Use fractional coordinates to improve angular movement accuracy

src/libsrc/turtle.pla

@@ -7,7 +7,7 @@ const CLIP_RIGHT  = 2
 const CLIP_TOP    = 4
 const CLIP_BOTTOM = 8
 
-var turtleX, turtleY, turtleAngle, drawLine
+var turtleXp4, turtleYp4, turtleAngle, drawLine
 byte pen
 var clipLeft, clipRight, clipTop, clipBottom
 
@@ -35,7 +35,7 @@ def lerp(a1, b1, a2, b2, n)#1
     word[2] i32
 
     if b2 == b1; return b1; fin // Horizontal lerp
-    if a2 == a1; return n;  fin // Vertical lerp 
+    if a2 == a1; return n;  fin // Vertical lerp
     loadi16(b2 - b1)
     muli16(n - a1)
     divi16(a2 - a1)
@@ -105,8 +105,8 @@ end
 export def initTurtle(width, height, linefunc)#0
     drawLine     = linefunc
     clipRect(0, 0, width - 1, height - 1)
-    turtleX      = width / 2
+    turtleXp4    = width << 3
-    turtleY      = height / 2
+    turtleYp4    = height << 3
     turtleAngle  = 0
     pen          = 1
 end
@@ -127,18 +127,26 @@ end
 
 export def moveTo(x, y)#0
     if pen
-        clipLine(turtleX, turtleY, x, y)
+        clipLine(turtleXp4 >> 4, turtleYp4 >> 4, x, y)
     fin
-    turtleX = x
+    turtleXp4 = x << 4
-    turtleY = y
+    turtleYp4 = y << 4
 end
 
 export def moveForward(dist)#0
-    moveTo(turtleX + mulcos(dist, turtleAngle), turtleY + mulsin(dist, turtleAngle))
+    var oldX, oldY
+
+    oldX      = turtleXp4 >> 4
+    oldY      = turtleYp4 >> 4
+    turtleXp4 = turtleXp4 + mulcos(dist << 4, turtleAngle)
+    turtleYp4 = turtleYp4 + mulsin(dist << 4, turtleAngle)
+    if pen
+        clipLine(turtleXp4 >> 4, turtleYp4 >> 4, oldX, oldY)
+    fin
 end
 
 export def moveBackward(dist)#0
-    moveTo(turtleX - mulcos(dist, turtleAngle), turtleY - mulsin(dist, turtleAngle))
+    moveForward(-dist)
 end
 
 export def penUp#0