937).
 * @module @linxin666/dsh-usage/core/spend
 */

/** A currency the spend display can render. */
export type SpendCurrency = 'CNY' | 'USD'

/** How to read a stored CNY cost. */
export interface SpendDisplay {
  /** The currency to render in. */
  currency: SpendCurrency
  /** CNY per USD; consulted only when the display currency is USD. */
  cnyPerUsd: number
}

/**
 * The rate used when the row configures none: a plain default, so an
 * unconfigured display still has a defined美元 figure instead of a blank card.
 * The plugin never fetches a live rate (its host routes are loopback-fenced and
 * its browser half has no network), so there is no fetch that could fail and
 * leave the card empty.
 */
export const DEFAULT_CNY_PER_USD = 7.2

/** The display used when the row carries no setting: the historical CNY rendering. */
export const DEFAULT_SPEND_DISPLAY: SpendDisplay = { currency: 'CNY', cnyPerUsd: DEFAULT_CNY_PER_USD }

/**
 * Normalize a configured display.
 * An unknown currency reads as CNY, and a rate that is not a positive finite
 * number falls back to the default rate — the display must never be able to
 * blank the card.
 * @param input - the configured currency and rate, if any.
 * @returns a display that can always render.
 */
export function resolveSpendDisplay(input?: { currency?: unknown; cnyPerUsd?: unknown }): SpendDisplay {
  const currency: SpendCurrency = input?.currency === 'USD' ? 'USD' : 'CNY'
  const rate = typeof input?.cnyPerUsd === 'number' && Number.isFinite(input.cnyPerUsd) && input.cnyPerUsd > 0
    ? input.cnyPerUsd
    : DEFAULT_CNY_PER_USD
  return { currency, cnyPerUsd: rate }
}

/**
 * Render one stored CNY cost in the display currency.
 * A non-finite or negative cost renders as zero rather than as `NaN`/`¥-0.10`.
 * @param costCny - the stored cost in CNY.
 * @param display - the display to render in; defaults to CNY.
 * @returns the formatted amount with its currency symbol.
 */
export function formatSpend(costCny: number, display: SpendDisplay = DEFAULT_SPEND_DISPLAY): string {
  const safe = Number.isFinite(costCny) && costCny > 0 ? costCny : 0
  if (display.currency === 'USD') {
    const rate = Number.isFinite(display.cnyPerUsd) && display.cnyPerUsd > 0 ? display.cnyPerUsd : DEFAULT_CNY_PER_USD
    return '$' + (safe / rate).toFixed(2)
  }
  return '¥' + safe.toFixed(2)
}