X11: Fix get_current_monitor (#515)

* X11: Fix get_current_monitor

Fixes #64

* impl Debug for MonitorId on all platforms

src/platform/linux/x11/util/randr.rs

@@ -0,0 +1,84 @@
+use std::slice;
+
+use super::*;
+use super::ffi::{
+    RROutput,
+    XRRCrtcInfo,
+    XRRMonitorInfo,
+    XRRScreenResources,
+};
+
+pub enum MonitorRepr {
+    Monitor(*mut XRRMonitorInfo),
+    Crtc(*mut XRRCrtcInfo),
+}
+
+impl MonitorRepr {
+    pub unsafe fn get_output(&self) -> RROutput {
+        match *self {
+            // Same member names, but different locations within the struct...
+            MonitorRepr::Monitor(monitor) => *((*monitor).outputs.offset(0)),
+            MonitorRepr::Crtc(crtc) => *((*crtc).outputs.offset(0)),
+        }
+    }
+
+    pub unsafe fn get_dimensions(&self) -> (u32, u32) {
+        match *self {
+            MonitorRepr::Monitor(monitor) => ((*monitor).width as u32, (*monitor).height as u32),
+            MonitorRepr::Crtc(crtc) => ((*crtc).width as u32, (*crtc).height as u32),
+        }
+    }
+
+    pub unsafe fn get_position(&self) -> (i32, i32) {
+        match *self {
+            MonitorRepr::Monitor(monitor) => ((*monitor).x as i32, (*monitor).y as i32),
+            MonitorRepr::Crtc(crtc) => ((*crtc).x as i32, (*crtc).y as i32),
+        }
+    }
+}
+
+impl From<*mut XRRMonitorInfo> for MonitorRepr {
+    fn from(monitor: *mut XRRMonitorInfo) -> Self {
+        MonitorRepr::Monitor(monitor)
+    }
+}
+
+impl From<*mut XRRCrtcInfo> for MonitorRepr {
+    fn from(crtc: *mut XRRCrtcInfo) -> Self {
+        MonitorRepr::Crtc(crtc)
+    }
+}
+
+pub fn calc_dpi_factor(
+    (width_px, height_px): (u32, u32),
+    (width_mm, height_mm): (u64, u64),
+) -> f64 {
+    let ppmm = (
+        (width_px as f64 * height_px as f64) / (width_mm as f64 * height_mm as f64)
+    ).sqrt();
+    // Quantize 1/12 step size
+    ((ppmm * (12.0 * 25.4 / 96.0)).round() / 12.0).max(1.0)
+}
+
+pub unsafe fn get_output_info(
+    xconn: &Arc<XConnection>,
+    resources: *mut XRRScreenResources,
+    repr: &MonitorRepr,
+) -> (String, f32) {
+    let output_info = (xconn.xrandr.XRRGetOutputInfo)(
+        xconn.display,
+        resources,
+        repr.get_output(),
+    );
+    let name_slice = slice::from_raw_parts(
+        (*output_info).name as *mut u8,
+        (*output_info).nameLen as usize,
+    );
+    let name = String::from_utf8_lossy(name_slice).into();
+    let hidpi_factor = calc_dpi_factor(
+        repr.get_dimensions(),
+        ((*output_info).mm_width as u64, (*output_info).mm_height as u64),
+    ) as f32;
+    (xconn.xrandr.XRRFreeOutputInfo)(output_info);
+    (name, hidpi_factor)
+}