# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1541865738 -3600
# Node ID 39834d788dcc06bbb37cfe1be562d41c3a7d3c56
# Parent  9e84b8d477a3cf4af12ebf3d1a8c76bc3dbf26fb
Introduced an abstraction for stored screen region details.

diff -r 9e84b8d477a3 -r 39834d788dcc examples/vga/main.c
--- a/examples/vga/main.c	Fri Nov 09 22:50:30 2018 +0100
+++ b/examples/vga/main.c	Sat Nov 10 17:02:18 2018 +0100
@@ -115,25 +115,65 @@
 
 
 
+/* Stored display regions. */
+
+typedef struct
+{
+    /* Stored region overplotted by the sprite. */
+
+    uint8_t *image;
+    int stored;
+
+    /* Position of the stored region. */
+
+    int x, y;
+
+} stored_region_t;
+
+/* Initialise the stored regions. */
+
+void init_stored_regions(stored_region_t *r, int frames,
+                         uint8_t *regions, uint32_t region_size)
+{
+    int frame;
+
+    for (frame = 0; frame < frames; frame++)
+    {
+        r->stored = 0;
+        r->image = regions;
+        regions += region_size;
+        r++;
+    }
+}
+
+
+
 /* Copy to the store from the display, then blit the image. */
 
-static void plot_sprite(uint8_t *background, int x, int y)
+static void plot_sprite(stored_region_t *r, int x, int y, int key)
 {
-    display_copy(&display_config, background,
+    display_copy(&display_config, r->image,
                  sprite_width, sprite_height / SOURCE_YSTEP, 1,
                  x, y, -1, 0);
     display_copy(&display_config, sprite,
                  sprite_width, sprite_height, SOURCE_YSTEP,
-                 x, y, 0x8c, 1);
+                 x, y, key, 1);
+
+    /* Record the stored background. */
+
+    r->x = x;
+    r->y = y;
+    r->stored = 1;
 }
 
 /* Copy to the display from the store, restoring the original background. */
 
-static void unplot_sprite(uint8_t *background, int x, int y)
+static void unplot_sprite(stored_region_t *r)
 {
-    display_copy(&display_config, background,
-                 sprite_width, sprite_height / SOURCE_YSTEP, 1,
-                 x, y, -1, 1);
+    if (r->stored)
+        display_copy(&display_config, r->image,
+                     sprite_width, sprite_height / SOURCE_YSTEP, 1,
+                     r->x, r->y, -1, 1);
 }
 
 /* Plot the revealed region at the edge of the screen after scrolling. */
@@ -211,14 +251,10 @@
 
 static void animate(uint32_t delay)
 {
-    /* Stored region behind the sprite. */
-
-    uint8_t background[FRAME_COUNT][(sprite_width * sprite_height) / SOURCE_YSTEP];
+    /* Stores of background details, replotted when moving the sprite. */
 
-    /* Positions of the stored regions for each frame. */
-
-    int background_x[FRAME_COUNT], background_y[FRAME_COUNT];
-    int background_stored[FRAME_COUNT];
+    uint8_t backgrounds[FRAME_COUNT][(sprite_width * sprite_height) / SOURCE_YSTEP];
+    stored_region_t regions[FRAME_COUNT];
 
     /* Sprite position. */
 
@@ -237,18 +273,14 @@
 
     viewport_t v;
 
-    /* Initialise the backing stores. */
-
-    int frame;
-
-    for (frame = 0; frame < display_config.frames; frame++)
-        background_stored[frame] = 0;
-
     /* Initialise the viewport. */
 
     init_viewport(&v, &display_config, xorigins, yorigins,
                   SCROLL_XSTEP, SOURCE_YSTEP, plot_screen_edge);
 
+    init_stored_regions(regions, display_config.frames, (uint8_t *) backgrounds,
+                        (sprite_width * sprite_height) / SOURCE_YSTEP);
+
     /* Animation loop. */
 
     while (1)
@@ -257,13 +289,7 @@
         {
             for (x = 0; x < display_config.line_length - sprite_width; x++)
             {
-                plot_sprite(background[display_config.frame], x, y);
-
-                /* Record the stored background. */
-
-                background_x[display_config.frame] = x;
-                background_y[display_config.frame] = y;
-                background_stored[display_config.frame] = 1;
+                plot_sprite(&regions[display_config.frame], x, y, 0x8c);
 
                 /* Update the display with the frame details. */
 
@@ -276,10 +302,7 @@
 
                 /* Prepare the frame for updates. */
 
-                if (background_stored[display_config.frame])
-                    unplot_sprite(background[display_config.frame],
-                                  background_x[display_config.frame],
-                                  background_y[display_config.frame]);
+                unplot_sprite(&regions[display_config.frame]);
 
                 /* Scroll in the indicated direction. */