Add an awesome random number generator made by lua :3

Co-authored-by: Lua MacDougall <lua@foxgirl.dev>

fox32rom.def

@@ -84,6 +84,10 @@ string_to_int:                            jmp [0xF0047000]
 play_audio:                               jmp [0xF0048000]
 stop_audio:                               jmp [0xF0048004]
 
+; random number jump table
+random:                                   jmp [0xF0049000]
+random_range:                             jmp [0xF0049004]
+
 ; event types
 const EVENT_TYPE_MOUSE_CLICK:    0x00000000
 const EVENT_TYPE_MOUSE_RELEASE:  0x00000001

main.asm

@@ -14,12 +14,25 @@ const TEXT_COLOR:       0xFFFFFFFF
 entry:
     mov rsp, SYSTEM_STACK
 
-    mov [0x000003FC], system_vsync_handler
-
     ; disable audio playback
     mov r0, 0x80000600
     out r0, 0
 
+    ; seed the random number generator
+entry_seed:
+    mov [0x000003FC], entry_seed_done
+    ise
+    mov r0, 2166136261
+entry_seed_loop:
+    mul r0, 16777619
+    rjmp entry_seed_loop
+entry_seed_done:
+    mov [RANDOM_STATE], r0
+
+    ; set up the vsync interrupt
+    mov [0x000003FC], system_vsync_handler
+    ise
+
     ; clear the interrupt vector for interrupt 0xFE - audio buffer switch
     mov [0x000003F8], 0x00000000
 
@@ -53,7 +66,6 @@ draw_startup_text:
     mov r12, FOX32ROM_VERSION_PATCH
     call draw_format_str_to_background
 
-    ise
 event_loop:
     call get_next_event
 
@@ -132,6 +144,7 @@ get_rom_version:
     #include "mouse.asm"
     #include "overlay.asm"
     #include "panic.asm"
+    #include "random.asm"
     #include "ryfs.asm"
     #include "string.asm"
     #include "vsync.asm"
@@ -235,6 +248,11 @@ get_rom_version:
     data.32 play_audio
     data.32 stop_audio
 
+    ; random number jump table
+    org.pad 0xF0049000
+    data.32 random
+    data.32 random_range
+
     org.pad 0xF004F000
 standard_font_width:
     data.16 8

random.asm

@@ -0,0 +1,40 @@
+; random number generation routines
+; by lua :3 - 2022 https://foxgirl.dev/
+
+const RANDOM_STATE: 0x0228218E ; 4 bytes
+
+; generate a random number
+; inputs:
+; none
+; outputs:
+; r0: random number
+random:
+    push r1
+    mov r0, [RANDOM_STATE]
+
+    mov r1, r0
+    sla r1, 13
+    xor r0, r1
+    mov r1, r0
+    srl r1, 17
+    xor r0, r1
+    mov r1, r0
+    sla r1, 5
+    xor r0, r1
+
+    mov [RANDOM_STATE], r0
+    pop r1
+    ret
+
+; generate a random number in the range [r1, r2)
+; inputs:
+; r1: minimum value, inclusive
+; r2: maximum value, exclusive
+; outputs:
+; r0: random number
+random_range:
+    call random
+    sub r2, r1
+    rem r0, r2
+    add r0, r1
+    ret