import Foundation
import ForgeGrade

/// Rate-limits camera look pushes. Preview updates instantly elsewhere;
/// camera gets at most one push per interval, trailing edge guaranteed —
/// final panel position always lands on camera.
public actor DebouncedPusher {
    private let driver: any CameraDriver
    private let interval: Duration
    private let latticeSize: Int

    private var latestGrade: GradeStack?
    private var lastPushAt: ContinuousClock.Instant?
    private var pending: Task<Void, Never>?
    private var stopped = false

    public init(driver: any CameraDriver, interval: Duration = .milliseconds(150), latticeSize: Int = 33) {
        self.driver = driver
        self.interval = interval
        self.latticeSize = latticeSize
    }

    /// Record new grade; push now if interval elapsed, else schedule trailing push.
    public func update(grade: GradeStack) {
        guard !stopped else { return }
        latestGrade = grade

        let now = ContinuousClock.now
        if let last = lastPushAt, last.duration(to: now) < interval {
            scheduleTrailing(after: interval - last.duration(to: now))
        } else {
            pushNow()
        }
    }

    private func scheduleTrailing(after delay: Duration) {
        guard pending == nil else { return } // one trailing push covers all updates
        pending = Task {
            try? await Task.sleep(for: delay)
            await self.firePending()
        }
    }

    private func firePending() {
        pending = nil
        guard !stopped else { return }
        pushNow()
    }

    private func pushNow() {
        guard let grade = latestGrade else { return }
        lastPushAt = ContinuousClock.now
        let caps = driver.capabilities
        let look = Flattener.flatten(
            stack: grade,
            latticeSize: caps.lut3dSizes.contains(latticeSize) ? latticeSize : (caps.lut3dSizes.max() ?? latticeSize),
            splitLeadingCDL: caps.supportsNativeCDL)
        let driver = self.driver
        Task {
            try? await driver.push(look: look)
        }
    }

    public func stop() {
        stopped = true
        pending?.cancel()
        pending = nil
    }
}