import Foundation

/// Memory footprint probe for the long-chapter optimization work
/// (Doc/LONG_CHAPTER_MEMORY_OPTIMIZATION_PLAN.md, P0-2). Enabled with the
/// `--demo-memory-probe` launch argument; logs phys_footprint at chapter
/// load, every 20 page turns, settings invalidation, and rotation.
enum RDEPUBMemoryProbe {

    static let isEnabled = ProcessInfo.processInfo.arguments.contains("--demo-memory-probe")

    private static let pageTurnLogStride = 20

    /// Main-thread only (page turns are delivered on main).
    private static var pageTurnCount = 0

    static func logPageTurn() {
        guard isEnabled else { return }
        pageTurnCount += 1
        guard pageTurnCount % pageTurnLogStride == 0 else { return }
        log("pageTurn count=\(pageTurnCount)")
    }

    static func log(_ event: String) {
        guard isEnabled else { return }
        print(String(format: "[EPUB][MemoryProbe] %@ footprint=%.1fMB", event, footprintMB))
    }

    /// Current phys_footprint in MB. Cheap enough (one task_info call) to
    /// surface in the demo state snapshot for automated memory assertions.
    static var footprintMB: Double {
        Double(currentFootprint()) / 1_048_576
    }

    /// phys_footprint matches the value Xcode's memory gauge and Jetsam use.
    private static func currentFootprint() -> UInt64 {
        var info = task_vm_info_data_t()
        var count = mach_msg_type_number_t(
            MemoryLayout<task_vm_info_data_t>.size / MemoryLayout<integer_t>.size
        )
        let result = withUnsafeMutablePointer(to: &info) { pointer in
            pointer.withMemoryRebound(to: integer_t.self, capacity: Int(count)) {
                task_info(mach_task_self_, task_flavor_t(TASK_VM_INFO), $0, &count)
            }
        }
        guard result == KERN_SUCCESS else { return 0 }
        return info.phys_footprint
    }
}