Data commit

Task/Partition-an-integer-x-into-n-primes/Java/partition-an-integer-x-into-n-primes.java

@@ -0,0 +1,77 @@
+import java.util.Arrays;
+import java.util.stream.IntStream;
+
+public class PartitionInteger {
+    private static final int[] primes = IntStream.concat(IntStream.of(2), IntStream.iterate(3, n -> n + 2))
+        .filter(PartitionInteger::isPrime)
+        .limit(50_000)
+        .toArray();
+
+    private static boolean isPrime(int n) {
+        if (n < 2) return false;
+        if (n % 2 == 0) return n == 2;
+        if (n % 3 == 0) return n == 3;
+        int d = 5;
+        while (d * d <= n) {
+            if (n % d == 0) return false;
+            d += 2;
+            if (n % d == 0) return false;
+            d += 4;
+        }
+        return true;
+    }
+
+    private static boolean findCombo(int k, int x, int m, int n, int[] combo) {
+        boolean foundCombo = false;
+        if (k >= m) {
+            if (Arrays.stream(combo).map(i -> primes[i]).sum() == x) {
+                String s = m > 1 ? "s" : "";
+                System.out.printf("Partitioned %5d with %2d prime%s: ", x, m, s);
+                for (int i = 0; i < m; ++i) {
+                    System.out.print(primes[combo[i]]);
+                    if (i < m - 1) System.out.print('+');
+                    else System.out.println();
+                }
+                foundCombo = true;
+            }
+        } else {
+            for (int j = 0; j < n; ++j) {
+                if (k == 0 || j > combo[k - 1]) {
+                    combo[k] = j;
+                    if (!foundCombo) {
+                        foundCombo = findCombo(k + 1, x, m, n, combo);
+                    }
+                }
+            }
+        }
+        return foundCombo;
+    }
+
+    private static void partition(int x, int m) {
+        if (x < 2 || m < 1 || m >= x) {
+            throw new IllegalArgumentException();
+        }
+        int[] filteredPrimes = Arrays.stream(primes).filter(it -> it <= x).toArray();
+        int n = filteredPrimes.length;
+        if (n < m) throw new IllegalArgumentException("Not enough primes");
+        int[] combo = new int[m];
+        boolean foundCombo = findCombo(0, x, m, n, combo);
+        if (!foundCombo) {
+            String s = m > 1 ? "s" : " ";
+            System.out.printf("Partitioned %5d with %2d prime%s: (not possible)\n", x, m, s);
+        }
+    }
+
+    public static void main(String[] args) {
+        partition(99809, 1);
+        partition(18, 2);
+        partition(19, 3);
+        partition(20, 4);
+        partition(2017, 24);
+        partition(22699, 1);
+        partition(22699, 2);
+        partition(22699, 3);
+        partition(22699, 4);
+        partition(40355, 3);
+    }
+}