Create RNG module

build.sh

@@ -8,4 +8,4 @@ defs="-D_POSIX_C_SOURCE=200809L"
 
 $cc $warn $flags $libs $defs \
 	-o asteroids \
-	fb.c game.c input.c main.c maths.c renderer.c
+	fb.c game.c input.c main.c maths.c renderer.c rng.c

game.c

@@ -2,12 +2,11 @@
 
 #include "input.h"
 #include "renderer.h"
+#include "rng.h"
 
 #include <assert.h>
 #include <linux/input-event-codes.h>
-#include <stdlib.h>
 #include <string.h>
-#include <sys/time.h>
 
 #define FIRE_MEAN -0.15
 #define FIRE_JITTER 0.01
@@ -204,11 +203,6 @@ static void remove_entity(unsigned id)
 	--entity_count;
 }
 
-static float rand_plusminus()
-{
-	return 2.0f * ((float)rand() / RAND_MAX - 0.5f);
-}
-
 static void ship_update()
 {
 	entity_t *ship = entities + ship_entity_id;
@@ -221,7 +215,7 @@ static void ship_update()
 
 	shape_t *fire = shapes + fire_shape_id;
 	fire->visible = input.fwd != 0;
-	fire->verts[0].y = FIRE_MEAN + FIRE_JITTER * rand_plusminus();
+	fire->verts[0].y = FIRE_MEAN + FIRE_JITTER * rng_plusminus();
 }
 
 void game_init(float _aspect)
@@ -230,10 +224,6 @@ void game_init(float _aspect)
 	input_on_release(key_release_callback);
 	input_on_repeat(key_repeat_callback);
 
-	struct timeval tv;
-	gettimeofday(&tv, NULL);
-	srand(tv.tv_usec);
-
 	aspect = _aspect;
 	quit = false;
 

main.c

@@ -1,6 +1,7 @@
 #include "game.h"
 #include "input.h"
 #include "renderer.h"
+#include "rng.h"
 
 #include <stdio.h>
 #include <sys/select.h>
@@ -9,6 +10,8 @@
 
 int main()
 {
+	rng_init();
+
 	const int input_fd = input_init();
 
 	const renderer_params_t renderer_params = renderer_init();

rng.c

@@ -0,0 +1,32 @@
+#include "rng.h"
+
+#include <assert.h>
+#include <sys/time.h>
+
+static uint32_t state;
+
+void rng_init()
+{
+	struct timeval tv;
+	const int res = gettimeofday(&tv, nullptr);
+	assert(res == 0);
+	state = tv.tv_usec;
+}
+
+uint32_t rng_uint32()
+{
+	state ^= state << 13;
+	state ^= state >> 17;
+	state ^= state << 5;
+	return state;
+}
+
+float rng_canon()
+{
+	return (float)rng_uint32() / UINT32_MAX;
+}
+
+float rng_plusminus()
+{
+	return 2.0 * rng_canon() - 1.0;
+}

rng.h

@@ -0,0 +1,12 @@
+#ifndef RNG_H
+#define RNG_H
+
+#include <stdint.h>
+
+void rng_init();
+
+uint32_t rng_uint32();
+float rng_canon();
+float rng_plusminus();
+
+#endif