add multimonitor

src/action.zig

@@ -32,6 +32,46 @@ pub fn tagSlot(tags: u32) usize {
     return @ctz(t) + 1;
 }
 
+/// The tags belonging to output `index` of `count`, when the tag set is split
+/// across the displays.
+///
+/// The tags are divided into contiguous ranges in the order the outputs are
+/// arranged on the desk, so with two monitors the left one owns 1–5 and the
+/// right 6–9. Contiguous rather than interleaved because the keys are what the
+/// user reaches for: 1–5 under the left hand for the left screen reads as one
+/// screen's worth of workspaces, 1,3,5,7,9 does not.
+///
+/// A lone output owns every tag, which is what makes the split invisible on a
+/// laptop with nothing plugged in.
+pub fn tagsForOutput(index: usize, count: usize) u32 {
+    if (count <= 1 or index >= count) return all_tags;
+
+    // More outputs than tags: one each, and the outputs left over share the
+    // last tag rather than getting none. An output owning no tag could show no
+    // window at all, which is worse than two screens showing the same one.
+    if (count >= tag_count) {
+        return @as(u32, 1) << @intCast(@min(index, tag_count - 1));
+    }
+
+    // Earlier outputs take one of the leftover tags each, so the ranges differ
+    // by at most one and it is never the first screen that comes up short.
+    const base = tag_count / count;
+    const rem = tag_count % count;
+    const start = index * base + @min(index, rem);
+    const len = base + @as(usize, @intFromBool(index < rem));
+
+    const ones: u32 = (@as(u32, 1) << @intCast(len)) - 1;
+    return ones << @intCast(start);
+}
+
+/// The lowest tag in a mask, as a mask of its own. What a view falls back to
+/// when the tags it was showing have moved to another screen.
+pub fn lowestTag(tags: u32) u32 {
+    const t = tags & all_tags;
+    if (t == 0) return 0;
+    return @as(u32, 1) << @intCast(@ctz(t));
+}
+
 /// Keyboard modifiers, matching the values of river_seat_v1.modifiers so the
 /// mask can be bit-cast straight into the protocol type.
 pub const Mods = struct {