/*
 * Copyright 2021 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
 *
 *      https://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.
 */

@file:Suppress("MemberVisibilityCanBePrivate")

package com.google.accompanist.pager

import androidx.annotation.FloatRange
import androidx.annotation.IntRange
import androidx.compose.animation.core.AnimationSpec
import androidx.compose.animation.core.spring
import androidx.compose.foundation.MutatePriority
import androidx.compose.foundation.gestures.ScrollScope
import androidx.compose.foundation.gestures.ScrollableState
import androidx.compose.foundation.interaction.InteractionSource
import androidx.compose.foundation.lazy.LazyListItemInfo
import androidx.compose.foundation.lazy.LazyListState
import androidx.compose.runtime.Composable
import androidx.compose.runtime.Stable
import androidx.compose.runtime.derivedStateOf
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.saveable.Saver
import androidx.compose.runtime.saveable.listSaver
import androidx.compose.runtime.saveable.rememberSaveable
import androidx.compose.runtime.setValue
import io.github.aakira.napier.DebugAntilog
import io.github.aakira.napier.Napier
import kotlin.math.abs
import kotlin.math.absoluteValue
import kotlin.math.roundToInt

/**
 * Creates a [PagerState] that is remembered across compositions.
 *
 * Changes to the provided values for [initialPage] will **not** result in the state being
 * recreated or changed in any way if it has already
 * been created.
 *
 * @param initialPage the initial value for [PagerState.currentPage]
 */
@ExperimentalPagerApi
@Composable
fun rememberPagerState(
    @IntRange(from = 0) initialPage: Int = 0,
): PagerState = rememberSaveable(saver = PagerState.Saver) {
    PagerState(
        currentPage = initialPage,
    )
}

/**
 * A state object that can be hoisted to control and observe scrolling for [HorizontalPager].
 *
 * In most cases, this will be created via [rememberPagerState].
 *
 * @param currentPage the initial value for [PagerState.currentPage]
 */
@ExperimentalPagerApi
@Stable
class PagerState(
    @IntRange(from = 0) currentPage: Int = 0,
) : ScrollableState {
    // Should this be public?
    internal val lazyListState = LazyListState(firstVisibleItemIndex = currentPage)

    private var _currentPage by mutableStateOf(currentPage)

    // finds the page which has larger visible area within the viewport not including paddings
    internal val mostVisiblePageLayoutInfo: LazyListItemInfo?
        get() {
            val layoutInfo = lazyListState.layoutInfo
            return layoutInfo.visibleItemsInfo.maxByOrNull {
                val start = maxOf(it.offset, 0)
                val end = minOf(it.offset + it.size, layoutInfo.viewportEndOffset - afterContentPadding)
                end - start
            }
        }

    internal var afterContentPadding = 0

    private val currentPageLayoutInfo: LazyListItemInfo?
        get() = lazyListState.layoutInfo.visibleItemsInfo.lastOrNull {
            it.index == currentPage
        }

    /**
     * [InteractionSource] that will be used to dispatch drag events when this
     * list is being dragged. If you want to know whether the fling (or animated scroll) is in
     * progress, use [isScrollInProgress].
     */
    val interactionSource: InteractionSource
        get() = lazyListState.interactionSource

    /**
     * The number of pages to display.
     */
    @get:IntRange(from = 0)
    val pageCount: Int by derivedStateOf {
        lazyListState.layoutInfo.totalItemsCount
    }

    /**
     * The index of the currently selected page. This may not be the page which is
     * currently displayed on screen.
     *
     * To update the scroll position, use [scrollToPage] or [animateScrollToPage].
     */
    @get:IntRange(from = 0)
    var currentPage: Int
        get() = _currentPage
        internal set(value) {
            if (value != _currentPage) {
                _currentPage = value
                if (DebugLog) {
                    Napier.d(message = "Current page changed: $_currentPage")
                }
            }
        }

    /**
     * The current offset from the start of [currentPage], as a ratio of the page width.
     *
     * To update the scroll position, use [scrollToPage] or [animateScrollToPage].
     */
    val currentPageOffset: Float by derivedStateOf {
        currentPageLayoutInfo?.let {
            // We coerce since itemSpacing can make the offset > 1f.
            // We don't want to count spacing in the offset so cap it to 1f
            (-it.offset / it.size.toFloat()).coerceIn(-1f, 1f)
        } ?: 0f
    }

    /**
     * The target page for any on-going animations.
     */
    private var animationTargetPage: Int? by mutableStateOf(null)

    internal var flingAnimationTarget: (() -> Int?)? by mutableStateOf(null)

    /**
     * The target page for any on-going animations or scrolls by the user.
     * Returns the current page if a scroll or animation is not currently in progress.
     */
    @Deprecated(
        "targetPage is deprecated in favor of currentPage as currentPage property is" +
            "now being updated right after we over scrolled the half of the previous current page." +
            "If you still think that you need targetPage, not currentPage please file a bug as " +
            "we are planning to remove this property in future.",
        ReplaceWith("currentPage")
    )
    val targetPage: Int
        get() = animationTargetPage
            ?: flingAnimationTarget?.invoke()
            ?: when {
                // If a scroll isn't in progress, return the current page
                !isScrollInProgress -> currentPage
                // If the offset is 0f (or very close), return the current page
                currentPageOffset.absoluteValue < 0.001f -> currentPage
                // If we're offset towards the start, guess the previous page
                currentPageOffset < 0f -> (currentPage - 1).coerceAtLeast(0)
                // If we're offset towards the end, guess the next page
                else -> (currentPage + 1).coerceAtMost(pageCount - 1)
            }

    @Deprecated(
        "Replaced with animateScrollToPage(page, pageOffset)",
        ReplaceWith("animateScrollToPage(page = page, pageOffset = pageOffset)")
    )
    @Suppress("UNUSED_PARAMETER")
    suspend fun animateScrollToPage(
        @IntRange(from = 0) page: Int,
        @FloatRange(from = 0.0, to = 1.0) pageOffset: Float = 0f,
        animationSpec: AnimationSpec<Float> = spring(),
        initialVelocity: Float = 0f,
        skipPages: Boolean = true,
    ) {
        animateScrollToPage(page = page, pageOffset = pageOffset)
    }

    /**
     * Animate (smooth scroll) to the given page to the middle of the viewport.
     *
     * Cancels the currently running scroll, if any, and suspends until the cancellation is
     * complete.
     *
     * @param page the page to animate to. Must be >= 0.
     * @param pageOffset the percentage of the page size to offset, from the start of [page].
     * Must be in the range -1f..1f.
     */
    suspend fun animateScrollToPage(
        @IntRange(from = 0) page: Int,
        @FloatRange(from = -1.0, to = 1.0) pageOffset: Float = 0f,
    ) {
        requireCurrentPage(page, "page")
        requireCurrentPageOffset(pageOffset, "pageOffset")
        try {
            animationTargetPage = page

            // pre-jump to nearby item for long jumps as an optimization
            // the same trick is done in ViewPager2
            val oldPage = lazyListState.firstVisibleItemIndex
            if (abs(page - oldPage) > 3) {
                lazyListState.scrollToItem(if (page > oldPage) page - 3 else page + 3)
            }

            if (pageOffset.absoluteValue <= 0.005f) {
                // If the offset is (close to) zero, just call animateScrollToItem and we're done
                lazyListState.animateScrollToItem(index = page)
            } else {
                // Else we need to figure out what the offset is in pixels...
                lazyListState.scroll { } // this will await for the first layout.
                val layoutInfo = lazyListState.layoutInfo
                var target = layoutInfo.visibleItemsInfo
                    .firstOrNull { it.index == page }

                if (target != null) {
                    // If we have access to the target page layout, we can calculate the pixel
                    // offset from the size
                    lazyListState.animateScrollToItem(
                        index = page,
                        scrollOffset = (target.size * pageOffset).roundToInt()
                    )
                } else if (layoutInfo.visibleItemsInfo.isNotEmpty()) {
                    // If we don't, we use the current page size as a guide
                    val currentSize = layoutInfo.visibleItemsInfo.first().size
                    lazyListState.animateScrollToItem(
                        index = page,
                        scrollOffset = (currentSize * pageOffset).roundToInt()
                    )

                    // The target should be visible now
                    target = lazyListState.layoutInfo.visibleItemsInfo.firstOrNull { it.index == page }

                    if (target != null && target.size != currentSize) {
                        // If the size we used for calculating the offset differs from the actual
                        // target page size, we need to scroll again. This doesn't look great,
                        // but there's not much else we can do.
                        lazyListState.animateScrollToItem(
                            index = page,
                            scrollOffset = (target.size * pageOffset).roundToInt()
                        )
                    }
                }
            }
        } finally {
            // We need to manually call this, as the `animateScrollToItem` call above will happen
            // in 1 frame, which is usually too fast for the LaunchedEffect in Pager to detect
            // the change. This is especially true when running unit tests.
            onScrollFinished()
        }
    }

    /**
     * Instantly brings the item at [page] to the middle of the viewport.
     *
     * Cancels the currently running scroll, if any, and suspends until the cancellation is
     * complete.
     *
     * @param page the page to snap to. Must be >= 0.
     * @param pageOffset the percentage of the page size to offset, from the start of [page].
     * Must be in the range -1f..1f.
     */
    suspend fun scrollToPage(
        @IntRange(from = 0) page: Int,
        @FloatRange(from = -1.0, to = 1.0) pageOffset: Float = 0f,
    ) {
        requireCurrentPage(page, "page")
        requireCurrentPageOffset(pageOffset, "pageOffset")
        try {
            animationTargetPage = page

            // First scroll to the given page. It will now be laid out at offset 0
            lazyListState.scrollToItem(index = page)
            updateCurrentPageBasedOnLazyListState()

            // If we have a start spacing, we need to offset (scroll) by that too
            if (pageOffset.absoluteValue > 0.0001f) {
                currentPageLayoutInfo?.let {
                    scroll {
                        scrollBy(it.size * pageOffset)
                    }
                }
            }
        } finally {
            // We need to manually call this, as the `scroll` call above will happen in 1 frame,
            // which is usually too fast for the LaunchedEffect in Pager to detect the change.
            // This is especially true when running unit tests.
            onScrollFinished()
        }
    }

    internal fun updateCurrentPageBasedOnLazyListState() {
        // Then update the current page to our layout page
        mostVisiblePageLayoutInfo?.let {
            currentPage = it.index
        }
    }

    internal fun onScrollFinished() {
        // Clear the animation target page
        animationTargetPage = null
    }

    override suspend fun scroll(
        scrollPriority: MutatePriority,
        block: suspend ScrollScope.() -> Unit
    ) = lazyListState.scroll(scrollPriority, block)

    override fun dispatchRawDelta(delta: Float): Float {
        return lazyListState.dispatchRawDelta(delta)
    }

    override val isScrollInProgress: Boolean
        get() = lazyListState.isScrollInProgress

    override fun toString(): String = "PagerState(" +
        "pageCount=$pageCount, " +
        "currentPage=$currentPage, " +
        "currentPageOffset=$currentPageOffset" +
        ")"

    private fun requireCurrentPage(value: Int, name: String) {
        require(value >= 0) { "$name[$value] must be >= 0" }
    }

    private fun requireCurrentPageOffset(value: Float, name: String) {
        require(value in -1f..1f) { "$name must be >= 0 and <= 1" }
    }

    companion object {
        /**
         * The default [Saver] implementation for [PagerState].
         */
        val Saver: Saver<PagerState, *> = listSaver(
            save = {
                listOf<Any>(
                    it.currentPage,
                )
            },
            restore = {
                PagerState(
                    currentPage = it[0] as Int,
                )
            }
        )

        init {
            if (DebugLog) {
                Napier.base(DebugAntilog(defaultTag = "Pager"))
            }
        }
    }
}
