/**
 * @desc 解决浮动运算问题，避免小数点后产生多位数和计算精度损失。
 *
 * 问题示例：2.3 + 2.4 = 4.699999999999999，1.0 - 0.9 = 0.09999999999999998
 */

type NumberType = number | string;

/**
 * Correct the given number to specifying significant digits.
 *
 * @param num The input number
 * @param precision An integer specifying the number of significant digits
 *
 * @example strip(0.09999999999999998) === 0.1 // true
 */
function strip(num: NumberType, precision = 15): number {
  return +parseFloat(Number(num).toPrecision(precision));
}

/**
 * Return digits length of a number.
 *
 * @param num The input number
 */
function digitLength(num: NumberType): number {
  // Get digit length of e
  const eSplit = num.toString().split(/[eE]/);
  const len = (eSplit[0].split('.')[1] || '').length - +(eSplit[1] || 0);
  return len > 0 ? len : 0;
}

/**
 * Convert the given number to integer, support scientific notation.
 * The number will be scale up if it is decimal.
 *
 * @param num The input number
 */
function float2Fixed(num: NumberType): number {
  if (num.toString().indexOf('e') === -1) {
    return Number(num.toString().replace('.', ''));
  }
  const dLen = digitLength(num);
  return dLen > 0 ? strip(Number(num) * Math.pow(10, dLen)) : Number(num);
}

/**
 * Log a warning if the given number is out of bounds.
 *
 * @param num The input number
 */
function checkBoundary(num: number) {
  if (_boundaryCheckingState) {
    if (num > Number.MAX_SAFE_INTEGER || num < Number.MIN_SAFE_INTEGER) {
      console.warn(`${num} is beyond boundary when transfer to integer, the results may not be accurate`);
    }
  }
}

/**
 * Create an operation to support rest params.
 *
 * @param operation The original operation
 */
function createOperation(operation: (n1: NumberType, n2: NumberType) => number): (...nums: NumberType[]) => number {
  return (...nums: NumberType[]) => {
    const [first, ...others] = nums;
    return others.reduce((prev, next) => operation(prev, next), first) as number;
  };
}

/**
 * Accurate multiplication.
 *
 * @param nums The numbers to multiply
 */
const times = createOperation((num1, num2) => {
  const num1Changed = float2Fixed(num1);
  const num2Changed = float2Fixed(num2);
  const baseNum = digitLength(num1) + digitLength(num2);
  const leftValue = num1Changed * num2Changed;

  checkBoundary(leftValue);

  return leftValue / Math.pow(10, baseNum);
});

/**
 * Accurate addition.
 *
 * @param nums The numbers to add
 */
const plus = createOperation((num1, num2) => {
  // 取最大的小数位
  const baseNum = Math.pow(10, Math.max(digitLength(num1), digitLength(num2)));
  // 把小数都转为整数然后再计算
  return (times(num1, baseNum) + times(num2, baseNum)) / baseNum;
});

/**
 * Accurate subtraction.
 *
 * @param nums The numbers to subtract
 */
const minus = createOperation((num1, num2) => {
  const baseNum = Math.pow(10, Math.max(digitLength(num1), digitLength(num2)));
  return (times(num1, baseNum) - times(num2, baseNum)) / baseNum;
});

/**
 * Accurate division.
 *
 * @param nums The numbers to divide
 */
const divide = createOperation((num1, num2) => {
  const num1Changed = float2Fixed(num1);
  const num2Changed = float2Fixed(num2);

  checkBoundary(num1Changed);
  checkBoundary(num2Changed);

  // fix: 类似 10 ** -4 为 0.00009999999999999999，strip 修正
  return times(num1Changed / num2Changed, strip(Math.pow(10, digitLength(num2) - digitLength(num1))));
});

/**
 * Accurate rounding method.
 *
 * @param num The number to round
 * @param decimal An integer specifying the decimal digits
 */
function round(num: NumberType, decimal: number): number {
  const base = Math.pow(10, decimal);
  let result = divide(Math.round(Math.abs(times(num, base))), base);

  if (num < 0 && result !== 0) {
    result = times(result, -1);
  }

  return result;
}

let _boundaryCheckingState = true;

/**
 * Whether to check the bounds of number, default is enabled.
 *
 * @param flag The value to indicate whether is enabled
 */
function enableBoundaryChecking(flag = true) {
  _boundaryCheckingState = flag;
}

export { strip, plus, minus, times, divide, round, digitLength, float2Fixed, enableBoundaryChecking };

export default {
  strip,
  plus,
  minus,
  times,
  divide,
  round,
  digitLength,
  float2Fixed,
  enableBoundaryChecking,
};