Add testGraphics.lua

Fix bug on exactly-zero/exactly-400 ballX

src/graphics.lua

@@ -10,7 +10,7 @@ function getDrawOffset(ballX, ballY)
     -- Keep the ball approximately in the center, once it's past C.Center.y - 30
     offsetY = math.max(0, (-1 * ballY) + C.Center.y - 30)
 
-    if ballX > 0 and ballX < C.Screen.W then
+    if ballX >= 0 and ballX <= C.Screen.W then
         offsetX = 0
     elseif ballX < 0 then
         offsetX = math.max(-1 * C.Screen.W, ballX * -1)

src/test/testGraphics.lua

@@ -0,0 +1,37 @@
+require("test/setup")
+require("graphics")
+
+local function assertSmallDifference(previous, current, ballValue, ballLabel)
+    local difference = math.abs(previous - current)
+    local baseError = "Expected a small difference, but received a difference of "
+    local fullDetails = luaunit.prettystr({ previous = previous, current = current, [ballLabel] = ballValue })
+    luaunit.assertIsTrue(difference < 2, baseError .. difference .. ":\n  " .. luaunit.prettystr(fullDetails))
+end
+
+function testNoJumpsInYOffset()
+    local startY = -240 * 3
+    local atXValues = { -400, 0, 400 }
+    for _, xValue in ipairs(atXValues) do
+        local _, lastYOffset = getDrawOffset(xValue, startY)
+        for ballY = startY, 240 do
+            local _, currentYOffset = getDrawOffset(xValue, ballY)
+            assertSmallDifference(lastYOffset, currentYOffset, ballY, "ballY")
+            lastYOffset = currentYOffset
+        end
+    end
+end
+
+function testNoJumpsInXOffset()
+    local startX = -800
+    local atYValues = { 240, 0, -240 }
+    for _, yValue in ipairs(atYValues) do
+        local lastXOffset = getDrawOffset(startX, yValue)
+        for ballX = startX, 800 do
+            local currentXOffset = getDrawOffset(ballX, yValue)
+            assertSmallDifference(lastXOffset, currentXOffset, ballX, "ballX")
+            lastXOffset = currentXOffset
+        end
+    end
+end
+
+os.exit(luaunit.LuaUnit.run())