From 638dfdca6e773d4b1ec573606cac90bdb1778667 Mon Sep 17 00:00:00 2001
From: Miquel Sabaté Solà <mikisabate@gmail.com>
Date: Thu, 15 May 2025 22:06:00 +0200
Subject: Initialize enemies at a random Y coordinate
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The randomness comes from a pre-computed table of "random" numbers. This
is of course the most simple technique you can come across a code base
for the NES/Famicom, but for the purposes of this game is good enough
and (most importantly) fast.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>
---
 src/background.s |  4 ++++
 src/enemies.s    |  4 +++-
 src/jetpac.s     |  1 +
 src/prng.s       | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/vectors.s    |  6 ++++++
 5 files changed, 76 insertions(+), 1 deletion(-)
 create mode 100644 src/prng.s

diff --git a/src/background.s b/src/background.s
index add7059..26d38c6 100644
--- a/src/background.s
+++ b/src/background.s
@@ -1,6 +1,10 @@
 .segment "CODE"
 
 .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.).
+    UPPER_MARGIN_Y_COORD = $10
+
     ;; Screen coordinates on the Y axis for the ground.
     GROUND_Y_COORD = $C8
 
diff --git a/src/enemies.s b/src/enemies.s
index ab67ccd..377a9a5 100644
--- a/src/enemies.s
+++ b/src/enemies.s
@@ -25,7 +25,9 @@
         sta zp_enemies_pool_base, x
 
         inx
-        lda #$80                ; TODO: random
+        stx Globals::zp_tmp0
+        jsr Prng::random_valid_y_coordinate
+        ldx Globals::zp_tmp0
         sta zp_enemies_pool_base, x
 
         inx
diff --git a/src/jetpac.s b/src/jetpac.s
index 3b7ac18..6708ed3 100644
--- a/src/jetpac.s
+++ b/src/jetpac.s
@@ -39,6 +39,7 @@
 
 .include "assets.s"
 .include "background.s"
+.include "prng.s"
 .include "player.s"
 .include "bullets.s"
 .include "enemies.s"
diff --git a/src/prng.s b/src/prng.s
new file mode 100644
index 0000000..e2653b7
--- /dev/null
+++ b/src/prng.s
@@ -0,0 +1,62 @@
+;;;
+;; Pseudo-random number implementation by using a pre-calculated table.
+;;
+;; I know, not too flashy, but it's good enough for this game :)
+
+.segment "CODE"
+
+.scope Prng
+    ;; Current random value. Initialized on the title to game transition.
+    zp_rand = $0A
+
+    ;; Updates the 'a' register with the next random number set after the
+    ;; current value of `zp_rand`, while also making sure that it is a valid
+    ;; screen Y coordinate.
+    .proc random_valid_y_coordinate
+        ;; Get the new random number and store it right away.
+        ldx zp_rand
+        lda 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
diff --git a/src/vectors.s b/src/vectors.s
index 75de30a..77ce0bc 100644
--- a/src/vectors.s
+++ b/src/vectors.s
@@ -104,6 +104,12 @@
     .endif
 
     ;; TODO: some actions here will depend on the status of the game...
+    lda Globals::zp_flags
+    and #%00000001
+    bne @ppu_registers
+
+    ;; Increase the random seed.
+    inc Prng::zp_rand
 
     ;; Decrease title timer.
     lda Title::zp_title_timer
-- 
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