# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1466189370 -7200
# Node ID b1f2c3de8b1526f4d2ba4a6de88ced78a1091e5f
# Parent  cc151c813762bccb8d9890cc08e5e602f7b09fe5
Introduced a pixel position counter, eliminating the horizontal position counter
and simplifying the logic slightly.

diff -r cc151c813762 -r b1f2c3de8b15 ula.py
--- a/ula.py	Tue Jun 07 14:54:07 2016 +0200
+++ b/ula.py	Fri Jun 17 20:49:30 2016 +0200
@@ -37,6 +37,9 @@
 
 MAX_PIXELPOS = MIN_PIXELPOS + MAX_WIDTH
 
+PIXEL_POSITIONS = 8     # the number of pixel positions per byte
+                        # (doubled or quadrupled in lower resolutions)
+
 SCREEN_LIMIT = 0x8000   # the first address after the screen memory
 MAX_MEMORY = 0x10000    # the number of addressable memory locations
 MAX_RAM = 0x10000       # the number of addressable RAM locations (64Kb in each IC)
@@ -268,10 +271,6 @@
         self.footer = rows * LINES_PER_ROW
         self.margin = MAX_SCANLINE - rows * (LINES_PER_ROW + self.spacing) + self.spacing
 
-        # Internal pixel buffer size.
-
-        self.buffer_limit = 8 / self.depth
-
     def vsync(self, value=0):
 
         "Signal the start of a frame."
@@ -380,7 +379,7 @@
             # Initialise the pixel buffer if appropriate.
 
             if not self.writing_pixels and self.have_pixels:
-                self.xcounter = self.xscale
+                self.pixelpos = 0
                 self.buffer_index = 0
                 self.fill_pixel_buffer()
                 self.writing_pixels = 1
@@ -509,21 +508,19 @@
         # For pixels within the frame, obtain and output the value.
 
         else:
-
-            self.xcounter -= 1
+            self.pixelpos += 1
             self.video.colour = self.buffer[self.buffer_index]
 
             # Scale pixels horizontally, only accessing the next pixel value
             # after the required number of scan positions.
 
-            if self.xcounter == 0:
-                self.xcounter = self.xscale
+            if self.pixelpos % self.xscale == 0:
                 self.buffer_index += 1
 
-                # Handle the buffer empty condition.
+            # Finish writing pixels.
 
-                if self.buffer_index >= self.buffer_limit:
-                    self.writing_pixels = 0
+            if self.pixelpos == PIXEL_POSITIONS:
+                self.writing_pixels = 0
 
         self.x += 1