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package com.github.underscore;

import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

public final class Optional {
    private static final Optional EMPTY = new Optional();
    private final T arg;
    private final boolean absent;

    private Optional() {
        this.arg = null;
        this.absent = true;
    }

    private Optional(final T arg) {
        this.arg = arg;
        this.absent = false;
    }

    public static  Optional of(final T arg) {
        return new Optional(arg);
    }

    public static  Optional fromNullable(final T nullableReference) {
        return nullableReference == null ? Optional.absent()
            : new Optional(nullableReference);
    }

    @SuppressWarnings("unchecked")
    public static Optional absent() {
        return (Optional) EMPTY;
    }

    public T get() {
        if (absent) {
            throw new IllegalStateException("Optional.get() cannot be called on an absent value");
        }
        return arg;
    }

    public T or(final T defaultValue) {
        if (absent) {
            return defaultValue;
        }
        return arg;
    }

    public T orNull() {
        if (absent) {
            return null;
        }
        return arg;
    }

    public boolean isEmpty() {
        return absent;
    }

    public boolean isPresent() {
        return !absent;
    }

    @SuppressWarnings("unchecked")
    public Optional filter(Predicate predicate) {
        U.checkNotNull(predicate);
        if (isPresent()) {
            return predicate.test(arg) ? this : Optional.absent();
        } else {
            return this;
        }
    }

    public  Optional map(Function mapper) {
        U.checkNotNull(mapper);
        if (isPresent()) {
            return Optional.fromNullable(mapper.apply(arg));
        } else {
            return absent();
        }
    }

    public  T orThrow(Supplier exceptionFunction) throws X {
        if (absent) {
            throw exceptionFunction.get();
        } else {
            return arg;
        }
    }

    @Override
    public boolean equals(final Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }

        final Optional optional = (Optional) o;

        return absent == optional.absent && !(arg == null ? optional.arg != null : !arg.equals(optional.arg));
    }

    @Override
    public int hashCode() {
        int result = arg == null ? 0 : arg.hashCode();
        result = 31 * result + (absent ? 1 : 0);
        return result;
    }

    @Override
    public String toString() {
        return absent ? "Optional.absent()" : "Optional.of(" + arg + ")";
    }
}