Use a lookup table for y position rather than a random function in asm

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+## [0.0.3] - 2018-09-17
+
+### Changed
+
+- Use a lookup table for y position rather than a random function in asm
+
 ## [0.0.2] - 2018-07-03
 
 ### Added

src/droplet.asm

@@ -1,6 +1,7 @@
 INCLUDE "gbhw.inc"
 
-MAX_DROPLETS            EQU 10
+MAX_DROPLETS            EQU 20
+INITIAL_DROPLETS        EQU 20
 INITIAL_SPAWN_SPRITE_Y  EQU 8
 IDLE_SPRITE_Y           EQU 0
 
@@ -25,29 +26,53 @@ init_droplets::
     call    mem_Set
 
     xor     a
-    ld      [total_droplets],a
+    ld      [total_droplets], a
     ld      [spawn_delay], a
-    ret
 
+    ; spawn max
+    jr      .skip
+.loop
+    call    spawn_droplet
+.skip
+    ld      a, [total_droplets]
+    cp      MAX_DROPLETS
+    jr      nz, .loop
 
-spawn_droplet::
-    ; has the spawn delay reached zero?
-    ld      a, [spawn_delay]
-    cp      0
-    jr      z, .skip_return
-    dec     a
-    ld      [spawn_delay], a
+    call init_droplets_y
     ret
-.skip_return
-    ; reset the spawn delay with a random range
+
+
+init_droplets_y:
+    ld      hl, droplets
+
+    ld      a, [total_droplets]
+    ld      c, a
+    inc     c
+    jp      .skip
+
+.loop
+    push    hl
     push    bc
-    call    fast_random
+    call    get_random_sprite_y
     pop     bc
+    pop     hl
+
+    ld      [hl+], a
+    inc     hl
+    inc     hl
+    inc     hl
+.skip
+    dec     c
+    jp      nz, .loop
+    ret
 
-    and     %00000011
-    add     16
-    ld      [spawn_delay], a
 
+spawn_droplet::
+    ld      a, [total_droplets]
+    cp      MAX_DROPLETS
+    jr      nz, .skip_return
+    ret
+.skip_return
     ; advance to the next idle droplet
     ld      hl, droplets
 
@@ -61,12 +86,10 @@ spawn_droplet::
     cp      IDLE_SPRITE_Y
     jr      nz, .next
 
-
     ; found an idle droplet
     ; set y
     ld      [hl], INITIAL_SPAWN_SPRITE_Y
 
-
     ; set x to random 0-19 multiplied by 8 to align with tiles
     inc     hl
     push    hl
@@ -107,7 +130,7 @@ spawn_droplet::
     ret
 
 
-; Move each active droplet down and rotate the tile
+; Move each active droplet down and cycle the tile
 ; Speed is based on memory position for variety
 move_droplets::
     ld      hl, droplets
@@ -181,13 +204,13 @@ move_droplets::
 .inc_y
     ld      a, [droplet_sprite_y]
     add     e
-    ; if we are now offscreen, set the droplet to idle
+    ; if we are now offscreen, reset the droplet
     cp      a, SCRN_Y + 16   ; carry flag set if 160 > y
     jp      c, .inc_y_set_idle_skip
     ld      a, [total_droplets]
     dec     a
     ld      [total_droplets], a
-    ld      a, IDLE_SPRITE_Y
+    ld      a, INITIAL_SPAWN_SPRITE_Y
 .inc_y_set_idle_skip
     ld      [droplet_sprite_y], a
 .dont_move:
@@ -197,9 +220,6 @@ move_droplets::
     cp      %00000011
     jp      nz, .dont_cycle_character
 
-    ld      a,  [droplet_sprite_y]
-    cp      IDLE_SPRITE_Y               ; is the droplet idle?
-    jp      nc, .skip_tile_reset
     push    bc
     call    fast_random
     pop     bc
@@ -222,7 +242,6 @@ move_droplets::
     inc     hl
 
 .skip
-
     dec     c
     jp      nz, .loop
     ret

src/main.asm

@@ -34,7 +34,6 @@ start::
     xor     a
     ld      [tile_command_list_length], a
     call    move_droplets
-    call    spawn_droplet
     call    set_droplets_to_bg
     call    update_tile_fade
     call    update_tile_fade

src/random.asm

@@ -1,16 +1,19 @@
 RANDOM_SEED            EQU 10
-RANDOM_LENGTH          EQU 40
+RANDOM_X_LENGTH        EQU 40
+RANDOM_Y_LENGTH        EQU 36
 
 SECTION "random vars", WRAM0
 
 seed:: DS 1
-next_random_offset:: DS 1
+next_random_x_offset:: DS 1
+next_random_y_offset:: DS 1
 
 SECTION "random", ROM0
 
 init_random::
     xor a
-    ld [next_random_offset], a
+    ld [next_random_x_offset], a
+    ld [next_random_y_offset], a
     ld      a, RANDOM_SEED
     ld      [seed],a
     ret
@@ -22,7 +25,7 @@ init_random::
 get_random_sprite_x::
     ld      hl, random_sprite_x      ; get start of random LUT
     ld      b, 0
-    ld      a, [next_random_offset]  ; current offset in LUT
+    ld      a, [next_random_x_offset]  ; current offset in LUT
     ld      c, a
     add     hl, bc
     ld      b, [hl]
@@ -31,11 +34,37 @@ get_random_sprite_x::
     ld      a, c
     inc     a
 
-    cp      a, RANDOM_LENGTH
+    cp      a, RANDOM_X_LENGTH
     jp      nz, .no_reset
     ld      a, 0
 .no_reset
-    ld      [next_random_offset], a
+    ld      [next_random_x_offset], a
+    ld      a, b
+    ret
+
+
+
+; Output:
+;   A - random tile-aligned x coord
+; Destroys:
+;   BC, HL
+get_random_sprite_y::
+    ld      hl, random_sprite_y      ; get start of random LUT
+    ld      b, 0
+    ld      a, [next_random_y_offset]  ; current offset in LUT
+    ld      c, a
+    add     hl, bc
+    ld      b, [hl]
+
+    ; inc offset
+    ld      a, c
+    inc     a
+
+    cp      a, RANDOM_X_LENGTH
+    jp      nz, .no_reset
+    ld      a, 0
+.no_reset
+    ld      [next_random_y_offset], a
     ld      a, b
     ret
 
@@ -74,6 +103,13 @@ fast_random::
 
 ; Table of tile-aligned sprite x values. Two sequences are used to improve
 ; randomness.
+; PowerShell: (0..19 | get-random -count 20 | %{'{0:X}' -f (8 + ($_ * 8)) }) -join ',$'
 random_sprite_x:
 DB $78,$40,$50,$58,$90,$70,$20,$48,$A0,$18,$88,$68,$8,$60,$80,$98,$28,$38,$30,$10
 DB $60,$78,$48,$18,$88,$50,$68,$90,$8,$58,$38,$20,$70,$40,$80,$30,$28,$10,$98,$A0
+
+; Table of tile-aligned sprite y values. Two sequences are used to improve
+; randomness.
+random_sprite_y:
+DB $50,$88,$60,$20,$10,$58,$70,$78,$80,$38,$68,$48,$40,$90,$30,$28,$98,$18
+DB $20,$50,$48,$30,$68,$88,$10,$38,$70,$60,$98,$80,$18,$78,$58,$40,$28,$90