// Exact-decimal money helpers of the `money` field and list column (Phase 18
// D-07, amended D-09, D-10). A money value is the decimal string the server
// stores ("39.8400", "85.4193"); it is never turned into a JS number here:
// parsing is text work on digits and separators, and formatting hands the
// string itself to Intl.NumberFormat, which accepts decimal strings without
// going through a float.

export type MoneyParseError = 'invalid' | 'too_precise'

export interface MoneyParseResult {
  /** The canonical dot-separated string, or null for empty text. */
  value: string | null
  /** Why the text was refused, or null when value holds the result. */
  error: MoneyParseError | null
}

const digitsOnly = /^[0-9]+$/

/**
 * Turns typed text into the canonical money string: spaces and U+00A0 are
 * stripped, one comma or dot is the decimal separator, an optional leading
 * minus is kept, digits only otherwise. Empty text is null. More than
 * `scale` significant fraction digits is `too_precise`: the value is never
 * rounded or truncated, and trailing zeros beyond the scale are kept and
 * accepted (they carry no precision), as the server does. Anything else
 * (letters, two separators, grouping) is `invalid`.
 */
export function parseMoneyInput(text: string, scale: number): MoneyParseResult {
  const compact = text.replace(/[\s ]/g, '')
  if (compact === '') {
    return { value: null, error: null }
  }
  let body = compact
  let negative = false
  if (body.startsWith('-')) {
    negative = true
    body = body.slice(1)
  }
  const separators = body.match(/[.,]/g) ?? []
  if (separators.length > 1) {
    return { value: null, error: 'invalid' }
  }
  const [whole, fraction = ''] = body.split(/[.,]/)
  if (!digitsOnly.test(whole ?? '') || (separators.length === 1 && !digitsOnly.test(fraction))) {
    return { value: null, error: 'invalid' }
  }
  if (fraction.replace(/0+$/, '').length > scale) {
    return { value: null, error: 'too_precise' }
  }
  const canonical = fraction === '' ? whole : `${whole}.${fraction}`
  return { value: `${negative ? '-' : ''}${canonical}`, error: null }
}

/**
 * The stored string for display in the input: trailing fraction zeros are
 * trimmed down to two decimals ("39.8400" → "39.84", "85.4193" unchanged,
 * "5.0000" → "5.00", "7" → "7"). The separator stays a dot; the field swaps
 * it for the locale's own.
 */
export function trimMoneyDisplay(stored: string): string {
  const [whole, fraction] = stored.split('.')
  if (fraction === undefined || fraction.length <= 2) {
    return stored
  }
  let end = fraction.length
  while (end > 2 && fraction.charAt(end - 1) === '0') {
    end -= 1
  }
  return `${whole}.${fraction.slice(0, end)}`
}

/** The decimal separator Intl uses for a locale ("," for pl, "." for en). */
export function decimalSeparator(locale: string): string {
  const parts = new Intl.NumberFormat(locale).formatToParts(1.1)
  return parts.find((part) => part.type === 'decimal')?.value ?? '.'
}

/**
 * Formats a money string for reading: Intl.NumberFormat with the ISO code
 * (`currencyDisplay: 'code'`), at least two and at most `scale` fraction
 * digits, so "99999999.9999" keeps all four decimals and "39.8400" shows two.
 * Without a currency the amount alone is formatted (`style: 'decimal'`). The
 * string is passed to format() as is; it is never converted to a number.
 */
export function formatMoney(value: string, currency: string, locale: string, scale = 4): string {
  const digits = { minimumFractionDigits: 2, maximumFractionDigits: Math.max(2, scale) }
  const options: Intl.NumberFormatOptions =
    currency === '' ? { style: 'decimal', ...digits } : { style: 'currency', currency, currencyDisplay: 'code', ...digits }
  let formatter: Intl.NumberFormat
  try {
    formatter = new Intl.NumberFormat(locale, options)
  } catch {
    // An unknown ISO code: show the amount with the code appended.
    const plain = new Intl.NumberFormat(locale, { style: 'decimal', ...digits })
    return `${plain.format(value as unknown as number)} ${currency}`.trim()
  }
  // Intl accepts decimal strings (ECMA-402 "intl mathematical value"); the
  // cast only satisfies the lib typings that predate it.
  return formatter.format(value as unknown as number)
}