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Draw the opposite arrow only while dragging

--- a/galaxies.c

+++ b/galaxies.c

@@ -2310,6 +2310,7 @@

     unsigned long *grid;

     int *dx, *dy;

     blitter *bl;

+    blitter *blmirror;

     int dragging, dragx, dragy;

@@ -2927,6 +2928,9 @@

     assert(!ds->bl);

     ds->bl = blitter_new(dr, TILE_SIZE, TILE_SIZE);

+    assert(!ds->blmirror);

+    ds->blmirror = blitter_new(dr, TILE_SIZE, TILE_SIZE);

+

     assert(!ds->cur_bl);

     ds->cur_bl = blitter_new(dr, TILE_SIZE, TILE_SIZE);

 }

@@ -3001,6 +3005,7 @@

     ds->dy = snewn(ds->w*ds->h, int);

     ds->bl = NULL;

+    ds->blmirror = NULL;

     ds->dragging = FALSE;

     ds->dragx = ds->dragy = 0;

@@ -3017,6 +3022,7 @@

 {

     if (ds->cur_bl) blitter_free(dr, ds->cur_bl);

     sfree(ds->colour_scratch);

+    if (ds->blmirror) blitter_free(dr, ds->blmirror);

     if (ds->bl) blitter_free(dr, ds->bl);

     sfree(ds->dx);

     sfree(ds->dy);

@@ -3142,6 +3148,16 @@

     draw_update(dr, lx, ly, TILE_SIZE, TILE_SIZE);

 }

+static void calculate_opposite_point(const game_ui *ui,

+                                     const game_drawstate *ds, const int x,

+                                     const int y, int *oppositex,

+                                     int *oppositey)

+{

+    /* oppositex - dotx = dotx - x <=> oppositex = 2 * dotx - x */

+    *oppositex = 2 * SCOORD(ui->dotx) - x;

+    *oppositey = 2 * SCOORD(ui->doty) - y;

+}

+

 static void game_redraw(drawing *dr, game_drawstate *ds,

                         const game_state *oldstate, const game_state *state,

                         int dir, const game_ui *ui,

@@ -3149,6 +3165,7 @@

 {

     int w = ds->w, h = ds->h;

     int x, y, flashing = FALSE;

+    int oppx, oppy;

     if (flashtime > 0) {

         int frame = (int)(flashtime / FLASH_TIME);

@@ -3157,8 +3174,15 @@

     if (ds->dragging) {

         assert(ds->bl);

+        assert(ds->blmirror);

+        calculate_opposite_point(ui, ds, ds->dragx + TILE_SIZE/2,

+                                 ds->dragy + TILE_SIZE/2, &oppx, &oppy);

+        oppx -= TILE_SIZE/2;

+        oppy -= TILE_SIZE/2;

         blitter_load(dr, ds->bl, ds->dragx, ds->dragy);

         draw_update(dr, ds->dragx, ds->dragy, TILE_SIZE, TILE_SIZE);

+        blitter_load(dr, ds->blmirror, oppx, oppy);

+        draw_update(dr, oppx, oppy, TILE_SIZE, TILE_SIZE);

         ds->dragging = FALSE;

     }

     if (ds->cur_visible) {

@@ -3313,10 +3337,13 @@

         ds->dragging = TRUE;

         ds->dragx = ui->dx - TILE_SIZE/2;

         ds->dragy = ui->dy - TILE_SIZE/2;

+        calculate_opposite_point(ui, ds, ui->dx, ui->dy, &oppx, &oppy);

         blitter_save(dr, ds->bl, ds->dragx, ds->dragy);

-        draw_arrow(dr, ds, ui->dx, ui->dy,

-                   SCOORD(ui->dotx) - ui->dx,

+        blitter_save(dr, ds->blmirror, oppx - TILE_SIZE/2, oppy - TILE_SIZE/2);

+        draw_arrow(dr, ds, ui->dx, ui->dy, SCOORD(ui->dotx) - ui->dx,

                    SCOORD(ui->doty) - ui->dy, COL_ARROW);

+        draw_arrow(dr, ds, oppx, oppy, SCOORD(ui->dotx) - oppx,

+                   SCOORD(ui->doty) - oppy, COL_ARROW);

     }

 #ifdef EDITOR

     {