Add some randomness when the spider changes direction.

BuGS/gameSpider.s

@@ -37,6 +37,7 @@ SPIDER_RHS_TILE_OFFSET      equ SPIDER_TOP_ROW_OFFSET+(GAME_NUM_TILES_WIDE-1)*SI
 SPIDER_STARTING_SHIFT               equ 2
 SPIDER_LHS_STARTING_SCREEN_OFFSET   equ SCREEN_BYTES_PER_ROW*SPIDER_STARTING_SHIFT+6
 
+; Every four frames, change the spider sprite
 SPIDER_SPRITE_REFRESH_COUNT     equ 4
 
 
@@ -366,7 +367,7 @@ updateSpider_tilesDown anop
         stx spiderTileOffsets+2
         lda tiles+TILE_BELOW_OFFSET,x
         cmp #INVALID_TILE_NUM
-        beq updateSpider_offScreen
+        beq updateSpider_tilesDownSkip
         sta spiderTileOffsets
         
         ldx spiderTileOffsets+4
@@ -374,6 +375,8 @@ updateSpider_tilesDown anop
         lda tiles+TILE_BELOW_OFFSET,x
         sta spiderTileOffsets+4
         
+; If the middle tile is a game time and it isn't empty, then
+; empty it.  Spiders "consume" mushrooms as they pass over them.
         cpx #RHS_FIRST_TILE_OFFSET
         bge updateSpider_tilesDownCont
         lda tiles+TILE_TYPE_OFFSET,x
@@ -387,6 +390,13 @@ updateSpider_tilesDownCont anop
         lda tiles+TILE_BELOW_OFFSET,x
         sta spiderTileOffsets+8
         
+updateSpider_tilesDownSkip anop
+        lda spiderCurrentRow
+        inc a
+        sta spiderCurrentRow
+        cmp spiderTargetRow
+        beq updateSpider_offScreen
+        
         rtl
         
 updateSpider_offScreen anop
@@ -437,6 +447,14 @@ addSpider entry
         lda #SPIDER_SPRITE_REFRESH_COUNT
         sta spiderSpriteRefresh
         
+        lda #SPIDER_TOP_ROW-1
+        sta spiderCurrentRow
+        
+        lda #SPIDER_NUM_POSSIBLE_ROWS-1
+        jsl randN
+        adc #SPIDER_TOP_ROW+1
+        sta spiderTargetRow
+        
 addSpider_done anop
         rtl
         
@@ -453,6 +471,9 @@ spiderScreenOffset  dc i2'0'
 spiderScreenShift   dc i2'0'
 spiderShiftInTile   dc i2'0'
 
+spiderCurrentRow    dc i2'0'
+spiderTargetRow     dc i2'0'
+
 
 ;  10  6  2
 ;   8  4  0

BuGS/random.s

@@ -51,6 +51,7 @@ randInit_store16 anop
         
         rtl
 
+
 ; Returns a number from 0 to 14
 rand0_to_14  entry
         lda seed16
@@ -62,6 +63,21 @@ skipEor16 anop
         dec a
         rtl
         
+; Returns a number from 0 to N-1 where N is <= 15, arrives in the accumulator
+randN entry
+        cmp #2
+        bge randN_doit
+        lda #0
+        rtl
+        
+randN_doit anop
+        sta upperLimit
+randN_retry anop
+        jsl rand0_to_14
+        cmp upperLimit
+        bge randN_retry
+        rtl
+
 
 ; Returns a number from 0 to 30
 rand0_to_30 entry
@@ -94,7 +110,9 @@ skipEor65535 anop
         rtl
         
         
-seed65535 dc i2'0'
-seed32    dc i2'0'
-seed16    dc i2'0'
+seed65535   dc i2'0'
+seed32      dc i2'0'
+seed16      dc i2'0'
+
+upperLimit  dc i2'0'
         end