#11

src/test/scala/org/bescala/projecteuler/problems/Problem007.scala

@@ -1,15 +1,66 @@
 package org.bescala.projecteuler.problems
 
-import org.bescala.projecteuler.EulerSuite
+import org.bescala.projecteuler._
 import org.bescala.projecteuler.ProjectEuler._
 
+import scala.annotation.tailrec
+
 class Problem007 extends EulerSuite {
 
   /**
    * By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
    * What is the 10 001st prime number?
    */
-  euler(problem(7)) {
-    TODO
+
+  euler(problem(7), "tailrec") {
+    def prime(nth: Int) = {
+      @tailrec def prime_(count: Int, lastPrime: Int): Int = {
+        if (count == 0)
+          lastPrime
+        else
+          prime_(count - 1, nextPrime(lastPrime))
+      }
+      prime_(nth - 1, nextPrime(1))
+    }
+    prime(10001)
+  }
+
+  euler(problem(7), "stream") {
+    def primes = {
+      def nextPrimes(n: Long): Stream[Long] = {
+        val np = nextPrime(n.toInt)
+        np #:: nextPrimes(np)
+      }
+      2L #:: nextPrimes(2)
+    }
+    primes(10001 - 1)
+  }
+
+  euler(problem(7), "mverbist") {
+    Stream.from(2).filter(isPrime)(10000)
+  }
+
+  euler(problem(7), "LucDupAtGitHub no sieve") {
+    import math._
+    def squareRootOf(z: Int) = round(floor(sqrt(z.toDouble))).toInt
+    def isPrime(z: Int): Boolean = (2 to squareRootOf(z)).forall(z % _ != 0)
+
+    Stream.from(2).filter(isPrime).drop(10000).head
+  }
+
+  // Stack overflows
+  euler(problem(7), "LucDupAtGitHub with sieve") {
+    def sieve(stream: Stream[Int], l: Int): Int = {
+      @tailrec  def loop(stream: Stream[Int])(acc: Int, m: Int): Int = {
+        if (m == 0)
+          acc
+        else {
+          val p = stream.head
+          loop(stream.filter(_ % p != 0))(p, m - 1)
+        }
+      }
+      loop(stream)(2, l)
+    }
+    sieve(Stream.from(2), 10001)
   }
 }