Fix awful.screen.getbycoord() for top-left corner of a screen

The problem was that get_square_distance() made the screen one pixel larger to
the bottom/right than it really was. Thus, the (x+0,y+0)-pixel of a screen that
was below or to the right of some other screen had distance zero to both of
these screens.

This commit fixes the screen size computation and adds a small unit test for
getbycoord() and get_square_distance().

Reported by Elv13 here:
https://github.com/awesomeWM/awesome/pull/878#issuecomment-219272864

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/screen.lua

@@ -68,12 +68,12 @@ function screen.object.get_square_distance(self, x, y)
     if x < geom.x then
         dist_x = geom.x - x
     elseif x >= geom.x + geom.width then
-        dist_x = x - geom.x - geom.width
+        dist_x = x - geom.x - geom.width + 1
     end
     if y < geom.y then
         dist_y = geom.y - y
     elseif y >= geom.y + geom.height then
-        dist_y = y - geom.y - geom.height
+        dist_y = y - geom.y - geom.height + 1
     end
     return dist_x * dist_x + dist_y * dist_y
 end

spec/awful/screen_spec.lua

@@ -0,0 +1,81 @@
+---------------------------------------------------------------------------
+-- @author Uli Schlachter
+-- @copyright 2016 Uli Schlachter
+---------------------------------------------------------------------------
+
+local fake_screens = {}
+
+_G.screen = setmetatable({
+    add_signal = function() end,
+    set_index_miss_handler = function() end,
+    set_newindex_miss_handler = function() end,
+}, {
+    __call = function(_, _, prev)
+        if not prev then
+            return fake_screens[1]
+        end
+        for k, v in ipairs(fake_screens) do
+            if v == prev then
+                return fake_screens[k+1]
+            end
+        end
+    end,
+
+    __index = function(_, arg)
+        if arg == "primary" then
+            return fake_screens[1]
+        end
+        if type(arg) == "number" then
+            return fake_screens[arg]
+        end
+        for _, v in ipairs(fake_screens) do
+            if v == arg then
+                return arg
+            end
+        end
+    end,
+
+    __newindex = error,
+})
+local ascreen = require("awful.screen")
+
+describe("awful.screen", function()
+    describe("two screens each 1x1", function()
+        before_each(function()
+            fake_screens = {
+                { geometry = { x = 0, y = 0, width = 1, height = 1 }, index = 1 },
+                { geometry = { x = 0, y = 1, width = 1, height = 1 }, index = 2 },
+            }
+        end)
+
+        it("get_square_distance", function()
+            assert.is.equal(0, ascreen.object.get_square_distance(fake_screens[1], 0, 0))
+            assert.is.equal(1, ascreen.object.get_square_distance(fake_screens[1], 0, 1))
+            assert.is.equal(2, ascreen.object.get_square_distance(fake_screens[1], 1, 1))
+            assert.is.equal(5, ascreen.object.get_square_distance(fake_screens[1], 2, 1))
+            assert.is.equal(2, ascreen.object.get_square_distance(fake_screens[1], -1, 1))
+
+            assert.is.equal(1, ascreen.object.get_square_distance(fake_screens[2], 0, 0))
+            assert.is.equal(0, ascreen.object.get_square_distance(fake_screens[2], 0, 1))
+            assert.is.equal(1, ascreen.object.get_square_distance(fake_screens[2], 1, 1))
+            assert.is.equal(4, ascreen.object.get_square_distance(fake_screens[2], 2, 1))
+            assert.is.equal(1, ascreen.object.get_square_distance(fake_screens[2], -1, 1))
+        end)
+
+        it("getbycoord", function()
+            -- Some exact matches
+            assert.is.equal(1, ascreen.getbycoord(0, 0))
+            assert.is.equal(2, ascreen.getbycoord(0, 1))
+
+            -- Some "find the closest screen"-cases
+            assert.is.equal(1, ascreen.getbycoord(-1, -1))
+            assert.is.equal(1, ascreen.getbycoord(-1, 0))
+            assert.is.equal(2, ascreen.getbycoord(-1, 1))
+            assert.is.equal(2, ascreen.getbycoord(-1, 2))
+
+            assert.is.equal(2, ascreen.getbycoord(1, 1))
+        end)
+    end)
+end)
+
+-- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80