package fj.data.fingertrees;

import fj.F;
import fj.F2;
import fj.Function;
import fj.data.vector.V2;
import fj.data.vector.V3;
import fj.data.vector.V4;
import static fj.data.fingertrees.FingerTree.mkTree;

/**
 * A digit is a vector of 1-4 elements. Serves as a pointer to the prefix or suffix of a finger tree.
 */
public abstract class Digit<V, A> {
  /**
   * Folds this digit to the right using the given function and the given initial value.
   *
   * @param f A function with which to fold this digit.
   * @param z An initial value to apply at the rightmost end of the fold.
   * @return The right reduction of this digit with the given function and the given initial value.
   */
  public abstract <B> B foldRight(final F<A, F<B, B>> f, final B z);

  /**
   * Folds this digit to the left using the given function and the given initial value.
   *
   * @param f A function with which to fold this digit.
   * @param z An initial value to apply at the leftmost end of the fold.
   * @return The left reduction of this digit with the given function and the given initial value.
   */
  public abstract <B> B foldLeft(final F<B, F<A, B>> f, final B z);

  /**
   * Folds this digit to the right using the given function.
   *
   * @param f A function with which to fold this digit.
   * @return The right reduction of this digit with the given function.
   */
  public final A reduceRight(final F<A, F<A, A>> f) {
    return match(new F<One<V, A>, A>() {
      public A f(final One<V, A> one) {
        return one.value();
      }
    }, new F<Two<V, A>, A>() {
      public A f(final Two<V, A> two) {
        final V2<A> v = two.values();
        return f.f(v._1()).f(v._2());
      }
    }, new F<Three<V, A>, A>() {
      public A f(final Three<V, A> three) {
        final V3<A> v = three.values();
        return f.f(v._1()).f(f.f(v._2()).f(v._3()));
      }
    }, new F<Four<V, A>, A>() {
      public A f(final Four<V, A> four) {
        final V4<A> v = four.values();
        return f.f(v._1()).f(f.f(v._2()).f(f.f(v._3()).f(v._4())));
      }
    });
  }

  /**
   * Folds this digit to the right using the given function.
   *
   * @param f A function with which to fold this digit.
   * @return The right reduction of this digit with the given function.
   */
  public final A reduceLeft(final F<A, F<A, A>> f) {
    return match(new F<One<V, A>, A>() {
      public A f(final One<V, A> one) {
        return one.value();
      }
    }, new F<Two<V, A>, A>() {
      public A f(final Two<V, A> two) {
        final V2<A> v = two.values();
        return f.f(v._1()).f(v._2());
      }
    }, new F<Three<V, A>, A>() {
      public A f(final Three<V, A> three) {
        final V3<A> v = three.values();
        return f.f(f.f(v._1()).f(v._2())).f(v._3());
      }
    }, new F<Four<V, A>, A>() {
      public A f(final Four<V, A> four) {
        final V4<A> v = four.values();
        return f.f(f.f(f.f(v._1()).f(v._2())).f(v._3())).f(v._4());
      }
    });
  }

  /**
   * Maps a function across the elements of this digit, measuring with the given measurement.
   *
   * @param f A function to map across the elements of this digit.
   * @param m A measuring for the function's domain (destination type).
   * @return A new digit with the same structure as this digit, but with all elements transformed
   *         with the given function and measured with the given measuring.
   */
  public final <B> Digit<V, B> map(final F<A, B> f, final Measured<V, B> m) {
    return match(new F<One<V, A>, Digit<V, B>>() {
      public Digit<V, B> f(final One<V, A> one) {
        return new One<V, B>(m, f.f(one.value()));
      }
    }, new F<Two<V, A>, Digit<V, B>>() {
      public Digit<V, B> f(final Two<V, A> two) {
        return new Two<V, B>(m, two.values().map(f));
      }
    }, new F<Three<V, A>, Digit<V, B>>() {
      public Digit<V, B> f(final Three<V, A> three) {
        return new Three<V, B>(m, three.values().map(f));
      }
    }, new F<Four<V, A>, Digit<V, B>>() {
      public Digit<V, B> f(final Four<V, A> four) {
        return new Four<V, B>(m, four.values().map(f));
      }
    });
  }

  /**
   * Structural pattern matching on digits. Applies the function that matches the structure of this digit.
   *
   * @param one   A function to apply to this digit if it's One.
   * @param two   A function to apply to this digit if it's Two.
   * @param three A function to apply to this digit if it's Three.
   * @param four  A function to apply to this digit if it's Four.
   * @return The result of applying the function matching this Digit.
   */
  public abstract <B> B match(final F<One<V, A>, B> one, final F<Two<V, A>, B> two, final F<Three<V, A>, B> three,
                              final F<Four<V, A>, B> four);

  private final Measured<V, A> m;

  Digit(final Measured<V, A> m) {
    this.m = m;
  }

  /**
   * Returns the sum of the measurements of this digit according to the monoid.
   *
   * @return the sum of the measurements of this digit according to the monoid.
   */
  public final V measure() {
    return foldLeft(Function.curry(new F2<V, A, V>() {
      public V f(final V v, final A a) {
        return m.sum(v, m.measure(a));
      }
    }), m.zero());
  }

  /**
   * Returns the tree representation of this digit.
   * @return the tree representation of this digit. 
   */
  public final FingerTree<V, A> toTree() {
    final MakeTree<V, A> mk = mkTree(m);
    return match(new F<One<V, A>, FingerTree<V, A>>() {
      public FingerTree<V, A> f(final One<V, A> one) {
        return mk.single(one.value());
      }
    }, new F<Two<V, A>, FingerTree<V, A>>() {
      public FingerTree<V, A> f(final Two<V, A> two) {
        return mk.deep(mk.one(two.values()._1()), new Empty<V, Node<V, A>>(m.nodeMeasured()), mk.one(two.values()._2()));
      }
    }, new F<Three<V, A>, FingerTree<V, A>>() {
      public FingerTree<V, A> f(final Three<V, A> three) {
        return mk.deep(mk.two(three.values()._1(), three.values()._2()), new Empty<V, Node<V, A>>(m.nodeMeasured()),
                       mk.one(three.values()._3()));
      }
    }, new F<Four<V, A>, FingerTree<V, A>>() {
      public FingerTree<V, A> f(final Four<V, A> four) {
        return mk.deep(mk.two(four.values()._1(), four.values()._2()), new Empty<V, Node<V, A>>(m.nodeMeasured()),
                       mk.two(four.values()._3(), four.values()._4()));
      }
    });
  }
}