Data update

Task/Smarandache-prime-digital-sequence/JavaScript/smarandache-prime-digital-sequence.js

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+function getSmarandachePrime(n) {
+    if (n < 10) {
+        switch (n) {
+            case 1: return 2;
+            case 2: return 3;
+            case 3: return 5;
+            case 4: return 7;
+        }
+    }
+    let s = getNextSmarandache(7);
+    let result = 0;
+    for (let count = 1; count <= n - 4; s = getNextSmarandache(s)) {
+        if (isPrime(s)) {
+            count++;
+            result = s;
+        }
+    }
+    return result;
+}
+
+function isPrime(test) {
+    if (test % 2 === 0) return false;
+    for (let i = 3; i <= Math.sqrt(test); i += 2) {
+        if (test % i === 0) {
+            return false;
+        }
+    }
+    return true;
+}
+
+function getNextSmarandache(n) {
+    // If 3, next is 7
+    if (n % 10 === 3) {
+        return n + 4;
+    }
+    let retVal = n - 4;
+
+    // Last digit 7. k = largest position from right where we have a 7.
+    let k = 0;
+    while (n % 10 === 7) {
+        k++;
+        n = Math.floor(n / 10);
+    }
+
+    // Determine first digit from right where digit != 7.
+    const digit = n % 10;
+
+    // Digit is 2, 3, or 5. 3-2 = 1, 5-3 = 2, 7-5 = 2, so digit = 2, coefficient = 1, otherwise 2.
+    const coeff = (digit === 2 ? 1 : 2);
+
+    // Compute next value
+    retVal += coeff * Math.pow(10, k);
+
+    // Subtract values for digit = 7.
+    while (k > 1) {
+        retVal -= 5 * Math.pow(10, k - 1);
+        k--;
+    }
+
+    // Even works for 777..777 --> 2222...223
+    return retVal;
+}
+
+// Main execution
+let s = getNextSmarandache(7);
+console.log("First 25 Smarandache prime-digital sequence numbers:");
+process.stdout.write("2 3 5 7 ");
+for (let count = 1; count <= 21; s = getNextSmarandache(s)) {
+    if (isPrime(s)) {
+        process.stdout.write(`${s} `);
+        count++;
+    }
+}
+console.log("\n");
+
+for (let i = 2; i <= 5; i++) {
+    const n = Math.pow(10, i);
+    console.log(`${n.toLocaleString()}th Smarandache prime-digital sequence number = ${getSmarandachePrime(n)}`);
+}