gears.matrix: Add create_rotate_at() (#1181)

We already have a variant of this function for transforming an actual
matrix. This adds the corresponding static factory.

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

lib/gears/matrix.lua

@@ -51,6 +51,17 @@ function matrix.create_rotate(angle)
     return matrix.create(c, s, -s, c, 0, 0)
 end
 
+--- Create a new rotation matrix rotating around a custom point
+-- @tparam number x The horizontal rotation point
+-- @tparam number y The vertical rotation point
+-- @tparam number angle The angle of the rotation in radians.
+-- @return A new matrix describing the given transformation.
+function matrix.create_rotate_at(x, y, angle)
+    return   matrix.create_translate( -x, -y )
+           * matrix.create_rotate   ( angle  )
+           * matrix.create_translate(  x,  y )
+end
+
 --- Translate this matrix
 -- @tparam number x The translation in x direction.
 -- @tparam number y The translation in y direction.
@@ -80,9 +91,7 @@ end
 -- @tparam number angle The angle (in radiant: -2*math.pi to 2*math.pi)
 -- @return A transformation object
 function matrix:rotate_at(x, y, angle)
-    return self * matrix.create_translate( -x, -y )
+    return self * matrix.create_rotate_at(x, y, angle)
-                * matrix.create_rotate   ( angle  )
-                * matrix.create_translate(  x,  y )
 end
 
 --- Invert this matrix

spec/gears/matrix_spec.lua

@@ -150,6 +150,30 @@ describe("gears.matrix", function()
         assert.is.equal(0, m.x0)
         assert.is.equal(0, m.y0)
     end)
+
+    describe("rotate_at", function()
+        it("create", function()
+            local m = matrix.create_rotate_at(5, 5, math.pi)
+            assert.is.equal(-1, m.xx)
+            -- Stupid rounding issues...
+            assert.is_true(math.abs(round(m.xy) - m.xy) < 0.00000001)
+            assert.is_true(math.abs(round(m.yx) - m.yx) < 0.00000001)
+            assert.is.equal(-1, m.yy)
+            assert.is.equal(10, m.x0)
+            assert.is.equal(10, m.y0)
+        end)
+
+        it("multiply", function()
+            local m = matrix.create_translate(-5, 0):rotate_at(5, 5, math.pi)
+            assert.is.equal(-1, m.xx)
+            -- Stupid rounding issues...
+            assert.is_true(math.abs(round(m.xy) - m.xy) < 0.00000001)
+            assert.is_true(math.abs(round(m.yx) - m.yx) < 0.00000001)
+            assert.is.equal(-1, m.yy)
+            assert.is.equal(15, m.x0)
+            assert.is.equal(10, m.y0)
+        end)
+    end)
 end)
 
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