Use portable xorshift PRNG for deterministic cross-platform RAM init

Replace libc rand()/srand() with a platform-independent xorshift32
PRNG. libc rand() produces different sequences on glibc vs macOS
libsystem, causing cross-platform baseline mismatches in regression
tests. The xorshift32 implementation guarantees identical RAM
initialization on all platforms.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: JoeMatt

src/dsp.c

@@ -822,7 +822,7 @@ void DSPReset(void)
 
 	// Contents of local RAM are quasi-stable; we simulate this by randomizing RAM contents
 	for(i=0; i<8192; i+=4)
-		*((uint32_t *)(&dsp_ram_8[i])) = rand();
+		*((uint32_t *)(&dsp_ram_8[i])) = JaguarRand();
 }
 
 void DSPDone(void)

src/gpu.c

@@ -666,7 +666,7 @@ void GPUReset(void)
 
    // Contents of local RAM are quasi-stable; we simulate this by randomizing RAM contents
    for(i=0; i<4096; i+=4)
-      *((uint32_t *)(&gpu_ram_8[i])) = rand();
+      *((uint32_t *)(&gpu_ram_8[i])) = JaguarRand();
 }
 
 uint32_t GPUReadPC(void)

src/jaguar.c

@@ -16,8 +16,6 @@
 #include <string.h>
 #include <stdlib.h>
 
-#include <time.h>
-
 #include "jaguar.h"
 
 #include "cdrom.h"
@@ -35,6 +33,24 @@
 
 static bool frameDone;
 
+// Platform-independent xorshift32 PRNG for deterministic RAM initialization.
+// libc rand() produces different sequences on different platforms (glibc vs
+// macOS libsystem), which causes cross-platform baseline mismatches.
+static uint32_t jaguar_prng_state = 12345;
+
+void JaguarSeedPRNG(uint32_t seed)
+{
+   jaguar_prng_state = seed ? seed : 1;
+}
+
+uint32_t JaguarRand(void)
+{
+   jaguar_prng_state ^= jaguar_prng_state << 13;
+   jaguar_prng_state ^= jaguar_prng_state >> 17;
+   jaguar_prng_state ^= jaguar_prng_state << 5;
+   return jaguar_prng_state;
+}
+
 #define ALPINE_FUNCTIONS
 
 // Private function prototypes
@@ -541,15 +557,12 @@ void JaguarSetScreenPitch(uint32_t pitch)
 void JaguarInit(void)
 {
    unsigned i;
-   // Seed with a fixed value so emulation is deterministic across runs.
-   // Real hardware has quasi-random RAM contents at power-on, but using
-   // a constant seed preserves that simulation while keeping output
-   // reproducible for regression testing and save state consistency.
-   srand(12345);
+   // Reset the platform-independent PRNG for deterministic RAM fill
+   JaguarSeedPRNG(12345);
 
    // Contents of local RAM are quasi-stable; we simulate this by randomizing RAM contents
    for(i=0; i<0x200000; i+=4)
-      *((uint32_t *)(&jaguarMainRAM[i])) = rand();
+      *((uint32_t *)(&jaguarMainRAM[i])) = JaguarRand();
 
    lowerField = false;							// Reset the lower field flag
    memset(jaguarMainRAM + 0x804, 0xFF, 4);
@@ -634,8 +647,9 @@ void JaguarReset(void)
 
    // Only problem with this approach: It wipes out RAM loaded files...!
    // Contents of local RAM are quasi-stable; we simulate this by randomizing RAM contents
+   JaguarSeedPRNG(12345);
    for(i=8; i<0x200000; i+=4)
-      *((uint32_t *)(&jaguarMainRAM[i])) = rand();
+      *((uint32_t *)(&jaguarMainRAM[i])) = JaguarRand();
 
    // New timer base code stuffola...
    InitializeEventList();

src/jaguar.h

@@ -16,6 +16,8 @@ void JaguarSetScreenPitch(uint32_t pitch);
 void JaguarInit(void);
 void JaguarReset(void);
 void JaguarDone(void);
+void JaguarSeedPRNG(uint32_t seed);
+uint32_t JaguarRand(void);
 
 uint8_t JaguarReadByte(uint32_t offset, uint32_t who);
 uint16_t JaguarReadWord(uint32_t offset, uint32_t who);

src/tom.c

@@ -256,7 +256,6 @@
 #include "tom.h"
 
 #include <string.h>								// For memset()
-#include <stdlib.h>								// For rand()
 #include "blitter.h"
 #include "event.h"
 #include "gpu.h"