ReadViewSDK/Sources/RDEpubReaderView/EPUBUI/ReaderController/RDEPUBBackgroundPriorityPolicy.swift

import Foundation

struct RDEPUBBackgroundPriorityPolicy {

    let hotRadius: Int

    let warmRadius: Int

    let maxWarmJumpAnchors: Int

    let coldLaneShare: Double

    static let `default` = RDEPUBBackgroundPriorityPolicy(
        hotRadius: 24,
        warmRadius: 96,
        maxWarmJumpAnchors: 2,
        coldLaneShare: 0.15
    )

    static func adaptive(totalBuildableChapters: Int) -> RDEPUBBackgroundPriorityPolicy {
        let hotRadius = min(max(12, Int(sqrt(Double(totalBuildableChapters)))), 48)
        let warmRadius = min(max(hotRadius * 3, 32), 192)
        return RDEPUBBackgroundPriorityPolicy(
            hotRadius: hotRadius,
            warmRadius: warmRadius,
            maxWarmJumpAnchors: 2,
            coldLaneShare: 0.15
        )
    }
}

enum RDEPUBPriorityBand: Int, Comparable {
    case hot = 0
    case warmPrimary = 1
    case warmSecondary = 2
    case cold = 3

    static func < (lhs: RDEPUBPriorityBand, rhs: RDEPUBPriorityBand) -> Bool {
        lhs.rawValue < rhs.rawValue
    }
}

struct RDEPUBWarmJumpAnchor {
    let spineIndex: Int
    let timestamp: CFAbsoluteTime
    let sequenceNumber: Int
}

struct RDEPUBMetadataParseWorkItem {
    let spineIndex: Int
    let generation: Int
    let priorityBand: RDEPUBPriorityBand

    var sortKey: (bandRank: Int, distanceToCurrent: Int, distanceToNewestJump: Int, spineIndex: Int) {
        (priorityBand.rawValue, 0, 0, spineIndex)
    }
}

final class RDEPUBBackgroundPriorityManager {

    private unowned let context: RDEPUBReaderContext

    private(set) var policy: RDEPUBBackgroundPriorityPolicy

    private var warmAnchors: [RDEPUBWarmJumpAnchor] = []
    private let warmAnchorsLock = NSLock()

    private(set) var currentGeneration: Int = 0

    private var coldCursor: Int = 0

    init(context: RDEPUBReaderContext) {
        self.context = context
        self.policy = .default
    }

    func updatePolicy(_ newPolicy: RDEPUBBackgroundPriorityPolicy) {
        policy = newPolicy
    }

    func addWarmAnchor(spineIndex: Int) {
        let anchor = RDEPUBWarmJumpAnchor(
            spineIndex: spineIndex,
            timestamp: CFAbsoluteTimeGetCurrent(),
            sequenceNumber: currentGeneration
        )

        warmAnchorsLock.lock()
        warmAnchors.insert(anchor, at: 0)

        if warmAnchors.count > policy.maxWarmJumpAnchors {
            warmAnchors = Array(warmAnchors.prefix(policy.maxWarmJumpAnchors))
        }
        warmAnchorsLock.unlock()

        currentGeneration += 1
        coldCursor = 0 

    }

    func makeMetadataPriorityOrder(
        allBuildableIndices: [Int],
        currentSpineIndex: Int?,
        cachedSpineIndices: Set<Int>
    ) -> [Int] {
        let uncachedIndices = allBuildableIndices.filter { !cachedSpineIndices.contains($0) }
        guard !uncachedIndices.isEmpty else { return [] }

        // M-03: Snapshot warmAnchors under lock since it's written on main thread
        // and read on background thread.
        warmAnchorsLock.lock()
        let warmAnchorsSnapshot = warmAnchors
        warmAnchorsLock.unlock()

        let items = uncachedIndices.map { spineIndex -> (spineIndex: Int, band: RDEPUBPriorityBand) in
            let band = classifySpineIndex(
                spineIndex: spineIndex,
                currentSpineIndex: currentSpineIndex,
                warmAnchors: warmAnchorsSnapshot
            )
            return (spineIndex, band)
        }

        let sorted = items.sorted { lhs, rhs in

            if lhs.band != rhs.band {
                return lhs.band < rhs.band
            }

            let lhsDistanceToCurrent = currentSpineIndex.map { abs(lhs.spineIndex - $0) } ?? Int.max
            let rhsDistanceToCurrent = currentSpineIndex.map { abs(rhs.spineIndex - $0) } ?? Int.max
            if lhsDistanceToCurrent != rhsDistanceToCurrent {
                return lhsDistanceToCurrent < rhsDistanceToCurrent
            }

            return lhs.spineIndex < rhs.spineIndex
        }

        return sorted.map { $0.spineIndex }
    }

    private func classifySpineIndex(
        spineIndex: Int,
        currentSpineIndex: Int?,
        warmAnchors: [RDEPUBWarmJumpAnchor]
    ) -> RDEPUBPriorityBand {

        if let current = currentSpineIndex {
            let distance = abs(spineIndex - current)
            if distance <= policy.hotRadius {
                return .hot
            }
        }

        for (index, anchor) in warmAnchors.enumerated() {
            let distance = abs(spineIndex - anchor.spineIndex)
            if distance <= policy.warmRadius {
                return index == 0 ? .warmPrimary : .warmSecondary
            }
        }

        return .cold
    }

    func currentWarmAnchors() -> [RDEPUBWarmJumpAnchor] {
        warmAnchorsLock.lock()
        defer { warmAnchorsLock.unlock() }
        return warmAnchors
    }

    func reset() {
        warmAnchorsLock.lock()
        warmAnchors.removeAll()
        warmAnchorsLock.unlock()
        currentGeneration = 0
        coldCursor = 0
    }
}