From fb23cf51040f06bfcfbaf318d7b452d76ffbedfb Mon Sep 17 00:00:00 2001
From: Miquel Sabaté Solà <mssola@mssola.com>
Date: Fri, 20 Mar 2026 21:52:36 +0100
Subject: Try to initialize enemies on unique Y coordinates
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That is, our PRNG algorithm is so stupid that sometimes it cycles into
the same value multiple times. While play testing, sometimes we were so
unlucky that we got all enemies on the same Y screen coordinate.

As funny as these situations can be, this shouldn't happen, so I have
introduced an (awesomely named) function that makes a harder effort at
finding a unique random number.

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>
---
 src/enemies.s |  2 +-
 src/jetpac.s  |  1 +
 src/prng.s    | 35 +++++++++++++++++++++++++++++++++++
 3 files changed, 37 insertions(+), 1 deletion(-)

(limited to 'src')

diff --git a/src/enemies.s b/src/enemies.s
index 6201416..2208758 100644
--- a/src/enemies.s
+++ b/src/enemies.s
@@ -244,7 +244,7 @@
 
         ;; The Y coordinate is also set at random within the bounds of the
         ;; playable screen.
-        jsr Prng::random_valid_y_coordinate
+        jsr Prng::random_non_repeated_valid_y_coordinate
         ldx Globals::zp_tmp0
         inx
         sta Enemies::zp_pool_base, x
diff --git a/src/jetpac.s b/src/jetpac.s
index f99c8b0..3cf8f0a 100644
--- a/src/jetpac.s
+++ b/src/jetpac.s
@@ -130,6 +130,7 @@
     sta Joypad::zp_prev
     sta Player::zp_state
     sta Items::zp_state
+    sta Prng::zp_last_rand
 
     ;; Initialize the level. We allow the build system to pass its own value for
     ;; this in `LEVEL`, just in case we want to debug the enemy of a specific
diff --git a/src/prng.s b/src/prng.s
index c5ec474..7aa313a 100644
--- a/src/prng.s
+++ b/src/prng.s
@@ -9,6 +9,14 @@
     ;; Current random value. Initialized on the title to game transition.
     zp_rand = $0A
 
+    ;; Last random value as stored by
+    ;; Prng::random_non_repeated_valid_y_coordinate().
+    zp_last_rand = $12
+
+    ;; How many attempts should Prng::random_non_repeated_valid_y_coordinate()
+    ;; take to find a unique value.
+    RAND_ATTEMPTS = 3
+
     ;; 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.
@@ -21,6 +29,33 @@
         sta zp_rand
         rts
     .endproc
+
+    ;; Calls Prng::random_valid_y_coordinate() multiple times until we get a
+    ;; different value than what we got the last time we called this
+    ;; function. Realistically this should just take 1 attempt for most cases,
+    ;; and in the worst case just 2. Nevertheless, we have set a generous value
+    ;; to 'RAND_ATTEMPTS' just in case.
+    ;;
+    ;; NOTE: the 'y' register is preserved.
+    .proc random_non_repeated_valid_y_coordinate
+        tya
+        pha
+        ldy #RAND_ATTEMPTS
+
+    @again:
+        jsr Prng::random_valid_y_coordinate
+        cmp Prng::zp_last_rand
+        bne @end
+        dey
+        bne @again
+
+    @end:
+        sta Prng::zp_last_rand
+        pla
+        tay
+        lda Prng::zp_last_rand
+        rts
+    .endproc
 .endscope
 
 
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