Multiplying numbers as strings: Part II

multiplying-numbers-as-strings-part-ii/index.js

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+// https://www.codewars.com/kata/multiplying-numbers-as-strings-part-ii/train/javascript
+
+function multiply(aa, bb) {
+  let [a, b] = [aa, bb].map(str => str.match(/(\-?)([0-9]+)\.?([0-9]*)$/));
+
+  let m = a[3].length + b[3].length;
+  let res = multiplySimple(a[2]+a[3], b[2]+b[3]);
+  
+  let zeros = '0'.repeat(Math.max(0, m-res.length));
+  if (m) res = (zeros+res).substr(0,res.length-m) + '.'+ (zeros+res).substr(-m);
+  let r = (a[1]+b[1]).replace(/\-\-/, '') + (res.replace(/\.([0-9]*?)0*$/, '.$1').replace(/\.$/, '').replace(/^\./, '0.') || '0')
+  return r === '-0' ? '0' : r;
+}
+
+// https://www.codewars.com/kata/multiplying-numbers-as-strings/solutions/javascript
+function multiplySimple(a, b) {
+  return a.split('').reduceRight((sum, d, i, arr) => {
+    return sumStrings(mul(b, +d) + '0'.repeat(arr.length - i - 1), sum);
+  }, '0');
+}
+
+// mltiple any big int (string) by one digit (0-9)
+function mul(str, d) {
+  return d.toString(2).split('').reduceRight((obj, b) => {
+    let {res, last} = obj;
+    if (b === '1') res = sumStrings(res, last);
+    last = sumStrings(last, last);
+    return {res, last}
+  }, {res: '0', last: str}).res;
+}
+
+// https://www.codewars.com/kata/sum-strings-as-numbers
+function sumStrings(aa,bb) { 
+  let a = aa.split('').reverse();
+  let b = bb.split('').reverse();
+  let c = 0;
+  let r = [];
+  
+  for (let i = 0; i < Math.max(a.length, b.length); ++i) {
+    let sum = (+a[i]|0) + (+b[i]|0) + c;
+    if (sum >= 10) {
+      r[i] = (sum - 10) + '';
+      c = 1;
+    } else {
+      r[i] = sum + '';
+      c = 0;
+    }
+  }
+  if (c) {
+    r.push('1');
+  }
+  return r.reverse().join('').replace(/^0*/, '') || '0';
+}