/*
 * Copyright 2023 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 org.tasks.compose.drawer

import androidx.compose.foundation.MutatePriority
import androidx.compose.runtime.Stable
import kotlinx.coroutines.CancellationException
import kotlinx.coroutines.Job
import kotlinx.coroutines.coroutineScope
import kotlinx.coroutines.sync.Mutex
import kotlinx.coroutines.sync.withLock

/**
 * Mutual exclusion for UI state mutation over time.
 *
 * [mutate] permits interruptible state mutation over time using a standard [MutatePriority].
 * A [InternalMutatorMutex] enforces that only a single writer can be active at a time for a particular
 * state resource. Instead of queueing callers that would acquire the lock like a traditional
 * [Mutex], new attempts to [mutate] the guarded state will either cancel the current mutator or
 * if the current mutator has a higher priority, the new caller will throw [CancellationException].
 *
 * [InternalMutatorMutex] should be used for implementing hoisted state objects that many mutators may
 * want to manipulate over time such that those mutators can coordinate with one another. The
 * [InternalMutatorMutex] instance should be hidden as an implementation detail. For example:
 *
 */
@Stable
internal class InternalMutatorMutex {
    private class Mutator(val priority: MutatePriority, val job: Job) {
        fun canInterrupt(other: Mutator) = priority >= other.priority

        fun cancel() = job.cancel()
    }

    private val currentMutator = InternalAtomicReference<Mutator?>(null)
    private val mutex = Mutex()

    private fun tryMutateOrCancel(mutator: Mutator) {
        while (true) {
            val oldMutator = currentMutator.get()
            if (oldMutator == null || mutator.canInterrupt(oldMutator)) {
                if (currentMutator.compareAndSet(oldMutator, mutator)) {
                    oldMutator?.cancel()
                    break
                }
            } else throw CancellationException("Current mutation had a higher priority")
        }
    }

    /**
     * Enforce that only a single caller may be active at a time.
     *
     * If [mutate] is called while another call to [mutate] or [mutateWith] is in progress, their
     * [priority] values are compared. If the new caller has a [priority] equal to or higher than
     * the call in progress, the call in progress will be cancelled, throwing
     * [CancellationException] and the new caller's [block] will be invoked. If the call in
     * progress had a higher [priority] than the new caller, the new caller will throw
     * [CancellationException] without invoking [block].
     *
     * @param priority the priority of this mutation; [MutatePriority.Default] by default.
     * Higher priority mutations will interrupt lower priority mutations.
     * @param block mutation code to run mutually exclusive with any other call to [mutate],
     * [mutateWith] or [tryMutate].
     */
    suspend fun <R> mutate(
        priority: MutatePriority = MutatePriority.Default,
        block: suspend () -> R
    ) = coroutineScope {
        val mutator = Mutator(priority, coroutineContext[Job]!!)

        tryMutateOrCancel(mutator)

        mutex.withLock {
            try {
                block()
            } finally {
                currentMutator.compareAndSet(mutator, null)
            }
        }
    }

    /**
     * Enforce that only a single caller may be active at a time.
     *
     * If [mutateWith] is called while another call to [mutate] or [mutateWith] is in progress,
     * their [priority] values are compared. If the new caller has a [priority] equal to or
     * higher than the call in progress, the call in progress will be cancelled, throwing
     * [CancellationException] and the new caller's [block] will be invoked. If the call in
     * progress had a higher [priority] than the new caller, the new caller will throw
     * [CancellationException] without invoking [block].
     *
     * This variant of [mutate] calls its [block] with a [receiver], removing the need to create
     * an additional capturing lambda to invoke it with a receiver object. This can be used to
     * expose a mutable scope to the provided [block] while leaving the rest of the state object
     * read-only. For example:
     *
     * @param receiver the receiver `this` that [block] will be called with
     * @param priority the priority of this mutation; [MutatePriority.Default] by default.
     * Higher priority mutations will interrupt lower priority mutations.
     * @param block mutation code to run mutually exclusive with any other call to [mutate],
     * [mutateWith] or [tryMutate].
     */
    suspend fun <T, R> mutateWith(
        receiver: T,
        priority: MutatePriority = MutatePriority.Default,
        block: suspend T.() -> R
    ) = coroutineScope {
        val mutator = Mutator(priority, coroutineContext[Job]!!)

        tryMutateOrCancel(mutator)

        mutex.withLock {
            try {
                receiver.block()
            } finally {
                currentMutator.compareAndSet(mutator, null)
            }
        }
    }

    /**
     * Attempt to mutate synchronously if there is no other active caller.
     * If there is no other active caller, the [block] will be executed in a lock. If there is
     * another active caller, this method will return false, indicating that the active caller
     * needs to be cancelled through a [mutate] or [mutateWith] call with an equal or higher
     * mutation priority.
     *
     * Calls to [mutate] and [mutateWith] will suspend until execution of the [block] has finished.
     *
     * @param block mutation code to run mutually exclusive with any other call to [mutate],
     * [mutateWith] or [tryMutate].
     * @return true if the [block] was executed, false if there was another active caller and the
     * [block] was not executed.
     */
    fun tryMutate(block: () -> Unit): Boolean {
        val didLock = mutex.tryLock()
        if (didLock) {
            try {
                block()
            } finally {
                mutex.unlock()
            }
        }
        return didLock
    }
}
