Decimal to Factorial and Back

decimal-to-factorial-and-back/index.js

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+// https://www.codewars.com/kata/decimal-to-factorial-and-back/javascript
+
+const [
+  dec2FactString,
+  factString2Dec
+] = (() => {
+  const bases = [1, 1];
+  let fact = 1;
+  for (let i = 2; i < 36; ++i) {
+    fact *= i;
+    bases[i] = fact;
+  }
+  let chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
+  return [
+    nb => {
+      const res = [];
+      for (let i = bases.length-1; 0 <= i; --i) {
+        if (1 <= nb/bases[i]) {
+          const tmp = Math.floor(nb/bases[i]);
+          nb = nb - tmp * bases[i];
+          res.push(tmp);
+        } else {
+          res.push(0);
+        }
+      }
+      return res.map(n=>chars[n]).join``.replace(/^0+/, '') || '0';
+    },
+    str => {
+      const r = [...str].reduceRight((acc, ch, i, arr) => {
+        return acc + bases[arr.length - i - 1] * chars.indexOf(ch);
+      }, 0);
+      return r;
+    }
+  ];
+})();