/*
 * Copyright (C) 2017 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 android.widget;

import android.annotation.NonNull;
import android.annotation.Nullable;
import android.annotation.UiThread;
import android.annotation.WorkerThread;
import android.app.RemoteAction;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.PointF;
import android.graphics.RectF;
import android.os.AsyncTask;
import android.os.Build;
import android.os.Bundle;
import android.os.LocaleList;
import android.text.Layout;
import android.text.Selection;
import android.text.Spannable;
import android.text.TextUtils;
import android.text.util.Linkify;
import android.util.Log;
import android.view.ActionMode;
import android.view.textclassifier.ExtrasUtils;
import android.view.textclassifier.SelectionEvent;
import android.view.textclassifier.SelectionEvent.InvocationMethod;
import android.view.textclassifier.TextClassification;
import android.view.textclassifier.TextClassificationConstants;
import android.view.textclassifier.TextClassificationContext;
import android.view.textclassifier.TextClassificationManager;
import android.view.textclassifier.TextClassifier;
import android.view.textclassifier.TextClassifierEvent;
import android.view.textclassifier.TextSelection;
import android.widget.Editor.SelectionModifierCursorController;

import com.android.internal.annotations.VisibleForTesting;
import com.android.internal.util.Preconditions;

import java.text.BreakIterator;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Objects;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.regex.Pattern;

/**
 * Helper class for starting selection action mode
 * (synchronously without the TextClassifier, asynchronously with the TextClassifier).
 * @hide
 */
@UiThread
@VisibleForTesting(visibility = VisibleForTesting.Visibility.PACKAGE)
public final class SelectionActionModeHelper {

    private static final String LOG_TAG = "SelectActionModeHelper";

    private final Editor mEditor;
    private final TextView mTextView;
    private final TextClassificationHelper mTextClassificationHelper;

    @Nullable private TextClassification mTextClassification;
    private AsyncTask mTextClassificationAsyncTask;

    private final SelectionTracker mSelectionTracker;

    // TODO remove nullable marker once the switch gating the feature gets removed
    @Nullable
    private final SmartSelectSprite mSmartSelectSprite;

    SelectionActionModeHelper(@NonNull Editor editor) {
        mEditor = Objects.requireNonNull(editor);
        mTextView = mEditor.getTextView();
        mTextClassificationHelper = new TextClassificationHelper(
                mTextView.getContext(),
                mTextView::getTextClassificationSession,
                getText(mTextView),
                0, 1, mTextView.getTextLocales());
        mSelectionTracker = new SelectionTracker(mTextView);

        if (getTextClassificationSettings().isSmartSelectionAnimationEnabled()) {
            mSmartSelectSprite = new SmartSelectSprite(mTextView.getContext(),
                    editor.getTextView().mHighlightColor, mTextView::invalidate);
        } else {
            mSmartSelectSprite = null;
        }
    }

    /**
     * Swap the selection index if the start index is greater than end index.
     *
     * @return the swap result, index 0 is the start index and index 1 is the end index.
     */
    private static int[] sortSelctionIndices(int selectionStart, int selectionEnd) {
        if (selectionStart < selectionEnd) {
            return new int[]{selectionStart, selectionEnd};
        }
        return new int[]{selectionEnd, selectionStart};
    }

    /**
     * The {@link TextView} selection start and end index may not be sorted, this method will swap
     * the {@link TextView} selection index if the start index is greater than end index.
     *
     * @param textView the selected TextView.
     * @return the swap result, index 0 is the start index and index 1 is the end index.
     */
    private static int[] sortSelctionIndicesFromTextView(TextView textView) {
        int selectionStart = textView.getSelectionStart();
        int selectionEnd = textView.getSelectionEnd();

        return sortSelctionIndices(selectionStart, selectionEnd);
    }

    /**
     * Starts Selection ActionMode.
     */
    public void startSelectionActionModeAsync(boolean adjustSelection) {
        // Check if the smart selection should run for editable text.
        adjustSelection &= getTextClassificationSettings().isSmartSelectionEnabled();
        int[] sortedSelectionIndices = sortSelctionIndicesFromTextView(mTextView);

        mSelectionTracker.onOriginalSelection(
                getText(mTextView),
                sortedSelectionIndices[0],
                sortedSelectionIndices[1],
                false /*isLink*/);
        cancelAsyncTask();
        if (skipTextClassification()) {
            startSelectionActionMode(null);
        } else {
            resetTextClassificationHelper();
            mTextClassificationAsyncTask = new TextClassificationAsyncTask(
                    mTextView,
                    mTextClassificationHelper.getTimeoutDuration(),
                    adjustSelection
                            ? mTextClassificationHelper::suggestSelection
                            : mTextClassificationHelper::classifyText,
                    mSmartSelectSprite != null
                            ? this::startSelectionActionModeWithSmartSelectAnimation
                            : this::startSelectionActionMode,
                    mTextClassificationHelper::getOriginalSelection)
                    .execute();
        }
    }

    /**
     * Starts Link ActionMode.
     */
    public void startLinkActionModeAsync(int start, int end) {
        int[] indexResult = sortSelctionIndices(start, end);
        mSelectionTracker.onOriginalSelection(getText(mTextView), indexResult[0], indexResult[1],
                true /*isLink*/);
        cancelAsyncTask();
        if (skipTextClassification()) {
            startLinkActionMode(null);
        } else {
            resetTextClassificationHelper(indexResult[0], indexResult[1]);
            mTextClassificationAsyncTask = new TextClassificationAsyncTask(
                    mTextView,
                    mTextClassificationHelper.getTimeoutDuration(),
                    mTextClassificationHelper::classifyText,
                    this::startLinkActionMode,
                    mTextClassificationHelper::getOriginalSelection)
                    .execute();
        }
    }

    public void invalidateActionModeAsync() {
        cancelAsyncTask();
        if (skipTextClassification()) {
            invalidateActionMode(null);
        } else {
            resetTextClassificationHelper();
            mTextClassificationAsyncTask = new TextClassificationAsyncTask(
                    mTextView,
                    mTextClassificationHelper.getTimeoutDuration(),
                    mTextClassificationHelper::classifyText,
                    this::invalidateActionMode,
                    mTextClassificationHelper::getOriginalSelection)
                    .execute();
        }
    }

    /** Reports a selection action event. */
    public void onSelectionAction(int menuItemId, @Nullable String actionLabel) {
        int[] sortedSelectionIndices = sortSelctionIndicesFromTextView(mTextView);
        mSelectionTracker.onSelectionAction(
                sortedSelectionIndices[0], sortedSelectionIndices[1],
                getActionType(menuItemId), actionLabel, mTextClassification);
    }

    public void onSelectionDrag() {
        int[] sortedSelectionIndices = sortSelctionIndicesFromTextView(mTextView);
        mSelectionTracker.onSelectionAction(
                sortedSelectionIndices[0], sortedSelectionIndices[1],
                SelectionEvent.ACTION_DRAG, /* actionLabel= */ null, mTextClassification);
    }

    public void onTextChanged(int start, int end) {
        int[] sortedSelectionIndices = sortSelctionIndices(start, end);
        mSelectionTracker.onTextChanged(sortedSelectionIndices[0], sortedSelectionIndices[1],
                mTextClassification);
    }

    public boolean resetSelection(int textIndex) {
        if (mSelectionTracker.resetSelection(textIndex, mEditor)) {
            invalidateActionModeAsync();
            return true;
        }
        return false;
    }

    @Nullable
    public TextClassification getTextClassification() {
        return mTextClassification;
    }

    public void onDestroyActionMode() {
        cancelSmartSelectAnimation();
        mSelectionTracker.onSelectionDestroyed();
        cancelAsyncTask();
    }

    public void onDraw(final Canvas canvas) {
        if (isDrawingHighlight() && mSmartSelectSprite != null) {
            mSmartSelectSprite.draw(canvas);
        }
    }

    public boolean isDrawingHighlight() {
        return mSmartSelectSprite != null && mSmartSelectSprite.isAnimationActive();
    }

    private TextClassificationConstants getTextClassificationSettings() {
        return TextClassificationManager.getSettings(mTextView.getContext());
    }

    private void cancelAsyncTask() {
        if (mTextClassificationAsyncTask != null) {
            mTextClassificationAsyncTask.cancel(true);
            mTextClassificationAsyncTask = null;
        }
        mTextClassification = null;
    }

    private boolean skipTextClassification() {
        // No need to make an async call for a no-op TextClassifier.
        final boolean noOpTextClassifier = mTextView.usesNoOpTextClassifier();
        // Do not call the TextClassifier if there is no selection.
        final boolean noSelection = mTextView.getSelectionEnd() == mTextView.getSelectionStart();
        // Do not call the TextClassifier if this is a password field.
        final boolean password = mTextView.hasPasswordTransformationMethod()
                || TextView.isPasswordInputType(mTextView.getInputType());
        return noOpTextClassifier || noSelection || password;
    }

    private void startLinkActionMode(@Nullable SelectionResult result) {
        startActionMode(Editor.TextActionMode.TEXT_LINK, result);
    }

    private void startSelectionActionMode(@Nullable SelectionResult result) {
        startActionMode(Editor.TextActionMode.SELECTION, result);
    }

    private void startActionMode(
            @Editor.TextActionMode int actionMode, @Nullable SelectionResult result) {
        final CharSequence text = getText(mTextView);
        if (result != null && text instanceof Spannable
                && (mTextView.isTextSelectable() || mTextView.isTextEditable())) {
            // Do not change the selection if TextClassifier should be dark launched.
            if (!getTextClassificationSettings().isModelDarkLaunchEnabled()) {
                Selection.setSelection((Spannable) text, result.mStart, result.mEnd);
                mTextView.invalidate();
            }
            mTextClassification = result.mClassification;
        } else if (result != null && actionMode == Editor.TextActionMode.TEXT_LINK) {
            mTextClassification = result.mClassification;
        } else {
            mTextClassification = null;
        }
        if (mEditor.startActionModeInternal(actionMode)) {
            final SelectionModifierCursorController controller = mEditor.getSelectionController();
            if (controller != null
                    && (mTextView.isTextSelectable() || mTextView.isTextEditable())) {
                controller.show();
            }
            if (result != null) {
                switch (actionMode) {
                    case Editor.TextActionMode.SELECTION:
                        mSelectionTracker.onSmartSelection(result);
                        break;
                    case Editor.TextActionMode.TEXT_LINK:
                        mSelectionTracker.onLinkSelected(result);
                        break;
                    default:
                        break;
                }
            }
        }
        mEditor.setRestartActionModeOnNextRefresh(false);
        mTextClassificationAsyncTask = null;
    }

    private void startSelectionActionModeWithSmartSelectAnimation(
            @Nullable SelectionResult result) {
        final Layout layout = mTextView.getLayout();

        final Runnable onAnimationEndCallback = () -> {
            final SelectionResult startSelectionResult;
            if (result != null && result.mStart >= 0 && result.mEnd <= getText(mTextView).length()
                    && result.mStart <= result.mEnd) {
                startSelectionResult = result;
            } else {
                startSelectionResult = null;
            }
            startSelectionActionMode(startSelectionResult);
        };
        // TODO do not trigger the animation if the change included only non-printable characters
        int[] sortedSelectionIndices = sortSelctionIndicesFromTextView(mTextView);
        final boolean didSelectionChange =
                result != null && (sortedSelectionIndices[0] != result.mStart
                        || sortedSelectionIndices[1] != result.mEnd);
        if (!didSelectionChange) {
            onAnimationEndCallback.run();
            return;
        }

        final List<SmartSelectSprite.RectangleWithTextSelectionLayout> selectionRectangles =
                convertSelectionToRectangles(layout, result.mStart, result.mEnd);

        final PointF touchPoint = new PointF(
                mEditor.getLastUpPositionX(),
                mEditor.getLastUpPositionY());

        final PointF animationStartPoint =
                movePointInsideNearestRectangle(touchPoint, selectionRectangles,
                        SmartSelectSprite.RectangleWithTextSelectionLayout::getRectangle);

        mSmartSelectSprite.startAnimation(
                animationStartPoint,
                selectionRectangles,
                onAnimationEndCallback);
    }

    private List<SmartSelectSprite.RectangleWithTextSelectionLayout> convertSelectionToRectangles(
            final Layout layout, final int start, final int end) {
        final List<SmartSelectSprite.RectangleWithTextSelectionLayout> result = new ArrayList<>();

        final Layout.SelectionRectangleConsumer consumer =
                (left, top, right, bottom, textSelectionLayout) -> mergeRectangleIntoList(
                        result,
                        new RectF(left, top, right, bottom),
                        SmartSelectSprite.RectangleWithTextSelectionLayout::getRectangle,
                        r -> new SmartSelectSprite.RectangleWithTextSelectionLayout(r,
                                textSelectionLayout)
                );

        layout.getSelection(start, end, consumer);

        result.sort(Comparator.comparing(
                SmartSelectSprite.RectangleWithTextSelectionLayout::getRectangle,
                SmartSelectSprite.RECTANGLE_COMPARATOR));

        return result;
    }

    // TODO: Move public pure functions out of this class and make it package-private.
    /**
     * Merges a {@link RectF} into an existing list of any objects which contain a rectangle.
     * While merging, this method makes sure that:
     *
     * <ol>
     * <li>No rectangle is redundant (contained within a bigger rectangle)</li>
     * <li>Rectangles of the same height and vertical position that intersect get merged</li>
     * </ol>
     *
     * @param list      the list of rectangles (or other rectangle containers) to merge the new
     *                  rectangle into
     * @param candidate the {@link RectF} to merge into the list
     * @param extractor a function that can extract a {@link RectF} from an element of the given
     *                  list
     * @param packer    a function that can wrap the resulting {@link RectF} into an element that
     *                  the list contains
     * @hide
     */
    @VisibleForTesting
    public static <T> void mergeRectangleIntoList(final List<T> list,
            final RectF candidate, final Function<T, RectF> extractor,
            final Function<RectF, T> packer) {
        if (candidate.isEmpty()) {
            return;
        }

        final int elementCount = list.size();
        for (int index = 0; index < elementCount; ++index) {
            final RectF existingRectangle = extractor.apply(list.get(index));
            if (existingRectangle.contains(candidate)) {
                return;
            }
            if (candidate.contains(existingRectangle)) {
                existingRectangle.setEmpty();
                continue;
            }

            final boolean rectanglesContinueEachOther = candidate.left == existingRectangle.right
                    || candidate.right == existingRectangle.left;
            final boolean canMerge = candidate.top == existingRectangle.top
                    && candidate.bottom == existingRectangle.bottom
                    && (RectF.intersects(candidate, existingRectangle)
                    || rectanglesContinueEachOther);

            if (canMerge) {
                candidate.union(existingRectangle);
                existingRectangle.setEmpty();
            }
        }

        for (int index = elementCount - 1; index >= 0; --index) {
            final RectF rectangle = extractor.apply(list.get(index));
            if (rectangle.isEmpty()) {
                list.remove(index);
            }
        }

        list.add(packer.apply(candidate));
    }


    /** @hide */
    @VisibleForTesting
    public static <T> PointF movePointInsideNearestRectangle(final PointF point,
            final List<T> list, final Function<T, RectF> extractor) {
        float bestX = -1;
        float bestY = -1;
        double bestDistance = Double.MAX_VALUE;

        final int elementCount = list.size();
        for (int index = 0; index < elementCount; ++index) {
            final RectF rectangle = extractor.apply(list.get(index));
            final float candidateY = rectangle.centerY();
            final float candidateX;

            if (point.x > rectangle.right) {
                candidateX = rectangle.right;
            } else if (point.x < rectangle.left) {
                candidateX = rectangle.left;
            } else {
                candidateX = point.x;
            }

            final double candidateDistance = Math.pow(point.x - candidateX, 2)
                    + Math.pow(point.y - candidateY, 2);

            if (candidateDistance < bestDistance) {
                bestX = candidateX;
                bestY = candidateY;
                bestDistance = candidateDistance;
            }
        }

        return new PointF(bestX, bestY);
    }

    private void invalidateActionMode(@Nullable SelectionResult result) {
        cancelSmartSelectAnimation();
        mTextClassification = result != null ? result.mClassification : null;
        final ActionMode actionMode = mEditor.getTextActionMode();
        if (actionMode != null) {
            actionMode.invalidate();
        }
        final int[] sortedSelectionIndices = sortSelctionIndicesFromTextView(mTextView);
        mSelectionTracker.onSelectionUpdated(
                sortedSelectionIndices[0], sortedSelectionIndices[1], mTextClassification);
        mTextClassificationAsyncTask = null;
    }

    private void resetTextClassificationHelper(int selectionStart, int selectionEnd) {
        if (selectionStart < 0 || selectionEnd < 0) {
            // Use selection indices
            int[] sortedSelectionIndices = sortSelctionIndicesFromTextView(mTextView);
            selectionStart = sortedSelectionIndices[0];
            selectionEnd = sortedSelectionIndices[1];
        }
        mTextClassificationHelper.init(
                mTextView::getTextClassificationSession,
                getText(mTextView),
                selectionStart, selectionEnd,
                mTextView.getTextLocales());
    }

    private void resetTextClassificationHelper() {
        resetTextClassificationHelper(-1, -1);
    }

    private void cancelSmartSelectAnimation() {
        if (mSmartSelectSprite != null) {
            mSmartSelectSprite.cancelAnimation();
        }
    }

    /**
     * Tracks and logs smart selection changes.
     * It is important to trigger this object's methods at the appropriate event so that it tracks
     * smart selection events appropriately.
     */
    private static final class SelectionTracker {

        private final TextView mTextView;
        private SelectionMetricsLogger mLogger;

        private int mOriginalStart;
        private int mOriginalEnd;
        private int mSelectionStart;
        private int mSelectionEnd;
        private boolean mAllowReset;
        private final LogAbandonRunnable mDelayedLogAbandon = new LogAbandonRunnable();

        SelectionTracker(TextView textView) {
            mTextView = Objects.requireNonNull(textView);
            mLogger = new SelectionMetricsLogger(textView);
        }

        /**
         * Called when the original selection happens, before smart selection is triggered.
         */
        public void onOriginalSelection(
                CharSequence text, int selectionStart, int selectionEnd, boolean isLink) {
            // If we abandoned a selection and created a new one very shortly after, we may still
            // have a pending request to log ABANDON, which we flush here.
            mDelayedLogAbandon.flush();

            mOriginalStart = mSelectionStart = selectionStart;
            mOriginalEnd = mSelectionEnd = selectionEnd;
            mAllowReset = false;
            maybeInvalidateLogger();
            mLogger.logSelectionStarted(
                    mTextView.getTextClassificationSession(),
                    mTextView.getTextClassificationContext(),
                    text,
                    selectionStart,
                    isLink ? SelectionEvent.INVOCATION_LINK : SelectionEvent.INVOCATION_MANUAL);
        }

        /**
         * Called when selection action mode is started and the results come from a classifier.
         */
        public void onSmartSelection(SelectionResult result) {
            onClassifiedSelection(result);
            mLogger.logSelectionModified(
                    result.mStart, result.mEnd, result.mClassification, result.mSelection);
        }

        /**
         * Called when link action mode is started and the classification comes from a classifier.
         */
        public void onLinkSelected(SelectionResult result) {
            onClassifiedSelection(result);
            // TODO: log (b/70246800)
        }

        private void onClassifiedSelection(SelectionResult result) {
            if (isSelectionStarted()) {
                mSelectionStart = result.mStart;
                mSelectionEnd = result.mEnd;
                mAllowReset = mSelectionStart != mOriginalStart || mSelectionEnd != mOriginalEnd;
            }
        }

        /**
         * Called when selection bounds change.
         */
        public void onSelectionUpdated(
                int selectionStart, int selectionEnd,
                @Nullable TextClassification classification) {
            if (isSelectionStarted()) {
                mSelectionStart = selectionStart;
                mSelectionEnd = selectionEnd;
                mAllowReset = false;
                mLogger.logSelectionModified(selectionStart, selectionEnd, classification, null);
            }
        }

        /**
         * Called when the selection action mode is destroyed.
         */
        public void onSelectionDestroyed() {
            mAllowReset = false;
            // Wait a few ms to see if the selection was destroyed because of a text change event.
            mDelayedLogAbandon.schedule(100 /* ms */);
        }

        /**
         * Called when an action is taken on a smart selection.
         */
        public void onSelectionAction(
                int selectionStart, int selectionEnd,
                @SelectionEvent.ActionType int action,
                @Nullable String actionLabel,
                @Nullable TextClassification classification) {
            if (isSelectionStarted()) {
                mAllowReset = false;
                mLogger.logSelectionAction(
                        selectionStart, selectionEnd, action, actionLabel, classification);
            }
        }

        /**
         * Returns true if the current smart selection should be reset to normal selection based on
         * information that has been recorded about the original selection and the smart selection.
         * The expected UX here is to allow the user to select a word inside of the smart selection
         * on a single tap.
         */
        public boolean resetSelection(int textIndex, Editor editor) {
            final TextView textView = editor.getTextView();
            if (isSelectionStarted()
                    && mAllowReset
                    && textIndex >= mSelectionStart && textIndex <= mSelectionEnd
                    && getText(textView) instanceof Spannable) {
                mAllowReset = false;
                boolean selected = editor.selectCurrentWord();
                if (selected) {
                    final int[] sortedSelectionIndices = sortSelctionIndicesFromTextView(textView);
                    mSelectionStart = sortedSelectionIndices[0];
                    mSelectionEnd = sortedSelectionIndices[1];
                    mLogger.logSelectionAction(
                            sortedSelectionIndices[0], sortedSelectionIndices[1],
                            SelectionEvent.ACTION_RESET,
                            /* actionLabel= */ null, /* classification= */ null);
                }
                return selected;
            }
            return false;
        }

        public void onTextChanged(int start, int end, TextClassification classification) {
            if (isSelectionStarted() && start == mSelectionStart && end == mSelectionEnd) {
                onSelectionAction(
                        start, end, SelectionEvent.ACTION_OVERTYPE,
                        /* actionLabel= */ null, classification);
            }
        }

        private void maybeInvalidateLogger() {
            if (mLogger.isEditTextLogger() != mTextView.isTextEditable()) {
                mLogger = new SelectionMetricsLogger(mTextView);
            }
        }

        private boolean isSelectionStarted() {
            return mSelectionStart >= 0 && mSelectionEnd >= 0 && mSelectionStart != mSelectionEnd;
        }

        /** A helper for keeping track of pending abandon logging requests. */
        private final class LogAbandonRunnable implements Runnable {
            private boolean mIsPending;

            /** Schedules an abandon to be logged with the given delay. Flush if necessary. */
            void schedule(int delayMillis) {
                if (mIsPending) {
                    Log.e(LOG_TAG, "Force flushing abandon due to new scheduling request");
                    flush();
                }
                mIsPending = true;
                mTextView.postDelayed(this, delayMillis);
            }

            /** If there is a pending log request, execute it now. */
            void flush() {
                mTextView.removeCallbacks(this);
                run();
            }

            @Override
            public void run() {
                if (mIsPending) {
                    mLogger.logSelectionAction(
                            mSelectionStart, mSelectionEnd,
                            SelectionEvent.ACTION_ABANDON,
                            /* actionLabel= */ null, /* classification= */ null);
                    mSelectionStart = mSelectionEnd = -1;
                    mLogger.endTextClassificationSession();
                    mIsPending = false;
                }
            }
        }
    }

    // TODO: Write tests
    /**
     * Metrics logging helper.
     *
     * This logger logs selection by word indices. The initial (start) single word selection is
     * logged at [0, 1) -- end index is exclusive. Other word indices are logged relative to the
     * initial single word selection.
     * e.g. New York city, NY. Suppose the initial selection is "York" in
     * "New York city, NY", then "York" is at [0, 1), "New" is at [-1, 0], and "city" is at [1, 2).
     * "New York" is at [-1, 1).
     * Part selection of a word e.g. "or" is counted as selecting the
     * entire word i.e. equivalent to "York", and each special character is counted as a word, e.g.
     * "," is at [2, 3). Whitespaces are ignored.
     *
     * NOTE that the definition of a word is defined by the TextClassifier's Logger's token
     * iterator.
     */
    private static final class SelectionMetricsLogger {

        private static final String LOG_TAG = "SelectionMetricsLogger";
        private static final Pattern PATTERN_WHITESPACE = Pattern.compile("\\s+");

        private final boolean mEditTextLogger;
        private final BreakIterator mTokenIterator;

        @Nullable private TextClassifier mClassificationSession;
        @Nullable private TextClassificationContext mClassificationContext;

        @Nullable private TextClassifierEvent mTranslateViewEvent;
        @Nullable private TextClassifierEvent mTranslateClickEvent;

        private int mStartIndex;
        private String mText;

        SelectionMetricsLogger(TextView textView) {
            Objects.requireNonNull(textView);
            mEditTextLogger = textView.isTextEditable();
            mTokenIterator = BreakIterator.getWordInstance(textView.getTextLocale());
        }

        public void logSelectionStarted(
                TextClassifier classificationSession,
                TextClassificationContext classificationContext,
                CharSequence text, int index,
                @InvocationMethod int invocationMethod) {
            try {
                Objects.requireNonNull(text);
                Preconditions.checkArgumentInRange(index, 0, text.length(), "index");
                if (mText == null || !mText.contentEquals(text)) {
                    mText = text.toString();
                }
                mTokenIterator.setText(mText);
                mStartIndex = index;
                mClassificationSession = classificationSession;
                mClassificationContext = classificationContext;
                if (hasActiveClassificationSession()) {
                    mClassificationSession.onSelectionEvent(
                            SelectionEvent.createSelectionStartedEvent(invocationMethod, 0));
                }
            } catch (Exception e) {
                // Avoid crashes due to logging.
                Log.e(LOG_TAG, "" + e.getMessage(), e);
            }
        }

        public void logSelectionModified(int start, int end,
                @Nullable TextClassification classification, @Nullable TextSelection selection) {
            try {
                if (hasActiveClassificationSession()) {
                    Preconditions.checkArgumentInRange(start, 0, mText.length(), "start");
                    Preconditions.checkArgumentInRange(end, start, mText.length(), "end");
                    int[] wordIndices = getWordDelta(start, end);
                    if (selection != null) {
                        mClassificationSession.onSelectionEvent(
                                SelectionEvent.createSelectionModifiedEvent(
                                        wordIndices[0], wordIndices[1], selection));
                    } else if (classification != null) {
                        mClassificationSession.onSelectionEvent(
                                SelectionEvent.createSelectionModifiedEvent(
                                        wordIndices[0], wordIndices[1], classification));
                    } else {
                        mClassificationSession.onSelectionEvent(
                                SelectionEvent.createSelectionModifiedEvent(
                                        wordIndices[0], wordIndices[1]));
                    }
                    maybeGenerateTranslateViewEvent(classification);
                }
            } catch (Exception e) {
                // Avoid crashes due to logging.
                Log.e(LOG_TAG, "" + e.getMessage(), e);
            }
        }

        public void logSelectionAction(
                int start, int end,
                @SelectionEvent.ActionType int action,
                @Nullable String actionLabel,
                @Nullable TextClassification classification) {
            try {
                if (hasActiveClassificationSession()) {
                    Preconditions.checkArgumentInRange(start, 0, mText.length(), "start");
                    Preconditions.checkArgumentInRange(end, start, mText.length(), "end");
                    int[] wordIndices = getWordDelta(start, end);
                    if (classification != null) {
                        mClassificationSession.onSelectionEvent(
                                SelectionEvent.createSelectionActionEvent(
                                        wordIndices[0], wordIndices[1], action,
                                        classification));
                    } else {
                        mClassificationSession.onSelectionEvent(
                                SelectionEvent.createSelectionActionEvent(
                                        wordIndices[0], wordIndices[1], action));
                    }

                    maybeGenerateTranslateClickEvent(classification, actionLabel);

                    if (SelectionEvent.isTerminal(action)) {
                        endTextClassificationSession();
                    }
                }
            } catch (Exception e) {
                // Avoid crashes due to logging.
                Log.e(LOG_TAG, "" + e.getMessage(), e);
            }
        }

        public boolean isEditTextLogger() {
            return mEditTextLogger;
        }

        public void endTextClassificationSession() {
            if (hasActiveClassificationSession()) {
                maybeReportTranslateEvents();
                mClassificationSession.destroy();
            }
        }

        private boolean hasActiveClassificationSession() {
            return mClassificationSession != null && !mClassificationSession.isDestroyed();
        }

        private int[] getWordDelta(int start, int end) {
            int[] wordIndices = new int[2];

            if (start == mStartIndex) {
                wordIndices[0] = 0;
            } else if (start < mStartIndex) {
                wordIndices[0] = -countWordsForward(start);
            } else {  // start > mStartIndex
                wordIndices[0] = countWordsBackward(start);

                // For the selection start index, avoid counting a partial word backwards.
                if (!mTokenIterator.isBoundary(start)
                        && !isWhitespace(
                        mTokenIterator.preceding(start),
                        mTokenIterator.following(start))) {
                    // We counted a partial word. Remove it.
                    wordIndices[0]--;
                }
            }

            if (end == mStartIndex) {
                wordIndices[1] = 0;
            } else if (end < mStartIndex) {
                wordIndices[1] = -countWordsForward(end);
            } else {  // end > mStartIndex
                wordIndices[1] = countWordsBackward(end);
            }

            return wordIndices;
        }

        private int countWordsBackward(int from) {
            Preconditions.checkArgument(from >= mStartIndex);
            int wordCount = 0;
            int offset = from;
            while (offset > mStartIndex) {
                int start = mTokenIterator.preceding(offset);
                if (!isWhitespace(start, offset)) {
                    wordCount++;
                }
                offset = start;
            }
            return wordCount;
        }

        private int countWordsForward(int from) {
            Preconditions.checkArgument(from <= mStartIndex);
            int wordCount = 0;
            int offset = from;
            while (offset < mStartIndex) {
                int end = mTokenIterator.following(offset);
                if (!isWhitespace(offset, end)) {
                    wordCount++;
                }
                offset = end;
            }
            return wordCount;
        }

        private boolean isWhitespace(int start, int end) {
            return PATTERN_WHITESPACE.matcher(mText.substring(start, end)).matches();
        }

        private void maybeGenerateTranslateViewEvent(@Nullable TextClassification classification) {
            if (classification != null) {
                final TextClassifierEvent event = generateTranslateEvent(
                        TextClassifierEvent.TYPE_ACTIONS_SHOWN,
                        classification, mClassificationContext, /* actionLabel= */null);
                mTranslateViewEvent = (event != null) ? event : mTranslateViewEvent;
            }
        }

        private void maybeGenerateTranslateClickEvent(
                @Nullable TextClassification classification, String actionLabel) {
            if (classification != null) {
                mTranslateClickEvent = generateTranslateEvent(
                        TextClassifierEvent.TYPE_SMART_ACTION,
                        classification, mClassificationContext, actionLabel);
            }
        }

        private void maybeReportTranslateEvents() {
            // Translate view and click events should only be logged once per selection session.
            if (mTranslateViewEvent != null) {
                mClassificationSession.onTextClassifierEvent(mTranslateViewEvent);
                mTranslateViewEvent = null;
            }
            if (mTranslateClickEvent != null) {
                mClassificationSession.onTextClassifierEvent(mTranslateClickEvent);
                mTranslateClickEvent = null;
            }
        }

        @Nullable
        private static TextClassifierEvent generateTranslateEvent(
                int eventType, TextClassification classification,
                TextClassificationContext classificationContext, @Nullable String actionLabel) {

            // The platform attempts to log "views" and "clicks" of the "Translate" action.
            // Views are logged if a user is presented with the translate action during a selection
            // session.
            // Clicks are logged if the user clicks on the translate action.
            // The index of the translate action is also logged to indicate whether it might have
            // been in the main panel or overflow panel of the selection toolbar.
            // NOTE that the "views" metric may be flawed if a TextView removes the translate menu
            // item via a custom action mode callback or does not show a selection menu item.

            final RemoteAction translateAction = ExtrasUtils.findTranslateAction(classification);
            if (translateAction == null) {
                // No translate action present. Nothing to log. Exit.
                return null;
            }

            if (eventType == TextClassifierEvent.TYPE_SMART_ACTION
                    && !translateAction.getTitle().toString().equals(actionLabel)) {
                // Clicked action is not a translate action. Nothing to log. Exit.
                // Note that we don't expect an actionLabel for "view" events.
                return null;
            }

            final Bundle foreignLanguageExtra = ExtrasUtils.getForeignLanguageExtra(classification);
            final String language = ExtrasUtils.getEntityType(foreignLanguageExtra);
            final float score = ExtrasUtils.getScore(foreignLanguageExtra);
            final String model = ExtrasUtils.getModelName(foreignLanguageExtra);
            return new TextClassifierEvent.LanguageDetectionEvent.Builder(eventType)
                    .setEventContext(classificationContext)
                    .setResultId(classification.getId())
                    .setEntityTypes(language)
                    .setScores(score)
                    .setActionIndices(classification.getActions().indexOf(translateAction))
                    .setModelName(model)
                    .build();
        }
    }

    /**
     * AsyncTask for running a query on a background thread and returning the result on the
     * UiThread. The AsyncTask times out after a specified time, returning a null result if the
     * query has not yet returned.
     */
    private static final class TextClassificationAsyncTask
            extends AsyncTask<Void, Void, SelectionResult> {

        private final int mTimeOutDuration;
        private final Supplier<SelectionResult> mSelectionResultSupplier;
        private final Consumer<SelectionResult> mSelectionResultCallback;
        private final Supplier<SelectionResult> mTimeOutResultSupplier;
        private final TextView mTextView;
        private final String mOriginalText;

        /**
         * @param textView the TextView
         * @param timeOut time in milliseconds to timeout the query if it has not completed
         * @param selectionResultSupplier fetches the selection results. Runs on a background thread
         * @param selectionResultCallback receives the selection results. Runs on the UiThread
         * @param timeOutResultSupplier default result if the task times out
         */
        TextClassificationAsyncTask(
                @NonNull TextView textView, int timeOut,
                @NonNull Supplier<SelectionResult> selectionResultSupplier,
                @NonNull Consumer<SelectionResult> selectionResultCallback,
                @NonNull Supplier<SelectionResult> timeOutResultSupplier) {
            super(textView != null ? textView.getHandler() : null);
            mTextView = Objects.requireNonNull(textView);
            mTimeOutDuration = timeOut;
            mSelectionResultSupplier = Objects.requireNonNull(selectionResultSupplier);
            mSelectionResultCallback = Objects.requireNonNull(selectionResultCallback);
            mTimeOutResultSupplier = Objects.requireNonNull(timeOutResultSupplier);
            // Make a copy of the original text.
            mOriginalText = getText(mTextView).toString();
        }

        @Override
        @WorkerThread
        protected SelectionResult doInBackground(Void... params) {
            final Runnable onTimeOut = this::onTimeOut;
            mTextView.postDelayed(onTimeOut, mTimeOutDuration);
            final SelectionResult result = mSelectionResultSupplier.get();
            mTextView.removeCallbacks(onTimeOut);
            return result;
        }

        @Override
        @UiThread
        protected void onPostExecute(SelectionResult result) {
            result = TextUtils.equals(mOriginalText, getText(mTextView)) ? result : null;
            mSelectionResultCallback.accept(result);
        }

        private void onTimeOut() {
            Log.d(LOG_TAG, "Timeout in TextClassificationAsyncTask");
            if (getStatus() == Status.RUNNING) {
                onPostExecute(mTimeOutResultSupplier.get());
            }
            cancel(true);
        }
    }

    /**
     * Helper class for querying the TextClassifier.
     * It trims text so that only text necessary to provide context of the selected text is
     * sent to the TextClassifier.
     */
    private static final class TextClassificationHelper {

        private static final int TRIM_DELTA = 120;  // characters

        private final Context mContext;
        private Supplier<TextClassifier> mTextClassifier;

        /** The original TextView text. **/
        private String mText;
        /** Start index relative to mText. */
        private int mSelectionStart;
        /** End index relative to mText. */
        private int mSelectionEnd;

        @Nullable
        private LocaleList mDefaultLocales;

        /** Trimmed text starting from mTrimStart in mText. */
        private CharSequence mTrimmedText;
        /** Index indicating the start of mTrimmedText in mText. */
        private int mTrimStart;
        /** Start index relative to mTrimmedText */
        private int mRelativeStart;
        /** End index relative to mTrimmedText */
        private int mRelativeEnd;

        /** Information about the last classified text to avoid re-running a query. */
        private CharSequence mLastClassificationText;
        private int mLastClassificationSelectionStart;
        private int mLastClassificationSelectionEnd;
        private LocaleList mLastClassificationLocales;
        private SelectionResult mLastClassificationResult;

        /** Whether the TextClassifier has been initialized. */
        private boolean mHot;

        TextClassificationHelper(Context context, Supplier<TextClassifier> textClassifier,
                CharSequence text, int selectionStart, int selectionEnd, LocaleList locales) {
            init(textClassifier, text, selectionStart, selectionEnd, locales);
            mContext = Objects.requireNonNull(context);
        }

        @UiThread
        public void init(Supplier<TextClassifier> textClassifier, CharSequence text,
                int selectionStart, int selectionEnd, LocaleList locales) {
            mTextClassifier = Objects.requireNonNull(textClassifier);
            mText = Objects.requireNonNull(text).toString();
            mLastClassificationText = null; // invalidate.
            Preconditions.checkArgument(selectionEnd > selectionStart);
            mSelectionStart = selectionStart;
            mSelectionEnd = selectionEnd;
            mDefaultLocales = locales;
        }

        @WorkerThread
        public SelectionResult classifyText() {
            mHot = true;
            return performClassification(null /* selection */);
        }

        @WorkerThread
        public SelectionResult suggestSelection() {
            mHot = true;
            trimText();
            final TextSelection selection;
            if (mContext.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.P) {
                final TextSelection.Request request = new TextSelection.Request.Builder(
                        mTrimmedText, mRelativeStart, mRelativeEnd)
                        .setDefaultLocales(mDefaultLocales)
                        .setDarkLaunchAllowed(true)
                        .build();
                selection = mTextClassifier.get().suggestSelection(request);
            } else {
                // Use old APIs.
                selection = mTextClassifier.get().suggestSelection(
                        mTrimmedText, mRelativeStart, mRelativeEnd, mDefaultLocales);
            }
            // Do not classify new selection boundaries if TextClassifier should be dark launched.
            if (!isDarkLaunchEnabled()) {
                mSelectionStart = Math.max(0, selection.getSelectionStartIndex() + mTrimStart);
                mSelectionEnd = Math.min(
                        mText.length(), selection.getSelectionEndIndex() + mTrimStart);
            }
            return performClassification(selection);
        }

        public SelectionResult getOriginalSelection() {
            return new SelectionResult(mSelectionStart, mSelectionEnd, null, null);
        }

        /**
         * Maximum time (in milliseconds) to wait for a textclassifier result before timing out.
         */
        // TODO: Consider making this a ViewConfiguration.
        public int getTimeoutDuration() {
            if (mHot) {
                return 200;
            } else {
                // Return a slightly larger number than usual when the TextClassifier is first
                // initialized. Initialization would usually take longer than subsequent calls to
                // the TextClassifier. The impact of this on the UI is that we do not show the
                // selection handles or toolbar until after this timeout.
                return 500;
            }
        }

        private boolean isDarkLaunchEnabled() {
            return TextClassificationManager.getSettings(mContext).isModelDarkLaunchEnabled();
        }

        private SelectionResult performClassification(@Nullable TextSelection selection) {
            if (!Objects.equals(mText, mLastClassificationText)
                    || mSelectionStart != mLastClassificationSelectionStart
                    || mSelectionEnd != mLastClassificationSelectionEnd
                    || !Objects.equals(mDefaultLocales, mLastClassificationLocales)) {

                mLastClassificationText = mText;
                mLastClassificationSelectionStart = mSelectionStart;
                mLastClassificationSelectionEnd = mSelectionEnd;
                mLastClassificationLocales = mDefaultLocales;

                trimText();
                final TextClassification classification;
                if (Linkify.containsUnsupportedCharacters(mText)) {
                    // Do not show smart actions for text containing unsupported characters.
                    android.util.EventLog.writeEvent(0x534e4554, "116321860", -1, "");
                    classification = TextClassification.EMPTY;
                } else if (mContext.getApplicationInfo().targetSdkVersion
                        >= Build.VERSION_CODES.P) {
                    final TextClassification.Request request =
                            new TextClassification.Request.Builder(
                                    mTrimmedText, mRelativeStart, mRelativeEnd)
                                    .setDefaultLocales(mDefaultLocales)
                                    .build();
                    classification = mTextClassifier.get().classifyText(request);
                } else {
                    // Use old APIs.
                    classification = mTextClassifier.get().classifyText(
                            mTrimmedText, mRelativeStart, mRelativeEnd, mDefaultLocales);
                }
                mLastClassificationResult = new SelectionResult(
                        mSelectionStart, mSelectionEnd, classification, selection);

            }
            return mLastClassificationResult;
        }

        private void trimText() {
            mTrimStart = Math.max(0, mSelectionStart - TRIM_DELTA);
            final int referenceEnd = Math.min(mText.length(), mSelectionEnd + TRIM_DELTA);
            mTrimmedText = mText.subSequence(mTrimStart, referenceEnd);
            mRelativeStart = mSelectionStart - mTrimStart;
            mRelativeEnd = mSelectionEnd - mTrimStart;
        }
    }

    /**
     * Selection result.
     */
    private static final class SelectionResult {
        private final int mStart;
        private final int mEnd;
        @Nullable private final TextClassification mClassification;
        @Nullable private final TextSelection mSelection;

        SelectionResult(int start, int end,
                @Nullable TextClassification classification, @Nullable TextSelection selection) {
            int[] sortedIndices = sortSelctionIndices(start, end);
            mStart = sortedIndices[0];
            mEnd = sortedIndices[1];
            mClassification = classification;
            mSelection = selection;
        }
    }

    @SelectionEvent.ActionType
    private static int getActionType(int menuItemId) {
        switch (menuItemId) {
            case TextView.ID_SELECT_ALL:
                return SelectionEvent.ACTION_SELECT_ALL;
            case TextView.ID_CUT:
                return SelectionEvent.ACTION_CUT;
            case TextView.ID_COPY:
                return SelectionEvent.ACTION_COPY;
            case TextView.ID_PASTE:  // fall through
            case TextView.ID_PASTE_AS_PLAIN_TEXT:
                return SelectionEvent.ACTION_PASTE;
            case TextView.ID_SHARE:
                return SelectionEvent.ACTION_SHARE;
            case TextView.ID_ASSIST:
                return SelectionEvent.ACTION_SMART_SHARE;
            default:
                return SelectionEvent.ACTION_OTHER;
        }
    }

    private static CharSequence getText(TextView textView) {
        // Extracts the textView's text.
        // TODO: Investigate why/when TextView.getText() is null.
        final CharSequence text = textView.getText();
        if (text != null) {
            return text;
        }
        return "";
    }
}