PrimeFactorization

src/main/java/com/odde/tdd/PrimeFactorization.java

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+package com.odde.tdd;
+
+import java.util.ArrayList;
+
+public class PrimeFactorization {
+    public ArrayList<Integer> run(int n)
+    {
+        ArrayList<Integer> factors = new ArrayList<Integer>();
+        //invalid cases
+        if( n < 2 )
+            return factors;
+
+        //take 2 out to speed up
+        while (n % 2 == 0) {
+            factors.add(2);
+            n /= 2;
+        }
+
+        for (int i = 3; i <= Math.sqrt(n); i += 2) {
+            while (n % i == 0) {
+                factors.add(i);
+                n /= i;
+            }
+        }
+
+        //the last one
+        if (n > 1)
+            factors.add(n);
+
+        return factors;
+    }
+}

src/test/java/com/odde/tdd/PrimeFactorsTest.java

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+package com.odde.tdd;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+
+import static org.junit.Assert.assertEquals;
+
+public class PrimeFactorsTest {
+    @Test
+    public void invalid_num()
+    {
+        PrimeFactorization pf = new PrimeFactorization();
+        ArrayList<Integer> result1 = pf.run(-1);
+        ArrayList<Integer> result2 = pf.run(0);
+        ArrayList<Integer> result3 = pf.run(1);
+        assertEquals(Arrays.asList(), result1);
+        assertEquals(Arrays.asList(), result2);
+        assertEquals(Arrays.asList(), result3);
+        return;
+    }
+
+    @Test
+    public void prime_num()
+    {
+        PrimeFactorization pf = new PrimeFactorization();
+        ArrayList<Integer> result1 = pf.run(2);
+        ArrayList<Integer> result2 = pf.run(3);
+        ArrayList<Integer> result3 = pf.run(17);
+        assertEquals(Arrays.asList(2), result1);
+        assertEquals(Arrays.asList(3), result2);
+        assertEquals(Arrays.asList(17), result3);
+        return;
+    }
+
+    @Test
+    public void nonprime_num()
+    {
+        PrimeFactorization pf = new PrimeFactorization();
+        ArrayList<Integer> result1 = pf.run(20);
+        ArrayList<Integer> result2 = pf.run(456);
+        ArrayList<Integer> result3 = pf.run(789);
+        assertEquals(Arrays.asList(2,2,5), result1);
+        assertEquals(Arrays.asList(2, 2, 2, 3, 19), result2);
+        assertEquals(Arrays.asList(3, 263), result3);
+        return;
+    }
+}