// This program contains some examples of Java 8 constructs.  Three of the most
// important higher-order functions to understand are map, filter, and reduce.
// higher order function are methods that take in methods as an argument

import java.util.*;
import java.util.stream.*;
import java.util.function.*;

public class Java8Demo {
   public static void main(String[] args) {
      listExample();
      primesExample();
      parallelExample();
   }

   // some Java 8 examples using a list of strings
   public static void listExample() {
      List<String> words =
         Arrays.asList("is", "mayonnaise", "an", "instrument?");
   
      // old fashioned way to print the words
      System.out.println("For loop approach");
      for (int i = 0; i < words.size(); i++) {
         System.out.print(words.get(i) + " ");
      }
      System.out.println();
      System.out.println();
   
      // Java 5 introduced the for-each loop and Iterable<T> interface
      System.out.println("For-each loop approach");
      for (String word : words) {
         System.out.print(word + " ");
      }
      System.out.println();
      System.out.println();
   
      // Java 8 introduced this idea of "Streams"
      System.out.println("Stream approach");
      words                            // List<String>
         .stream()                     // Stream<String>
         .forEach(Java8Demo::printWithSpace);
         
      words                            // List<String>
         .stream()                     // Stream<String>
         .forEach(Java8Demo::printWithSpace);
         
      words                            // List<String>
         .stream()                     // Stream<String>
         .forEach((word) -> System.out.print(word + " "));
      // anonymous function / lambda
      
      words                            // List<String> ["i", "m", "a", "i"]
         .stream()                     // Stream<String> "i", "m", "a" ,"i"
         .map((word) -> word + " ")    // Stream<String> "i ", "m ", "a " ,"i "
         .forEach(System.out::print);
   }
   
   public static void printWithSpace(String word) {
      System.out.println(word + " ");
   }

   // example of finding the sum of the primes between 1 and 20,000
   public static void primesExample() {
      System.out.println("Loop-based approach");
      // perform a slow operation with the iterative approach
      printSumIterative(20001);
   
      System.out.println("Stream approach");
      // perform a slow operation and report elapsed time
      printSum(20001);
      
      System.out.println();
   }
   
   // Prints the sum of all the primes between 1 and n
   // along with the time it took, done iteratively
   public static void printSumIterative(int n) {
      long start = System.currentTimeMillis();
      int sum = 0;
      for (int i = 1; i < 20001; i++) {
         if (isPrimeIterative(i)) {
            sum += i;
         }
      }
      double elapsed = (System.currentTimeMillis() - start) / 1000.0;
      System.out.println("n = " + sum + ", time = " + elapsed);
   }

   // Returns true if n is a prime number, determined with in an
   // iterative manner, and false otherwise
   public static boolean isPrimeIterative(int n) {
      int factors = 0;
      for (int i = 1; i <= n; i++) {
         if (n % i == 0) {
            factors++;
         }
      }
      return factors == 2;
   }
   
   // times how long it takes to find the sum of the primes in the stream
   public static void printSum(int n) {
      // run the algorithm and time it
      long start = System.currentTimeMillis();
      int sum = IntStream.range(1, n + 1)
         .parallel()
         .filter(Java8Demo::isPrime)
         .sum();
      double elapsed = (System.currentTimeMillis() - start) / 1000.0;
      System.out.println("n = " + sum + ", time = " + elapsed);
   }

   // Returns true if n is a prime number, determined with a
   // stream, and false otherwise
   public static boolean isPrime(int n) {
      return IntStream.range(1, n + 1)
         .filter((num) -> n % num == 0)
         .reduce(0, (count, element) -> count + 1) == 2;
   }

   // it's great to be able to make a stream parallel to make a program run
   // faster, but sometimes that doesn't make sense, as in this example
   public static void parallelExample() {
      System.out.println("Let's print 1 through 20");
   
      System.out.print("sequentially:");
      IntStream.range(1, 21)
         .forEach(n -> System.out.print(" " + n));
      System.out.println();
   
      System.out.print("in parallel :");
      IntStream.range(1, 21)
         .parallel()
         .forEach(n -> System.out.print(" " + n));
      System.out.println();
      System.out.println();
   }
}