/*
 * Copyright 2018 The Android Open Source Project
 *
 * 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 androidx.core.animation;

import android.annotation.SuppressLint;

import androidx.annotation.NonNull;

/**
 * This evaluator can be used to perform type interpolation between integer
 * values that represent ARGB colors.
 */
public final class ArgbEvaluator implements TypeEvaluator<Integer> {
    private static final ArgbEvaluator sInstance = new ArgbEvaluator();

    /**
     * Returns an instance of <code>ArgbEvaluator</code> that may be used in
     * {@link ValueAnimator#setEvaluator(TypeEvaluator)}. The same instance may
     * be used in multiple <code>Animator</code>s because it holds no state.
     * @return An instance of <code>ArgbEvaluator</code>.
     */
    @NonNull
    public static ArgbEvaluator getInstance() {
        return sInstance;
    }

    private ArgbEvaluator() {
    }

    /**
     * This function returns the calculated in-between value for a color
     * given integers that represent the start and end values in the four
     * bytes of the 32-bit int. Each channel is separately linearly interpolated
     * and the resulting calculated values are recombined into the return value.
     *
     * @param fraction The fraction from the starting to the ending values
     * @param startValue A 32-bit int value representing colors in the
     * separate bytes of the parameter
     * @param endValue A 32-bit int value representing colors in the
     * separate bytes of the parameter
     * @return A value that is calculated to be the linearly interpolated
     * result, derived by separating the start and end values into separate
     * color channels and interpolating each one separately, recombining the
     * resulting values in the same way.
     */
    @SuppressLint("AutoBoxing") /* Generics */
    @Override
    @NonNull
    public Integer evaluate(
            float fraction,
            @SuppressLint("AutoBoxing") @NonNull Integer startValue,
            @SuppressLint("AutoBoxing") @NonNull Integer endValue
    ) {
        int startInt = startValue;
        float startA = ((startInt >> 24) & 0xff) / 255.0f;
        float startR = ((startInt >> 16) & 0xff) / 255.0f;
        float startG = ((startInt >>  8) & 0xff) / 255.0f;
        float startB =  (startInt        & 0xff) / 255.0f;

        int endInt = endValue;
        float endA = ((endInt >> 24) & 0xff) / 255.0f;
        float endR = ((endInt >> 16) & 0xff) / 255.0f;
        float endG = ((endInt >>  8) & 0xff) / 255.0f;
        float endB =  (endInt        & 0xff) / 255.0f;

        // convert from sRGB to linear
        startR = (float) Math.pow(startR, 2.2);
        startG = (float) Math.pow(startG, 2.2);
        startB = (float) Math.pow(startB, 2.2);

        endR = (float) Math.pow(endR, 2.2);
        endG = (float) Math.pow(endG, 2.2);
        endB = (float) Math.pow(endB, 2.2);

        // compute the interpolated color in linear space
        float a = startA + fraction * (endA - startA);
        float r = startR + fraction * (endR - startR);
        float g = startG + fraction * (endG - startG);
        float b = startB + fraction * (endB - startB);

        // convert back to sRGB in the [0..255] range
        a = a * 255.0f;
        r = (float) Math.pow(r, 1.0 / 2.2) * 255.0f;
        g = (float) Math.pow(g, 1.0 / 2.2) * 255.0f;
        b = (float) Math.pow(b, 1.0 / 2.2) * 255.0f;

        return Math.round(a) << 24 | Math.round(r) << 16 | Math.round(g) << 8 | Math.round(b);
    }
}