Fixed error in Sieve Of Eratosthenes calculation and updated testing code

Author: Justin Wetherell

src/com/jwetherell/algorithms/mathematics/Primes.java

@@ -79,18 +79,15 @@ public static final boolean sieveOfEratosthenes(int number) {
                 sieve = new boolean[number+1];
             } else if (number >= sieve.length) {
                 sieve = Arrays.copyOf(sieve, number+1);
-                start = sieve.length;
             }
-            sieve[0] = true;
-            sieve[1] = false;
-            for (int i = start; (int) Math.pow(i, 2) < sieve.length; i++) {
+
+            for (int i = start; i < Math.sqrt(number); i++) {
                 if (!sieve[i]) {
-                    for (int j = (int) Math.pow(i, 2); j < sieve.length; j += i) {
+                    for (int j = i*2; j <= number; j += i) {
                         sieve[j] = true;
                     }
                 }
             }
-            sieve[2] = true;
         }
         return !sieve[number];
     }

src/com/jwetherell/algorithms/mathematics/test/Mathematics.java

@@ -51,7 +51,7 @@ public void multiplication() {
     @Test
     public void division() {
         int a = nextRandomInt(MIN, MAX);
-        int b = nextRandomInt(MIN, MAX);
+        int b = nextRandomInt(a, MAX);
         long result = Division.divisionUsingLoop(a, b);
         long check = Division.division(a, b);
         assertTrue("Division using a loop. a=" + a + " b=" + b + " result=" + result + " check=" + check, (result == check));
@@ -154,7 +154,7 @@ public void sieveOfEratosthenes() {
 
         number = 4178;
         isPrime = Primes.sieveOfEratosthenes(number);
-        assertTrue("Sieve Of Eratosthenes error.", isPrime);
+        assertFalse("Sieve Of Eratosthenes error.", isPrime);
 
         number = 7919;
         isPrime = Primes.sieveOfEratosthenes(number);
@@ -163,5 +163,17 @@ public void sieveOfEratosthenes() {
         number = 556;
         isPrime = Primes.sieveOfEratosthenes(number);
         assertFalse("Sieve Of Eratosthenes error.", isPrime);
+
+        number = 6091;
+        isPrime = Primes.sieveOfEratosthenes(number);
+        assertTrue("Sieve Of Eratosthenes error.", isPrime);
+
+        number = 6090;
+        isPrime = Primes.sieveOfEratosthenes(number);
+        assertFalse("Sieve Of Eratosthenes error.", isPrime);
+
+        number = 6089;
+        isPrime = Primes.sieveOfEratosthenes(number);
+        assertTrue("Sieve Of Eratosthenes error.", isPrime);
     }
 }