added reciprocal to docs and max_decimal_digits to multiply

README.md

@@ -259,6 +259,19 @@ pow("0", "0", { zero_to_the_power_of_zero: undefined })
 undefined
 ```
 
+### reciprocal
+Calculate [reciprocal](https://en.wikipedia.org/wiki/Multiplicative_inverse) or [multiplicative inverse](https://en.wikipedia.org/wiki/Multiplicative_inverse)
+```js
+import reciprocal from "preciso/reciprocal.js"
+
+reciprocal("0.1");
+"10"
+
+// return fraction if applicable
+reciprocal("3/4", { fraction: true })
+"4/3"
+```
+
 ### remainder
 ```js
 import remainder from "preciso/truncate.js";

multiply.js

@@ -3,8 +3,10 @@
 const multiply_array = require("./multiply_array.js");
 
 function multiply() {
-  const nums = Array.isArray(arguments[0]) ? arguments[0] : Array.from(arguments);
-  return multiply_array(nums);
+  const args = Array.from(arguments);
+  const options = typeof args[args.length - 1] === "object" ? args[args.length - 1] : undefined;
+  const nums = Array.isArray(args[0]) ? args[0] : options ? args.slice(0, args.length - 1) : args;
+  return multiply_array(nums, options);
 }
 
 module.exports = multiply;

multiply_array.js

@@ -11,9 +11,12 @@ const multiply_rational = require("./multiply_rational.js");
  * @param {Array.<String>} nums - array of numerical strings
  * @returns {String} product as a numerical string
  */
-function multiply_array(nums) {
+function multiply_array(nums, { max_decimal_digits } = {}) {
   const imaginary = is_odd(nums.filter(n => is_imaginary(n)).length.toString());
-  let product = multiply_rational(nums.map(n => n.replace(/i$/, "")));
+  let product = multiply_rational(
+    nums.map(n => n.replace(/i$/, "")),
+    { max_decimal_digits }
+  );
   if (imaginary) product += "i";
   return product;
 }

multiply_rational.js

@@ -4,12 +4,13 @@ const absolute = require("./absolute.js");
 const clean = require("./clean.js");
 const compare_positive = require("./compare_positive.js");
 const long_multiplication = require("./long_multiplication.js");
+const round = require("./round.js");
 
 /**
  * @name multiply
  * @returns {String} product
  */
-function multiply_rational(nums, options = {}) {
+function multiply_rational(nums, { max_decimal_digits } = {}) {
   let product = clean(nums[0]);
   let product_absolute = absolute(product);
   let product_sign = product[0] === "-" ? "-" : "";
@@ -27,6 +28,7 @@ function multiply_rational(nums, options = {}) {
 
     product = product_sign + product_absolute;
   }
+  if (typeof max_decimal_digits === "number") product = round(product, { digits: max_decimal_digits });
   return product;
 }
 

preciso.d.ts

@@ -14,7 +14,7 @@ export function max(nums: string[]): string;
 // you can also pass in min(a, b, c, d) but I'm not sure how to type that
 export function min(nums: string[]): string;
 
-export function multiply(a: string, b: string): string;
+export function multiply(a: string, b: string, options?: { max_decimal_digits?: number }): string;
 export function pow(base: string, exponent: string, options: { ellipsis?: boolean; max_decimal_digits?: number; zero_to_the_power_of_zero?: string }): string;
 export function remainder(dividend: string, divisor: string): string;
 export function root(radicand: string, index: string, options?: { imaginary?: boolean; max_decimal_digits?: number }): string;

test.js

@@ -228,6 +228,8 @@ test("multiply", ({ eq }) => {
   //                                      3573540430663160
   eq(multiply("7147008086132632", "5"), "35735040430663160");
   eq(multiply("-714.7008086132632", "8135.725531"), "-5814609.6156612701214627592");
+  eq(multiply("-714.7008086132632", "8135.725531", { max_decimal_digits: 0 }), "-5814610");
+  eq(multiply("-714.7008086132632", "8135.725531", { max_decimal_digits: 7 }), "-5814609.6156613");
   //                                       46456025598621080
   eq(multiply("7147008086132632", "65"), "464555525598621080");