Add support for short/long colors

This adds support to gears.color.parse_color to parse things like "#fff" (one
character per color component, without alpha) and "#ffff0000ffff0000" (four
characters per component, with alpha).

This makes sense on its own, but should also help with
https://github.com/awesomeWM/awesome/issues/585.

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

lib/gears/color.lua

@@ -28,13 +28,28 @@ local pattern_cache
 -- @usage -- This will return 0, 1, 0, 1
 -- gears.color.parse_color("#00ff00ff")
 function color.parse_color(col)
+    -- Get all hex chars
     local rgb = {}
-    for pair in string.gmatch(col, "[^#].") do
-        local i = tonumber(pair, 16)
-        if i then
-            table.insert(rgb, i / 255)
-        end
+    for char in string.gmatch(col, "[^#]") do
+        table.insert(rgb, tonumber(char, 16) / 0xf)
     end
+    -- Merge consecutive values until we have at most four groups (rgba)
+    local factor = 0xf
+    while #rgb > 4 do
+        local merged = {}
+        local key, value = next(rgb, nil)
+        local next_factor = (factor + 1)*(factor + 1) - 1
+        while key do
+            local key2, value2 = next(rgb, key)
+            local v1, v2 = value * factor, value2 * factor
+            local new = v1 * (factor + 1) + v2
+            table.insert(merged, new / next_factor)
+            key, value = next(rgb, key2)
+        end
+        rgb = merged
+        factor = next_factor
+    end
+    -- Add missing groups (missing alpha)
     while #rgb < 4 do
         table.insert(rgb, 1)
     end

spec/gears/color_spec.lua

@@ -11,26 +11,62 @@ describe("gears.color", function()
     describe("parse_color", function()
         local function test(e_r, e_g, e_b, e_a, input)
             local o_r, o_g, o_b, o_a, unused = color.parse_color(input)
-            assert.is.same(o_r, e_r / 255)
-            assert.is.same(o_g, e_g / 255)
-            assert.is.same(o_b, e_b / 255)
+            assert.is.same(o_r, e_r / 0xff)
+            assert.is.same(o_g, e_g / 0xff)
+            assert.is.same(o_b, e_b / 0xff)
+            assert.is.same(o_a, e_a / 0xff)
             assert.is_nil(unused)
         end
 
         it("black", function()
-            test(0, 0, 0, 255, "#000000")
+            test(0, 0, 0, 0xff, "#000000")
         end)
 
         it("opaque black", function()
-            test(0, 0, 0, 255, "#000000ff")
+            test(0, 0, 0, 0xff, "#000000ff")
         end)
 
         it("transparent gray", function()
-            test(128, 128, 128, 128, "#80808080")
+            test(0x80, 0x80, 0x80, 0x80, "#80808080")
         end)
 
         it("transparent white", function()
-            test(255, 255, 255, 127, "#ffffff7f")
+            test(0xff, 0xff, 0xff, 0x7f, "#ffffff7f")
+        end)
+
+        describe("different lengths", function()
+            local function gray(e, e_a, input)
+                local o_r, o_g, o_b, o_a, unused = color.parse_color(input)
+                assert.is.same(o_r, e)
+                assert.is.same(o_g, e)
+                assert.is.same(o_b, e)
+                assert.is.same(o_a, e_a)
+                assert.is_nil(unused)
+            end
+
+            it("rgb", function()
+                gray(0x8/0xf, 1, "#888")
+            end)
+
+            it("rgba", function()
+                gray(0x8/0xf, 0x8/0xf, "#8888")
+            end)
+
+            it("rrggbb", function()
+                gray(0x80/0xff, 1, "#808080")
+            end)
+
+            it("rrggbbaa", function()
+                gray(0x80/0xff, 0x80/0xff, "#80808080")
+            end)
+
+            it("rrrrggggbbbb", function()
+                gray(0x8000/0xffff, 1, "#800080008000")
+            end)
+
+            it("rrrrggggbbbbaaaa", function()
+                gray(0x8000/0xffff, 0x8000/0xffff, "#8000800080008000")
+            end)
         end)
     end)