Guess the (prime) number, digit by digit

guess-the-prime-number-digit-by-digit/index.js

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+
+// https://www.codewars.com/kata/6320b05f269dbb001fb409f1
+
+const Sieve = (max = 1e5) => {
+  const maxi = Math.sqrt(max);
+  const notPrime = new Int8Array(max);
+  notPrime[0] = notPrime[1] = 1;
+  for (let i = 2; i < maxi; ++i) {
+    if (notPrime[i] === 0) {
+      for (let j = 2*i; j < max; j += i) {
+        notPrime[j] = 1;
+      }
+    }
+  }
+  return {
+    isPrime: n => !notPrime[n],
+    getPrimes: n => {
+      const res = [];
+      let cnt = 0;
+      const limit = Math.min(max, n);
+      for (let i = 0; i < limit; ++i) {
+        if (!notPrime[i]) {
+          res[cnt++] = i;
+        }
+      }
+      return res;
+    }
+  };
+};
+
+const sieve = Sieve(1e5);
+
+const getHelper = n => {
+  const target = n + '';
+  return m => {
+    const cmp = m + '';
+    return [...cmp].map((d, i) => {
+      if (+target[i] > +d) {
+        return '<';
+      } else if (+target[i] < +d) {
+        return '>';
+      }
+      return '=';
+    }).join('')
+  };
+};
+
+const findPrime = helper => {
+  let d5 = sieve.getPrimes(1e5).filter(n => n > 1e4);
+  let max = 10;
+  while (--max) {
+    const mid = d5.length >> 1;
+    let prime = d5[mid];
+    const hint = helper(prime);
+    d5 = d5.filter(n => {
+      const ps = prime + '';
+      const ns = n + '';
+      for (let i = 0; i < 5; ++i) {
+        if (hint[i] === '>') {
+          if (ps[i] < ns[i]) return false;
+        } else if (hint[i] === '<') {
+          if (ps[i] > ns[i]) return false;
+        } else {
+          if (ps[i] !== ns[i]) return false;
+        }
+      }
+      return true;
+    });
+    if (d5.length === 1) return d5[0];
+  }
+};
+
+findPrime(getHelper(21767));