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// Copyright 2015 PlanBase Inc. & Glen Peterson
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package org.pkl.thirdparty.paguro.oneOf;

import org.pkl.thirdparty.jetbrains.annotations.NotNull;
import org.pkl.thirdparty.paguro.collections.ImList;
import org.pkl.thirdparty.paguro.function.Fn1;
import org.pkl.thirdparty.paguro.type.RuntimeTypes;

import java.util.Objects;

import static org.pkl.thirdparty.paguro.type.RuntimeTypes.union2Str;

/**
 This is designed to represent a union of 2 types, meaning an object that can be one type, or another.
 Instead of a get() method, pass 2 functions to match(), one to handle the case where this contains the first thing,
 the other if it contains the second thing.  In theory, this could work with two things of the same type, but
 Java has polymorphism to handle that more easily.  Before using a OneOf2, make sure you don't really need a
 {@link org.pkl.thirdparty.paguro.tuple.Tuple2}.

 OneOf2 is designed to be subclassed to add descriptive names.  The safest way
 to use Union classes is to always call match() because it forces you to think about how to
 handle each type you could possibly receive.

 Usage:
 {@code
thingy.match(fst -> fst.doOneThing(),
             sec -> sec.doSomethingElse());
}

 Sometimes it's a programming error to pass one type or another, and you may want to throw an
 exception.
 {@code
oneOf.match(fst -> fst.doOneThing(),
            sec -> { throw new IllegalStateException("Asked for a 2nd; only had a 1st."); });
}

 For the shortest syntax and best names, define your own subclass.  This is similar to sub-classing Tuples.
 {@code
static class String_Integer extends OneOf2 {

    // Private Constructor because the o parameter is not type safe.
    private String_Integer(Object o, int n) { super(o, String.class, Integer.class, n); }

    // Static factory methods ensure type-safe construction.
    public static String_Integer ofStr(String o) { return new String_Integer(o, 0); }
    public static String_Integer ofInt(Integer o) { return new String_Integer(o, 1); }
}
}

 equals(), hashcode(), and toString() are all taken care of for you.

 Now you use descriptive and extremely brief syntax:
 {@code
// Type-safe switching - always works at runtime.
x.match(s -> (s == null) ? null : s.lowerCase(),
        n -> "This is the number " + n);

// If not a String at runtime throws "Expected a(n) String but found a(n) Integer"
x.str().contains("goody!");

// If not an Integer at runtime throws "Expected a(n) Integer but found a(n) String"
3 + x.integer();
}

 Instead of putting a Null object in here, either return a null OneOf2 or wrap OneOf2 in an {@link Option}
 */
public class OneOf2 {
    protected final @NotNull Object item;
    private final int sel;
    @SuppressWarnings("rawtypes")
    private final @NotNull ImList types;

    /**
     * Protected constructor for subclassing.
     * Be extremely careful to pass the correct index!
     *
     * @param o the item
     * @param aClass class 0
     * @param bClass class 1
     * @param index 0 means this represents an A, 1 represents a B
     */
    protected OneOf2(
            @NotNull Object o,
            @NotNull Class aClass,
            @NotNull Class bClass,
            int index
    ) {
        types = RuntimeTypes.registerClasses(aClass, bClass);
        sel = index;
        item = o;
        if (index < 0) {
            throw new IllegalArgumentException("Selected item index must be 0-1");
        } else if (index > 1) {
            throw new IllegalArgumentException("Selected item index must be 0-1");
        }
        if (!types.get(index).isInstance(o)) {
            throw new ClassCastException("You specified index " + index + ", indicating a(n) " +
                                         types.get(index).getCanonicalName() + "," +
                                         " but passed a " + o.getClass().getCanonicalName());
        }
    }

    /**
     * Languages that have union types built in have a match statement that works like this method.
     * Exactly one of these functions will be executed - determined by which type of item this object holds.
     * @param fa applied iff this stores the first type.
     * @param fb applied iff this stores the second type.
     * @return the return value of whichever function is executed.
     */
    // We only store one item and its type is erased, so we have to cast it at runtime.
    // If sel is managed correctly, this ensures that cast is accurate.
    @SuppressWarnings("unchecked")
    public  R match(
            @NotNull Fn1 fa,
            @NotNull Fn1 fb
    ) {
        if (sel == 0) {
            return fa.apply((A) item);
        }
        return fb.apply((B) item);
    }

    public int hashCode() {
        // Simplest way to make the two items different.
        return Objects.hashCode(item) + sel;
    }

    @Override public boolean equals(Object other) {
        if (this == other) { return true; }
        if (!(other instanceof OneOf2)) { return false; }

        @SuppressWarnings("rawtypes")
        OneOf2 that = (OneOf2) other;
        return (sel == that.sel) &&
               Objects.equals(item, that.item);
    }

    @Override
    public @NotNull String toString() { return union2Str(item, types); }
}