random: Try to ensure rand() state is thread-local

Signed-off-by: Andrew Opalach <andrew@akon.city>

3 files changed, 47 insertions, 3 deletions

diff --git a/include/al/random.h b/include/al/random.h
index f92a77b..3df3140 100644
--- a/include/al/random.h
+++ b/include/al/random.h
@@ -5,23 +5,53 @@
 #include "macros.h"
 
 #define AL_RAND_MAX RAND_MAX
+#define AL_RAND_INIT_SEED 1738
 
 extern u32 al_rand_seed;
+
 extern __thread bool al_rand_set;
+#ifndef AL_SRAND_THREAD_LOCAL
+extern __thread u32 al_rand_state;
+#endif
 
 static inline void al_rand_init(u32 seed)
 {
     al_rand_seed = seed;
 }
 
+// @TODO: Replace this with an implementation of any PRNG algorithm where
+//  we can control the state and make it thread-local. Trying to predict the
+//  state in libc is pretty dumb.
+// al_rand() is equivalent to libc rand(), as such there is no expectation
+// of it being an actually good random number generator.
+#ifdef AL_SRAND_THREAD_LOCAL
+#define rrand() rand()
+#else
+#define rrand() rand_r(&al_rand_state)
+#endif
 static inline s32 al_rand(void)
 {
     if (UNLIKELY(!al_rand_set)) {
-        // srand() is thread-local in Windows CRT.
+#ifdef AL_SRAND_THREAD_LOCAL
         srand(al_rand_seed);
+#else
+        al_rand_state = al_rand_seed;
+#endif
+        // Call rand() once after setting the seed incase it returns the seed
+        // on the first call, like on Windows. Having the first result potentially
+        // be so similar run-to-run would no longer even be pseudo-random.
+        // https://stackoverflow.com/a/76676396
+        rrand();
         al_rand_set = true;
     }
-    return rand();
+    return rrand();
+}
+#undef rrand
+
+// https://c-faq.com/lib/randrange.html
+static inline s32 al_random_int(s32 min, s32 max) // [MIN, MAX]
+{
+    return min + al_rand() / (AL_RAND_MAX / (max - min + 1) + 1);
 }
 
 #endif // _AL_RANDOM_H
diff --git a/meson.build b/meson.build
index c948887..0dffee3 100644
--- a/meson.build
+++ b/meson.build
@@ -8,6 +8,7 @@ compiler = meson.get_compiler('c')
 is_debug = get_option('debug')
 is_msvc = compiler.get_id() == 'msvc' or compiler.get_id() == 'clang-cl'
 is_msvc_specifically = compiler.cmd_array()[0] == 'cl'
+is_windows = host_machine.system() == 'windows'
 
 supports_needed_gnu_extensions = not is_msvc_specifically
 
@@ -57,6 +58,15 @@ if is_msvc_specifically
   alabaster_args += ['/arch:AVX']
 endif
 
+if compiler.has_function('rand_r', prefix: '#define _POSIX_C_SOURCE 200112L\n#include <stdlib.h>')
+elif is_windows # This is supposed to be a check for if libc = Microsoft CRT.
+  # https://learn.microsoft.com/en-us/previous-versions/f0d4wb4t(v=vs.140)?redirectedfrom=MSDN
+  # "The srand function sets ... in the current thread."
+  alabaster_args += ['-DAL_SRAND_THREAD_LOCAL']
+else
+  error('No thread-local rand().')
+endif
+
 if supports_needed_gnu_extensions
   alabaster_args += ['-DAL_HAVE_GNU_EXTENSIONS']
 endif
diff --git a/src/random.c b/src/random.c
index 1bbf152..fb059a0 100644
--- a/src/random.c
+++ b/src/random.c
@@ -1,4 +1,8 @@
 #include "../include/al/random.h"
 
-u32 al_rand_seed = 1738;
+u32 al_rand_seed = AL_RAND_INIT_SEED;
+
 __thread bool al_rand_set = false;
+#ifndef AL_SRAND_THREAD_LOCAL
+__thread u32 al_rand_state = AL_RAND_INIT_SEED;
+#endif