//
//  Copyright RevenueCat Inc. All Rights Reserved.
//
//  Licensed under the MIT License (the "License");
//  you may not use this file except in compliance with the License.
//  You may obtain a copy of the License at
//
//      https://opensource.org/licenses/MIT
//
//  CheckpointWorkflowResolver.swift
//
//  Created by Rick van der Linden.
//

// swiftlint:disable file_length

import Foundation

// swiftlint:disable file_length

/// The result of resolving a checkpoint against RevenueCat configuration.
@_spi(Internal) public enum CheckpointResolution {

    /// A workflow was selected for the checkpoint.
    case matchedWorkflow(ResolvedCheckpointWorkflow)
    /// An offering was selected for the checkpoint, with no RevenueCat-managed UI to present. The app
    /// decides whether and how to use it. `traceID` identifies the hit, and the offering's packages carry it too.
    case matchedOffering(Offering, traceID: String)
    /// An ad was selected for the checkpoint, with no RevenueCat-managed UI to present. A registered
    /// ad presenter decides how to show it.
    case matchedAd(ResolvedAdStep)
    /// No workflow should run for the checkpoint.
    case noAction(CheckpointResolutionReason)

}

/// The reason that checkpoint resolution selected no workflow.
@_spi(Internal) public enum CheckpointResolutionReason: Sendable {

    /// No targeting rule matched.
    case noMatch
    /// Checkpoint configuration could not be loaded.
    case configurationUnavailable
    /// The checkpoint identifier is not configured.
    case unknownCheckpoint

}

/// A ``CheckpointResolution`` together with the rule that produced it. The rule id only attributes the hit
/// event, so it travels here rather than on ``CheckpointResolution``, which RevenueCatUI consumes.
struct ResolvedCheckpoint {

    let resolution: CheckpointResolution
    let checkpointRuleID: String?
    /// The matched workflow's or offering's trace id, so the hit and its purchases join on it. Fresh otherwise.
    let traceID: String

    init(_ resolution: CheckpointResolution, checkpointRuleID: String? = nil) {
        self.resolution = resolution
        self.checkpointRuleID = checkpointRuleID
        switch resolution {
        case let .matchedWorkflow(workflow):
            self.traceID = workflow.traceId
        case let .matchedOffering(_, traceID):
            self.traceID = traceID
        case .matchedAd, .noAction:
            self.traceID = UUID().uuidString
        }
    }

    init(_ resolution: CheckpointResolution, checkpointRuleID: String?, traceID: String) {
        self.resolution = resolution
        self.checkpointRuleID = checkpointRuleID
        self.traceID = traceID
    }

    /// Reports `rule` only when it was actually served.
    init(_ resolution: CheckpointResolution, servedBy rule: CheckpointRule, traceID: String) {
        switch resolution {
        case .matchedWorkflow, .matchedOffering, .matchedAd:
            self.init(resolution, checkpointRuleID: rule.id, traceID: traceID)

        case .noAction:
            self.init(resolution, checkpointRuleID: nil, traceID: traceID)
        }
    }

}

/// Resolves a checkpoint to the workflow that should run, or the reason no workflow should run.
protocol CheckpointWorkflowResolver: AnyObject {

    func resolve(identifier: String, params: CheckpointParams) async throws -> ResolvedCheckpoint

}

final class DisabledCheckpointWorkflowResolver: CheckpointWorkflowResolver {

    func resolve(identifier: String, params: CheckpointParams) async throws -> ResolvedCheckpoint {
        return .init(.noAction(.configurationUnavailable))
    }

}

/// Resolves checkpoints through the ordered rules served by the `checkpoint_rules` remote-config topic, taking
/// the first rule whose audience the customer matches.
///
/// The matched rule's workflow body is read first, because its shape decides what else the rule needs: a
/// workflow whose only step is a terminal `offering` step is handed back to the app as an offering, with
/// nothing presented, while every other workflow fetches the complete offerings bundle and resolves each
/// screen step only when it is reached.
///
/// The match is final either way. A matched rule that turns out to be unservable resolves to
/// ``CheckpointResolutionReason/configurationUnavailable`` instead of falling through to a rule this
/// customer wasn't the first choice for.
final class DefaultCheckpointWorkflowResolver: CheckpointWorkflowResolver {

    private let checkpointsConfigProvider: CheckpointsConfigProviderType
    private let audiencesConfigProvider: AudiencesConfigProviderType
    private let localRulesEvaluator: LocalRulesEvaluator
    private let workflowManager: WorkflowManager
    private let offeringsProvider: () async throws -> Offerings
    private let trackWorkflowEvent: (WorkflowEvent) async -> Void

    init(
        checkpointsConfigProvider: CheckpointsConfigProviderType,
        audiencesConfigProvider: AudiencesConfigProviderType,
        localRulesEvaluator: LocalRulesEvaluator,
        workflowManager: WorkflowManager,
        offeringsProvider: @escaping () async throws -> Offerings,
        trackWorkflowEvent: @escaping (WorkflowEvent) async -> Void
    ) {
        self.checkpointsConfigProvider = checkpointsConfigProvider
        self.audiencesConfigProvider = audiencesConfigProvider
        self.localRulesEvaluator = localRulesEvaluator
        self.workflowManager = workflowManager
        self.offeringsProvider = offeringsProvider
        self.trackWorkflowEvent = trackWorkflowEvent
    }

    func resolve(identifier: String, params: CheckpointParams) async throws -> ResolvedCheckpoint {
        #if DEBUG
        // Debug-only simulated error for testers. Config-backed resolution has no natural throwing case yet.
        if identifier == Self.simulatedErrorCheckpointIdentifier {
            throw ErrorUtils.configurationError(
                message: "Simulated error: checkpoint workflow not presentable."
            )
        }
        #endif

        // One id per hit, shared by every event it produces, so a retried attempt keeps it.
        let traceID = UUID().uuidString

        if let resolved = try await self.attemptResolveConfiguredWorkflow(
            identifier: identifier,
            params: params,
            traceID: traceID
        ) {
            return resolved
        }
        Logger.verbose(Strings.checkpoints.resolutionRetry(identifier: identifier))

        // A `nil` attempt means its configuration became stale while resolving.
        // Retry once against the latest generation before treating repeated staleness as unavailable.
        if let resolved = try await self.attemptResolveConfiguredWorkflow(
            identifier: identifier,
            params: params,
            traceID: traceID
        ) {
            return resolved
        }
        Logger.error(Strings.checkpoints.resolutionRepeatedlyStale(identifier: identifier))
        return .init(.noAction(.configurationUnavailable))
    }

    // A resolution attempt has several distinct terminal states plus the stale sentinel (`nil`).
    // swiftlint:disable:next cyclomatic_complexity function_body_length
    private func attemptResolveConfiguredWorkflow(
        identifier: String,
        params: CheckpointParams,
        traceID: String
    ) async throws -> ResolvedCheckpoint? {
        let rulesSnapshot: CheckpointRulesSnapshot
        do {
            guard let snapshot = try await self.checkpointsConfigProvider.rules(for: identifier) else {
                Logger.info(Strings.checkpoints.unknownCheckpoint(identifier: identifier))
                return .init(.noAction(.unknownCheckpoint))
            }
            rulesSnapshot = snapshot
        } catch let error as CancellationError {
            throw error
        } catch CheckpointRulesProviderError.stale {
            return nil
        } catch {
            return .init(.noAction(.configurationUnavailable))
        }

        guard self.checkpointsConfigProvider.isCurrent(rulesSnapshot) else { return nil }
        guard !rulesSnapshot.ruleSet.rules.isEmpty else {
            return .init(.noAction(.noMatch))
        }

        let audienceConfiguration: AudienceConfigurationSnapshot
        do {
            guard let configuration = try await self.audiencesConfigProvider.configuration() else {
                guard self.checkpointsConfigProvider.isCurrent(rulesSnapshot) else { return nil }
                return .init(.noAction(.configurationUnavailable))
            }
            audienceConfiguration = configuration
        } catch let error as CancellationError {
            throw error
        } catch {
            guard self.checkpointsConfigProvider.isCurrent(rulesSnapshot) else { return nil }
            Logger.error(Strings.checkpoints.audiencesNotEvaluated(
                checkpointID: identifier,
                reason: "\(error)"
            ))
            return .init(.noAction(.configurationUnavailable))
        }

        guard self.isCurrent(rulesSnapshot, audienceConfiguration) else {
            return nil
        }

        let ruleEvaluation = try await self.evaluateRules(
            for: identifier,
            in: rulesSnapshot.ruleSet.rules,
            params: params,
            audienceConfiguration: audienceConfiguration
        )
        guard self.isCurrent(rulesSnapshot, audienceConfiguration) else {
            return nil
        }

        let rule: CheckpointRule?
        switch ruleEvaluation {
        case let .completed(matchedRule):
            rule = matchedRule
        case let .unavailable(error):
            // Only return `noMatch` when audience evaluation succeeds and no rule matches. If evaluation fails,
            // the SDK can't determine whether the app user matches, so treat the checkpoint configuration as
            // unavailable.
            Logger.error(Strings.checkpoints.audiencesNotEvaluated(
                checkpointID: identifier,
                reason: "\(error)"
            ))
            return .init(.noAction(.configurationUnavailable))
        }

        // The offering mapping is resolved per branch now, since only a UI workflow needs it.
        guard let rule else { return .init(.noAction(.noMatch)) }

        let (resolution, stepEvents) = try await self.resolve(
            rule,
            checkpointIdentifier: identifier,
            traceID: traceID
        )
        guard self.isCurrent(rulesSnapshot, audienceConfiguration) else {
            return nil
        }

        // Held back until the result is known to be current, so a stale attempt that gets retried emits nothing.
        for event in stepEvents {
            await self.trackWorkflowEvent(event)
        }

        return .init(resolution, servedBy: rule, traceID: traceID)
    }

    private func isCurrent(
        _ rulesSnapshot: CheckpointRulesSnapshot,
        _ audienceConfiguration: AudienceConfigurationSnapshot
    ) -> Bool {
        return rulesSnapshot.configGeneration == audienceConfiguration.configGeneration
            && self.checkpointsConfigProvider.isCurrent(rulesSnapshot)
            && self.audiencesConfigProvider.isCurrent(audienceConfiguration)
    }

    private func evaluateRules(
        for identifier: String,
        in rules: [CheckpointRule],
        params: CheckpointParams,
        audienceConfiguration: AudienceConfigurationSnapshot
    ) async throws -> AudienceRuleEvaluation {
        do {
            return .completed(try await self.matchingRule(
                for: identifier,
                in: rules,
                params: params,
                audienceConfiguration: audienceConfiguration
            ))
        } catch let error as CancellationError {
            throw error
        } catch {
            return .unavailable(error)
        }
    }

    /// Walks the served rules in priority order and returns the first one whose audience matches.
    private func matchingRule(
        for identifier: String,
        in rules: [CheckpointRule],
        params: CheckpointParams,
        audienceConfiguration: AudienceConfigurationSnapshot
    ) async throws -> CheckpointRule? {
        return try await self.localRulesEvaluator.match(
            in: rules,
            // Already filtered to valid keys by `DimensionResolver`, which exposes them under `custom.*`.
            customVariables: params.customVariables.mapValues(\.dimensionValue),
            logPrefix: "[Checkpoint '\(identifier)'] "
        ) { rule in
            guard let audience = audienceConfiguration.audiences[rule.audienceId] else {
                throw AudienceUnavailableError(audienceID: rule.audienceId)
            }

            return audience.rules
        }
    }

    func loadOfferings() async -> Offerings? {
        do {
            return try await self.offeringsProvider()
        } catch {
            Logger.error(error.localizedDescription)
            return nil
        }
    }

    private func resolve(
        _ rule: CheckpointRule,
        checkpointIdentifier: String,
        traceID: String
    ) async throws -> (resolution: CheckpointResolution, stepEvents: [WorkflowEvent]) {
        // Deliberately the non-prewarming read: an offering workflow renders nothing, and prewarming takes
        // its fonts from `uiConfig` rather than from the workflow's screens, so it isn't free here.
        let workflowData: WorkflowDataResult
        do {
            workflowData = try await self.workflowManager.workflowData(workflowId: rule.workflowId)
        } catch let error as CancellationError {
            throw error
        } catch {
            return (Self.unservable(rule, reason: error.localizedDescription), [])
        }

        let workflow = workflowData.workflow
        guard let initialStep = workflow.steps[workflow.initialStepId] else {
            return (Self.unservable(rule, reason: "its initial step was not found"), [])
        }
        if let step = workflow.steps.values.first(where: { $0.gating == .invalid }) {
            return (Self.unservable(rule, reason: "step '\(step.id)' has invalid gating"), [])
        }

        if initialStep.isOfferingStep {
            return await self.resolveOfferingStep(
                rule, step: initialStep, in: workflowData,
                checkpointIdentifier: checkpointIdentifier, traceID: traceID
            )
        }

        if initialStep.type == Self.adStepType {
            return await self.resolveAdStep(rule, step: initialStep, in: workflowData, traceID: traceID)
        }

        if workflow.steps.values.contains(where: \.isOfferingStep) {
            return (Self.unservable(rule, reason: "a UI workflow cannot contain offering steps"), [])
        }

        if workflow.steps.values.contains(where: { $0.type == Self.adStepType }) {
            return (Self.unservable(rule, reason: "a UI workflow cannot contain ad steps"), [])
        }

        return (await self.resolveWorkflow(workflowData: workflowData, traceID: traceID), [])
    }

    private func resolveWorkflow(workflowData: WorkflowDataResult, traceID: String) async -> CheckpointResolution {
        guard let offerings = await self.loadOfferings() else {
            return .noAction(.configurationUnavailable)
        }

        self.workflowManager.scheduleAssetPrewarming(for: workflowData)

        return .matchedWorkflow(
            ResolvedCheckpointWorkflow(
                workflow: workflowData.workflow,
                uiConfig: workflowData.uiConfig,
                offerings: offerings,
                workflowBlobRef: workflowData.workflowBlobRef,
                traceId: traceID
            )
        )
    }

    @discardableResult
    static func unservable(_ rule: CheckpointRule, reason: String) -> CheckpointResolution {
        Logger.warn(Strings.checkpoints.workflowRuleSkipped(
            workflowID: rule.workflowId,
            reason: reason
        ))
        return .noAction(.configurationUnavailable)
    }

    #if DEBUG
    private static let simulatedErrorCheckpointIdentifier = "f0rc3_3rr0r"
    #endif
}
/// An audience referenced by a checkpoint rule that the SDK couldn't read, so the rule can't be evaluated.
private struct AudienceUnavailableError: Error, CustomStringConvertible {
    let audienceID: String
    var description: String {
        return "audience '\(self.audienceID)' could not be read"
    }
}
private enum AudienceRuleEvaluation {
    case completed(CheckpointRule?)
    case unavailable(any Error)
}