export interface BigIntFormatOptions {
  /**
   * The number of digits starting from the right of `value` to consider as decimal places.
   *
   * @default 18n
   */
  precision?: bigint | undefined
  /** @default 0 */
  minimumFractionDigits?: number | undefined
  /** @default 20 */
  maximumFractionDigits?: number | undefined
  /** @default 'auto' */
  trailingZeroDisplay?: 'auto' | 'stripIfInteger' | undefined
}

/**
 * Formats a bigint as a string.
 *
 * @param value - The bigint to format.
 * @param options - Formatting options.
 * @returns The string representation of `value`
 */
export function formatBigInt(value: bigint, options?: BigIntFormatOptions): string {
  const {
    precision = 18n,
    minimumFractionDigits = 0,
    maximumFractionDigits = 20,
    trailingZeroDisplay = 'auto',
  } = options ?? {}

  if (precision < 0n) {
    throw new Error(`[formatBigInt] \`precision\` must be positive, got ${precision}`)
  }
  if (minimumFractionDigits > maximumFractionDigits) {
    throw new Error(
      `[formatBigInt] \`minimumFractionDigits\` (${minimumFractionDigits}) cannot be greater than \`maximumFractionDigits\` (${maximumFractionDigits})`,
    )
  }
  const isNegative = value < 0n
  const absValue = isNegative ? value * -1n : value
  const precisionFactor = 10n ** precision
  const integerPart = absValue / precisionFactor
  const integerPartString = `${isNegative ? '-' : ''}${integerPart.toLocaleString('en-US')}`
  const decimalPart = absValue % precisionFactor
  let decimalPartString = ''
  if ((decimalPart > 0n || minimumFractionDigits > 0) && maximumFractionDigits > 0) {
    decimalPartString = decimalPart
      .toLocaleString('en-US', {
        useGrouping: false,
      })
      .padStart(Number(precision), '0')
      .replace(/0+$/, '')
      .padEnd(minimumFractionDigits, '0')
      .slice(0, maximumFractionDigits)
    if (trailingZeroDisplay === 'stripIfInteger' && decimalPartString.replace(/0/g, '') === '') {
      decimalPartString = ''
    }
  }
  return `${integerPartString}${decimalPartString ? `.${decimalPartString}` : ''}`
}