/*                        __    __  __  __    __  ___
    *                       \  \  /  /    \  \  /  /  __/
    *                        \  \/  /  /\  \  \/  /  /
    *                         \____/__/  \__\____/__/.ɪᴏ
    * ᶜᵒᵖʸʳᶦᵍʰᵗ ᵇʸ ᵛᵃᵛʳ ⁻ ˡᶦᶜᵉⁿˢᵉᵈ ᵘⁿᵈᵉʳ ᵗʰᵉ ᵃᵖᵃᶜʰᵉ ˡᶦᶜᵉⁿˢᵉ ᵛᵉʳˢᶦᵒⁿ ᵗʷᵒ ᵈᵒᵗ ᶻᵉʳᵒ
    */
   package io.vavr.collection.euler;
   
   import io.vavr.Function1;
  import io.vavr.collection.CharSeq;
  import io.vavr.collection.List;
  import io.vavr.collection.Stream;
  import org.assertj.core.api.Assertions;
  import org.junit.Test;
  
  import static io.vavr.collection.euler.Utils.file;
  import static io.vavr.collection.euler.Utils.readLines;
  import static org.assertj.core.api.Assertions.assertThat;
  
  public class Euler42Test {
  
      /**
       * <strong>Problem 42 Coded triangle numbers</strong>
       * <p>
       * The <i>n</i><sup>th</sup> term of the sequence of triangle numbers is
       * given by, <i>t</i><sub>n</sub> = ½<i>n</i>(<i>n</i>+1); so the first ten
       * triangle numbers are:
       * <pre>
       * 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ...
       * </pre>
       * <p>
       * By converting each letter in a word to a number corresponding to its
       * alphabetical position and adding these values we form a word value. For
       * example, the word value for SKY is 19 + 11 + 25 = 55 = t<sub>10</sub>. If
       * the word value is a triangle number then we shall call the word a
       * triangle word.
       * <p>
       * Using p042_words.txt, a 16K text file containing nearly two-thousand
       * common English words, how many are triangle words?
       * <p>
       * See also <a href="https://projecteuler.net/problem=42">projecteuler.net
       * problem 42</a>.
       */
      @Test
      public void shouldSolveProblem42() {
          assertThat(isTriangleWord("SKY")).isTrue();
          assertThat(sumOfAlphabeticalPositions("SKY")).isEqualTo(55);
          assertThat(alphabeticalPosition('S')).isEqualTo(19);
          assertThat(alphabeticalPosition('K')).isEqualTo(11);
          assertThat(alphabeticalPosition('Y')).isEqualTo(25);
          assertThat(TRIANGLE_NUMBERS.take(10)).containsExactly(1, 3, 6, 10, 15, 21, 28, 36, 45, 55);
          List.rangeClosed(1, 60).forEach(n -> Assertions.assertThat(isTriangleNumberMemoized.apply(n)).isEqualTo(List.of(1, 3, 6, 10, 15, 21, 28, 36, 45, 55).contains(n)));
  
          assertThat(numberOfTriangleNumbersInFile()).isEqualTo(162);
      }
  
      private static int numberOfTriangleNumbersInFile() {
          return readLines(file("p042_words.txt"))
                  .map(l -> l.replaceAll("\"", ""))
                  .flatMap(l -> List.of(l.split(",")))
                  .filter(Euler42Test::isTriangleWord)
                  .length();
      }
  
      private static boolean isTriangleWord(String word) {
          return isTriangleNumber(sumOfAlphabeticalPositions(word));
      }
  
      private static boolean isTriangleNumber(int n) {
          return TRIANGLE_NUMBERS
                  .takeWhile(t -> t <= n)
                  .exists(t -> t == n);
      }
  
      private static final Function1<Integer, Boolean> isTriangleNumberMemoized = Function1.of(Euler42Test::isTriangleNumber).memoized();
  
      private static final Stream<Integer> TRIANGLE_NUMBERS = Stream.from(1).map(n -> 0.5 * n * (n + 1)).map(Double::intValue);
  
      private static int sumOfAlphabeticalPositions(String word) {
          return CharSeq.of(word)
                  .map(Euler42Test::alphabeticalPosition)
                  .sum().intValue();
      }
  
      private static int alphabeticalPosition(char c) {
          return c - 'A' + 1;
      }
  }