/// A display mode reduced to what matters for picking one. Mirrors CGDisplayMode.
public struct DisplayModeInfo: Hashable, Sendable {
    public var width: Int          // points
    public var height: Int
    public var pixelWidth: Int
    public var pixelHeight: Int
    public var refreshRate: Double // Hz, 0 for some built-in panels

    public init(width: Int, height: Int, pixelWidth: Int, pixelHeight: Int, refreshRate: Double) {
        self.width = width
        self.height = height
        self.pixelWidth = pixelWidth
        self.pixelHeight = pixelHeight
        self.refreshRate = refreshRate
    }

    public var isHiDPI: Bool { pixelWidth > width }
}

public enum DisplayModeSelection {
    /// Picks the mode with the same size as `current` and a refresh rate within
    /// `tolerance` of `targetHz`. Returns nil if `current` already matches or no such mode exists.
    public static func mode(
        matching targetHz: Double,
        current: DisplayModeInfo,
        available: [DisplayModeInfo],
        tolerance: Double = 0.1
    ) -> DisplayModeInfo? {
        if abs(current.refreshRate - targetHz) <= tolerance { return nil }
        return available
            .filter {
                $0.width == current.width && $0.height == current.height
                    && abs($0.refreshRate - targetHz) <= tolerance
            }
            // Prefer the same backing resolution, then the closest rate.
            .min { lhs, rhs in
                let lhsSame = lhs.pixelWidth == current.pixelWidth && lhs.pixelHeight == current.pixelHeight
                let rhsSame = rhs.pixelWidth == current.pixelWidth && rhs.pixelHeight == current.pixelHeight
                if lhsSame != rhsSame { return lhsSame }
                return abs(lhs.refreshRate - targetHz) < abs(rhs.refreshRate - targetHz)
            }
    }
}