From 8943a12cd1e8ba2cd9b35b81f6fa3a3af5057b35 Mon Sep 17 00:00:00 2001
From: Lukas Krickl <lukas@krickl.dev>
Date: Sat, 15 Feb 2025 15:17:18 +0100
Subject: [PATCH] rand: Replaced rand with a very basic feedback shift register

This is by no means perfect but it is more random than what was here
before
---
 src/rand.s | 85 +++++++++++++++++++++++++++---------------------------
 src/wram.s |  4 ++-
 2 files changed, 45 insertions(+), 44 deletions(-)

diff --git a/src/rand.s b/src/rand.s
index f0a22db..fc6007b 100644
--- a/src/rand.s
+++ b/src/rand.s
@@ -2,9 +2,14 @@
 rand_init:
   call mbc1_ram_enable
 
+  ; restore seed from 
+  ; sram 
   ld a, [sram_srand] 
   ld [srand], a
 
+  ld a, [sram_srand+1]
+  ld [srand+1], a
+
   call mbc1_ram_disable
   ret
 
@@ -12,65 +17,59 @@ rand_init:
 rand_save_srand:
   call mbc1_ram_enable
   
+  ; save seed in sram
   ld a, [srand]
   ld [sram_srand], a
 
+  ld a, [srand+1]
+  ld [sram_srand+1], a
+
   call mbc1_ram_disable
   ret
 
   ; gets a pseudo-random number
   ; and advances to the next seed
+  ; using xorshift LFSR 
   ; uses:
   ;   a, hl, de
   ; returns:
   ;   a: random value
 rand:
+  ; load seed
   ld a, [srand]
-  cp a, (rand_table_end - rand_table)
-  jr nz, @not_end REL
-    ld a, 0
-@not_end:
- 
-  ld d, 0
-  ld e, a ; de = offset into table 
+  ld h, a
+  ld a, [srand+1]
+  ld l, a
+
+
+  ; perform lfsr 
+  ; TODO: this can be made much better
 
-  ld hl, rand_table
-  add hl, de
-  inc a
-  ld [srand], a ; srand++
+  ; h
+  ld a, h
+  rra 
+  ld a, l
+  rra
+  xor a, h
+  swap a
+  ld h, a
 
-  ld a, [hl]
+  ; l 
+  ld a, l
+  rra 
+  ld a, h
+  rra
+  xor a, l
+  swap a
+  ld l, a
+
+  ; store seed back
+  ld a, h
+  ld [srand], a
+  ld a, l
+  ld [srand+1], a
+  
+  ld a, [srand]
 
   ret
 
-rand_table:
-.db 0x09, 0xd7, 0xf4, 0xb1, 0x77, 0x61, 0xdd, 0x0e, 0xee 
-.db 0x21, 0x6e, 0x86, 0x44, 0x49, 0xc5, 0x89, 0x52, 0x76 
-.db 0x1b, 0x92, 0x90, 0x6b, 0x2b, 0xcb, 0x97, 0xae, 0x11 
-.db 0x1f, 0xb6, 0x5b, 0x95, 0x5a, 0x05, 0xfb, 0x01, 0x19 
-.db 0x0d, 0xd8, 0xdc, 0x6f, 0xc6, 0x41, 0x25, 0x68, 0x16 
-.db 0x50, 0x82, 0x2f, 0x6a, 0x7e, 0x57, 0xa9, 0x9c, 0x27 
-.db 0xbd, 0xb8, 0x60, 0xa8, 0xfa, 0x32, 0xf7, 0xe2, 0xde 
-.db 0xa2, 0xda, 0xf3, 0x67, 0x7c, 0x46, 0x4f, 0xea, 0xa5 
-.db 0x9e, 0x1c, 0x43, 0xcd, 0xd2, 0x1a, 0x71, 0x7a, 0xc3 
-.db 0x8b, 0x4e, 0xa0, 0xa6, 0xf9, 0x33, 0xca, 0x59, 0x30 
-.db 0xaa, 0xb2, 0x42, 0x91, 0xf8, 0x47, 0x24, 0x3c, 0xa4 
-.db 0xe8, 0x5e, 0xff, 0xcf, 0x8d, 0xd6, 0x9a, 0x15, 0x58 
-.db 0x22, 0xfe, 0x3e, 0x2d, 0xd5, 0x9f, 0x4b, 0x07, 0x7f 
-.db 0x88, 0x14, 0x6d, 0xb9, 0x02, 0x29, 0xab, 0x38, 0x37 
-.db 0x17, 0x6c, 0x64, 0x18, 0xe4, 0x2c, 0x56, 0xd4, 0x31 
-.db 0x0f, 0xa7, 0xd0, 0xe7, 0x74, 0xc1, 0xd1, 0xb5, 0xaf 
-.db 0x65, 0xf2, 0xba, 0xa1, 0x99, 0xe0, 0x69, 0x75, 0x10 
-.db 0x3a, 0x06, 0x9d, 0xe3, 0x7b, 0xeb, 0x9b, 0xe1, 0x81 
-.db 0x20, 0xf5, 0xe9, 0x5f, 0xe6, 0xdf, 0xa3, 0x2a, 0x35 
-.db 0xd9, 0xf6, 0xdb, 0x26, 0x72, 0x79, 0x36, 0x96, 0x3b 
-.db 0xb4, 0xe5, 0x04, 0x8f, 0x8a, 0xcc, 0xbe, 0xbc, 0x78 
-.db 0x2e, 0x3f, 0x1e, 0x84, 0x4c, 0xf0, 0x7d, 0x28, 0x03 
-.db 0xf1, 0x53, 0x8c, 0x0a, 0xb0, 0x3d, 0x94, 0xc9, 0x4a 
-.db 0x93, 0x45, 0x0b, 0xc8, 0x83, 0x4d, 0xbf, 0x5d, 0x34 
-.db 0xef, 0xd3, 0xfd, 0x98, 0x51, 0x54, 0x66, 0x40, 0xce 
-.db 0xc7, 0x12, 0x80, 0xfc, 0x48, 0xbb, 0x55, 0xec, 0x00 
-.db 0xc2, 0x13, 0x39, 0x87, 0xc4, 0x0c, 0xc0, 0xad, 0x1d 
-.db 0x63, 0x23, 0xac, 0x5c, 0x8e, 0xed, 0xb3, 0x85, 0x08 
-.db 0x73, 0x70, 0x62
-rand_table_end:
diff --git a/src/wram.s b/src/wram.s
index 1ec0924..4cad0a2 100644
--- a/src/wram.s
+++ b/src/wram.s
@@ -75,7 +75,9 @@ game_over_timer: .adv 1
 
 game_mode: .adv 1
 
-srand: .adv 1
+  ; 16 bit srand seed
+  ; seed must never be 0
+srand: .adv 2
 
   ; who is currently in control of the game
   ; if set to FF
-- 
2.30.2