Simplify the random_valid_y_coordinate function

This function relies on a pre-computed table to get the "random" numbers
from, but most of the times this is used, it needs to be between some
safe boundaries.

Before this commit, this was done inside of the function, correcting the
fetched value to be above or below these limits. But there was a bug on
the below the grounds limit, in which the 'sbc' instruction could
subtract too much from the fetched value and make enemies appear below
the sky.

Now, this is easy to correct, but since we are cheating with a
pre-computed table, I thought it would be vastly easier to just get a
random table with the "proper" values. That is, random but within the
required boundaries already. Plus, it makes the
'random_valid_y_coordinate' function much faster.

This commit adds a ruby script in bin/ which produces a pseudo-random
table when called, making sure all the requirements are met. We have to
make sure to call this script if we ever change the boundaries at some
point.

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>

3 files changed, 42 insertions, 38 deletions

diff --git a/bin/rand.rb b/bin/rand.rb
new file mode 100644
index 0000000..f627748
--- /dev/null
+++ b/bin/rand.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+# See values on background.s
+UPPER_MARGIN_Y_COORD = 0x1A
+GROUND_Y_COORD = 0xC8 - 32 # NOTE: As in background.s - twice the size of the enemy.
+
+# The available values for the Y axis for enemies are above ground, below the
+# sky, and avoiding the left-most and right-most platforms.
+available = (UPPER_MARGIN_Y_COORD..GROUND_Y_COORD).to_a - (0x58..0x69).to_a - (0x40..0x50).to_a
+
+# With this produce the array containing a randomized sample from the
+# 'available' values.
+random_byte_array = Array.new(256) { '$%02X' % available.sample }
+
+# And now print it in the assembler format.
+random_byte_array.each_slice(16) do |row|
+  puts ".byte #{row.join(', ')}"
+end
diff --git a/src/background.s b/src/background.s
index a9dfbaf..0facde6 100644
--- a/src/background.s
+++ b/src/background.s
@@ -3,9 +3,15 @@
 .scope Background
     ;; Screen coordinate on the Y axis where elements can begin to appear (e.g.
     ;; upper bound for new enemies, starting point for falling items, etc.).
+    ;;
+    ;; NOTE: if you change this value, you should re-generate the random values
+    ;; from prng.s as well.
     UPPER_MARGIN_Y_COORD = $1A
 
     ;; Screen coordinates on the Y axis for the ground.
+    ;;
+    ;; NOTE: if you change this value, you should re-generate the random values
+    ;; from prng.s as well.
     GROUND_Y_COORD = $C8
 
     ;; Returns whether the given tile position collides with a background
diff --git a/src/prng.s b/src/prng.s
index e2653b7..8211957 100644
--- a/src/prng.s
+++ b/src/prng.s
@@ -15,48 +15,28 @@
     .proc random_valid_y_coordinate
         ;; Get the new random number and store it right away.
         ldx zp_rand
-        lda rand_table, x
+        lda valid_y_rand_table, x
         sta zp_rand
-
-        ;; Is this value below ground?
-        cmp #(Background::GROUND_Y_COORD - 16)
-        bcc @check_sky
-
-        ;; Yes! Return something that is at least above it.
-        sec
-        sbc #(Background::GROUND_Y_COORD - 16)
-        rts
-
-    @check_sky:
-        ;; Is this value above the upper screen margin?
-        cmp #Background::UPPER_MARGIN_Y_COORD
-        bcs @end
-
-        ;; Yes! Return something that is at least below it.
-        clc
-        adc #Background::UPPER_MARGIN_Y_COORD
-
-    @end:
         rts
     .endproc
 .endscope
 
 
 ;; The pre-computed table.
-rand_table:
-    .byte $D7, $3A, $1C, $8F, $09, $B2, $E6, $54, $A3, $91, $2B, $F5, $78, $0D, $4C, $6E
-    .byte $FF, $C0, $52, $33, $6A, $E9, $9B, $1A, $47, $88, $7D, $21, $0E, $F4, $B3, $9C
-    .byte $15, $67, $A8, $41, $D2, $39, $80, $76, $C9, $E5, $0A, $1B, $5F, $22, $73, $DA
-    .byte $B4, $96, $3C, $E0, $8D, $F7, $2A, $05, $9E, $43, $11, $6D, $A7, $58, $C1, $32
-    .byte $28, $0F, $79, $BE, $51, $64, $9D, $A9, $3B, $71, $8E, $C6, $4A, $13, $F0, $27
-    .byte $E2, $5C, $06, $D3, $95, $B8, $4F, $70, $19, $A4, $6B, $38, $82, $C7, $5E, $01
-    .byte $F3, $2D, $9A, $65, $7C, $D1, $0B, $E8, $57, $36, $84, $1F, $B0, $92, $45, $AC
-    .byte $60, $7E, $A1, $53, $C8, $29, $D4, $FB, $07, $42, $E3, $99, $16, $8A, $3D, $C5
-    .byte $24, $B1, $6F, $03, $7A, $E7, $8C, $59, $D0, $46, $93, $1E, $A5, $2C, $B7, $F1
-    .byte $89, $55, $C3, $30, $62, $98, $04, $D6, $7F, $A0, $E4, $12, $3B, $81, $F9, $23
-    .byte $C4, $0D, $5A, $71, $9F, $B6, $2E, $85, $37, $A9, $18, $6C, $E1, $4B, $D9, $02
-    .byte $F8, $63, $B5, $40, $97, $0C, $7A, $51, $A2, $3E, $8F, $D5, $14, $69, $E0, $B8
-    .byte $4D, $77, $25, $9B, $0A, $F2, $3C, $86, $E9, $1F, $68, $A3, $50, $C1, $7D, $04
-    .byte $B2, $8E, $56, $1D, $73, $9C, $F5, $2A, $61, $D7, $09, $3E, $84, $A0, $E6, $1B
-    .byte $3F, $C8, $94, $05, $72, $D6, $A7, $4C, $1A, $5F, $B3, $29, $80, $E1, $6D, $9E
-    .byte $0C, $43, $F7, $8B, $52, $16, $A8, $3D, $91, $2B, $E5, $70, $C6, $4A, $D9, $F8
+valid_y_rand_table:
+    .byte $25, $87, $B7, $6A, $23, $77, $6D, $71, $6D, $B6, $86, $93, $2B, $97, $A8, $39
+    .byte $26, $AE, $A6, $70, $9F, $2D, $74, $B2, $8E, $A5, $33, $3E, $6D, $75, $91, $6B
+    .byte $A1, $2E, $A4, $7C, $53, $28, $1E, $79, $1B, $6B, $A9, $7E, $76, $74, $90, $B3
+    .byte $26, $9B, $2C, $94, $9C, $86, $7A, $B7, $1D, $B4, $72, $82, $7C, $23, $33, $78
+    .byte $B0, $70, $B5, $A9, $1B, $70, $83, $28, $73, $24, $7E, $28, $B1, $8A, $75, $9B
+    .byte $20, $26, $22, $8D, $7C, $3F, $29, $3D, $1F, $72, $22, $A4, $86, $34, $6F, $9C
+    .byte $20, $A9, $3A, $77, $39, $6F, $3B, $86, $95, $3D, $96, $9F, $56, $84, $53, $77
+    .byte $B2, $23, $72, $1C, $99, $B4, $B8, $32, $8E, $A3, $21, $20, $25, $97, $92, $2C
+    .byte $3E, $96, $B5, $28, $84, $7E, $A7, $8A, $A3, $3E, $A4, $72, $A7, $A8, $6D, $A3
+    .byte $AE, $7E, $23, $6E, $6A, $72, $35, $3C, $99, $B3, $37, $22, $7F, $79, $1C, $97
+    .byte $52, $77, $77, $95, $B4, $7F, $2D, $28, $71, $9E, $76, $9A, $A0, $32, $23, $7F
+    .byte $94, $9F, $90, $3C, $AF, $A7, $9C, $B5, $87, $9A, $9E, $84, $B0, $AD, $7D, $9D
+    .byte $A2, $24, $89, $6A, $79, $81, $A2, $1E, $A9, $1E, $AB, $8D, $6E, $54, $90, $AE
+    .byte $A4, $B5, $1E, $B4, $53, $9D, $7D, $78, $74, $AD, $6A, $96, $2F, $6F, $29, $9F
+    .byte $98, $1D, $3B, $57, $6D, $B0, $1C, $88, $B0, $55, $99, $8F, $B1, $3F, $AD, $23
+    .byte $92, $7A, $26, $55, $A3, $75, $34, $23, $20, $79, $B0, $A6, $B6, $2A, $20, $8C