const uppercaseDigits = ['零', '壹', '贰', '叁', '肆', '伍', '陆', '柒', '捌', '玖'] as const
const positionUnits = ['', '拾', '佰', '仟'] as const
const sectionUnits = ['', '万', '亿', '兆', '京'] as const

export function formatChineseUppercaseAmount(value: string) {
  const normalizedValue = value.trim().replaceAll(',', '')
  const matchedValue = /^([+-]?)(\d+)(?:\.(\d*))?$/.exec(normalizedValue)

  if (!matchedValue) return ''

  const negative = matchedValue[1] === '-'
  const decimal = matchedValue[3] || ''
  let integer = BigInt(matchedValue[2] || '0')
  let fenValue = BigInt(decimal.slice(0, 2).padEnd(2, '0'))

  if ((decimal[2] || '0') >= '5') fenValue += 1n
  if (fenValue >= 100n) {
    integer += fenValue / 100n
    fenValue %= 100n
  }

  if (integer === 0n && fenValue === 0n) return '零元整'

  const uppercaseInteger = formatUppercaseInteger(integer.toString())
  if (integer > 0n && !uppercaseInteger) return '金额超出转换范围'

  const jiao = Number(fenValue / 10n)
  const fen = Number(fenValue % 10n)
  let result = uppercaseInteger ? `${uppercaseInteger}元` : ''

  if (jiao === 0 && fen === 0) {
    result = `${result || '零元'}整`
  } else {
    if (jiao > 0) {
      result += `${uppercaseDigits[jiao] || ''}角`
    } else if (result && fen > 0) {
      result += '零'
    }

    if (fen > 0) result += `${uppercaseDigits[fen] || ''}分`
  }

  return negative ? `负${result}` : result
}

function formatUppercaseInteger(value: string) {
  if (value === '0') return ''

  const sections: string[] = []
  for (let end = value.length; end > 0; end -= 4) {
    sections.push(value.slice(Math.max(0, end - 4), end))
  }

  if (sections.length > sectionUnits.length) return ''

  let result = ''
  let needsZero = false

  for (let index = sections.length - 1; index >= 0; index -= 1) {
    const section = sections[index] || ''
    const sectionValue = Number(section)

    if (sectionValue === 0) {
      if (result) needsZero = true
      continue
    }

    if (result && (needsZero || sectionValue < 1000)) result += '零'

    const sectionUnit = sectionUnits[index]
    if (sectionUnit === undefined) return ''

    result += `${formatFourDigitSection(section)}${sectionUnit}`
    needsZero = false
  }

  return result
}

function formatFourDigitSection(value: string) {
  let result = ''
  let needsZero = false

  for (let index = 0; index < value.length; index += 1) {
    const digit = Number(value.charAt(index))

    if (digit === 0) {
      if (result) needsZero = true
      continue
    }

    if (needsZero) {
      result += '零'
      needsZero = false
    }

    const digitText = uppercaseDigits[digit] || ''
    const positionUnit = positionUnits[value.length - index - 1] || ''
    result += `${digitText}${positionUnit}`
  }

  return result
}