import CoreGraphics
import MusubiCore
import OSLog

/// Switches a display to a target refresh rate for the lifetime of the app and restores it on request.
public enum DisplayModeSwitcher {
    private static let log = Logger(subsystem: "studio.zielke.musubi", category: "display")

    /// Returns the refresh rate in effect afterwards, and the original mode to restore later (nil if unchanged).
    @discardableResult
    public static func setRefreshRate(_ hz: Double, on display: CGDirectDisplayID) -> (refreshRate: Double, original: CGDisplayMode?) {
        guard let current = CGDisplayCopyDisplayMode(display) else { return (0, nil) }
        let options = [kCGDisplayShowDuplicateLowResolutionModes: true] as CFDictionary
        let modes = (CGDisplayCopyAllDisplayModes(display, options) as? [CGDisplayMode]) ?? []

        guard let target = DisplayModeSelection.mode(matching: hz, current: info(current), available: modes.map(info)),
              let mode = modes.first(where: { info($0) == target })
        else {
            if abs(current.refreshRate - hz) > 0.1 {
                log.notice("Display \(display): no \(hz) Hz mode, staying at \(current.refreshRate) Hz")
            }
            return (current.refreshRate, nil)
        }

        guard apply(mode, to: display) else { return (current.refreshRate, nil) }
        log.info("Display \(display): \(current.refreshRate) Hz → \(mode.refreshRate) Hz")
        return (mode.refreshRate, current)
    }

    public static func restore(_ mode: CGDisplayMode, on display: CGDirectDisplayID) {
        if apply(mode, to: display) {
            log.info("Display \(display): restored \(mode.refreshRate) Hz")
        }
    }

    public static func currentRefreshRate(of display: CGDirectDisplayID) -> Double {
        CGDisplayCopyDisplayMode(display)?.refreshRate ?? 0
    }

    private static func apply(_ mode: CGDisplayMode, to display: CGDirectDisplayID) -> Bool {
        var config: CGDisplayConfigRef?
        guard CGBeginDisplayConfiguration(&config) == .success else { return false }
        CGConfigureDisplayWithDisplayMode(config, display, mode, nil)
        // .forAppOnly: macOS reverts the mode when Musubi quits or crashes.
        let result = CGCompleteDisplayConfiguration(config, .forAppOnly)
        if result != .success { log.error("Display \(display): mode change failed (\(result.rawValue))") }
        return result == .success
    }

    private static func info(_ mode: CGDisplayMode) -> DisplayModeInfo {
        DisplayModeInfo(width: mode.width, height: mode.height,
                        pixelWidth: mode.pixelWidth, pixelHeight: mode.pixelHeight,
                        refreshRate: mode.refreshRate)
    }
}