The position of a digital string in a infinite digital string

the-position-of-a-digital-string-in-a-infinite-digital-string/the-position-of-a-digital-string-in-a-infinite-dig.js

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+// The position of a digital string in a infinite digital string
+// https://www.codewars.com/kata/the-position-of-a-digital-string-in-a-infinite-digital-string/train/javascript
+
+function findPosition(num) {
+  let next = (function() {
+    let n = 1;
+    let cnt = 0;
+    let buf = [];
+    return (len) => {
+      while (buf.length < len) {
+        buf.push(...(''+(n++)).split(''));
+      }
+      let res = buf.join('').substr(0, len);
+      buf.shift();
+      return [res, cnt++];
+    };
+  }());
+  
+  while (1) {
+    let n = next(num.length);
+    // console.log(n[0]);
+    if (n[0] === num) {
+      return n[1];
+    }
+    if (1e8 < n[1]) {
+      console.log(num);
+      break;
+    }
+  }
+}
+
+
+console.log(findPosition('1000000071'));