From 6fafe08b7dc884960582d272d35650d1c9dfdf87 Mon Sep 17 00:00:00 2001
From: "Juan J. Martinez" <jjm@usebox.net>
Date: Thu, 11 May 2023 23:00:53 +0100
Subject: Add some randomness

---
 game/main.asm      |  1 +
 game/random.asm    | 24 ++++++++++++++++++++++++
 game/starfield.asm | 10 ++++++++++
 3 files changed, 35 insertions(+)
 create mode 100644 game/random.asm

(limited to 'game')

diff --git a/game/main.asm b/game/main.asm
index e779a95..cdf0844 100644
--- a/game/main.asm
+++ b/game/main.asm
@@ -52,6 +52,7 @@ fill_loop:
 int_handler:
     iret
 
+.include "random.asm"
 .include "starfield.asm"
 .include "entities.asm"
 .include "player.asm"
diff --git a/game/random.asm b/game/random.asm
new file mode 100644
index 0000000..0cb74db
--- /dev/null
+++ b/game/random.asm
@@ -0,0 +1,24 @@
+;
+; Random number generator
+;
+; x[i + 1] = (5 * x[i] + 1) mod 256
+;
+; out: y random value
+rnd:
+    ld x, <seed
+    ld a, >seed
+
+    ld y, [a : x]
+    ld b, y
+
+    add y, y
+    add y, y
+    add y, b
+    add y, 7
+
+    ld [a : x], y
+
+    ret
+
+seed:
+    .db 0
diff --git a/game/starfield.asm b/game/starfield.asm
index e8b8ed1..ac8fe32 100644
--- a/game/starfield.asm
+++ b/game/starfield.asm
@@ -40,6 +40,16 @@ stars_update_addr:
     cmp y, 0x80
     bc
     ld b, 0xbf
+
+    ; add some randomness
+    push a
+    push x
+    push b
+    call rnd
+    pop b
+    pop x
+    pop a
+
 stars_no_wrap:
     dec x
     bnz
-- 
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