From dcfd4c01ffa2a0c7469b8d20608c0e0daa35bd51 Mon Sep 17 00:00:00 2001
From: Imbus <>
Date: Tue, 9 Sep 2025 11:00:25 +0200
Subject: [PATCH] More rigorous testing and helpful comments

---
 freelist/main.c | 83 +++++++++++++++++++++++++++++++++++++------------
 1 file changed, 63 insertions(+), 20 deletions(-)

diff --git a/freelist/main.c b/freelist/main.c
index 79a39dd..ffb2068 100644
--- a/freelist/main.c
+++ b/freelist/main.c
@@ -18,20 +18,42 @@ void printvec(const Vec3 *v) {
     printf("Vec3: (%d, %d, %d)\n", v->x, v->y, v->z);
 }
 
+/* For testing crc poly 0x04C11DB7 */
+uint32_t crc32(const uint8_t *data, size_t length);
+
 #define BUFFER_SIZE (128)
 
 int main() {
-    FreeList    fl;
-    const char *mem = malloc(BUFFER_SIZE);
-    memset((void *)mem, 0, BUFFER_SIZE);
+    FreeList fl;
+
+    /* Here we split our malloc'ed memory into three parts, we pass one of
+     * them to our freelist, and keep the other two (head/tail) to check for
+     * overwrites:
+     *
+     *   | Headblock |      Freelist block     | Tailblock |
+     *   |    1x     |            2x           |     1x    |
+     */
+    void *headblock = malloc(BUFFER_SIZE * 2);
+    void *tailblock = headblock + (BUFFER_SIZE + BUFFER_SIZE / 2);
+    void *mem = headblock + (BUFFER_SIZE / 2);
+
+    /* Set these buffers to something known and randomize the buffer we give to the freelist */
+    memset(mem, arc4random(), BUFFER_SIZE);
+    memset(headblock, 0xAB, BUFFER_SIZE / 2);
+    memset(tailblock, 0xF0, BUFFER_SIZE / 2);
+
+    /* Store these for reference at the end of the testing */
+    const uint32_t tail_crc = crc32(tailblock, BUFFER_SIZE / 2);
+    const uint32_t head_crc = crc32(headblock, BUFFER_SIZE / 2);
+
+    /* Display a nice hexdump so you can see the layout */
+    hexdump(headblock, BUFFER_SIZE * 2);
 
     if (!fl_init(&fl, (uintptr_t)mem, BUFFER_SIZE, sizeof(Vec3))) {
-        printf("Allocation error\n");
+        printf("Freelist failed to initialize!\n");
         exit(EXIT_FAILURE);
     }
 
-    hexdump(mem, BUFFER_SIZE);
-
     const size_t cap = fl_capacity(&fl);
 
     printf("Item size: %lu\n", sizeof(Vec3));
@@ -42,23 +64,17 @@ int main() {
     printf("Space utilization external: %.2f%%\n", 100.0 * ((float)fl.size * cap) / BUFFER_SIZE);
 
     assert(fl_available(&fl) == cap);
-
-    if (!fl_check(&fl))
-        printf("Check failed!\n");
-
-    printf("Check: %zu\n", fl_check(&fl));
+    assert(fl_check(&fl) == cap);
 
     Vec3 *a = fl_alloc(&fl);
     Vec3 *b = fl_alloc(&fl);
     Vec3 *c = fl_alloc(&fl);
 
-    hexdump(mem, BUFFER_SIZE);
+    memset(a, 0xCA, sizeof(Vec3));
+    memset(b, 0xCB, sizeof(Vec3));
+    memset(c, 0xCC, sizeof(Vec3));
 
-    memset(a, 0x23, sizeof(Vec3));
-    memset(b, 0x24, sizeof(Vec3));
-    memset(c, 0x25, sizeof(Vec3));
-
-    *a = vec3_new(12, 13, 435);
+    *a = vec3_new(0xAAAA, 0xAAAA, 0xAAAA);
     *b = vec3_new(192, 199, 435);
     *c = vec3_new(432, 11, 435);
 
@@ -91,12 +107,10 @@ int main() {
 
     /* Write some data to the last element to catch out of bounds writes */
     size_t last_elem = fl_capacity(&fl) - 1;
-    *(Vec3 *)ptr_buf[last_elem] = *a;
+    memset((void *)ptr_buf[last_elem], 0x42, sizeof(Vec3));
 
     assert(0 == fl_available(&fl));
     assert(fl_allocated(&fl) == fl_capacity(&fl));
-
-    printf("Check: %zu\n", fl_check(&fl));
     assert(fl_check(&fl) == 0);
 
     /* Return it all */
@@ -104,10 +118,39 @@ int main() {
         fl_free(&fl, (void *)ptr_buf[i]);
     }
 
+    free(ptr_buf);
+
+    /* All slots are free here */
     assert(cap == fl_available(&fl));
     assert(fl_allocated(&fl) == 0);
     assert(fl_check(&fl) == cap);
 
+    /* Check so our head and tail blocks are intact */
+    assert(crc32(headblock, BUFFER_SIZE / 2) == head_crc);
+    assert(crc32(tailblock, BUFFER_SIZE / 2) == tail_crc);
+
+    /* Hexdump the heap, free the block and declare victory */
+    hexdump(headblock, BUFFER_SIZE * 2);
+    free(headblock);
     printf("All tests passed!\n");
     return 0;
 }
+
+uint32_t crc32(const uint8_t *data, size_t length) {
+    uint32_t crc = 0xFFFFFFFF;
+    uint32_t poly = 0x04C11DB7;
+
+    for (size_t i = 0; i < length; i++) {
+        crc ^= ((uint32_t)data[i]) << 24;
+
+        for (int j = 0; j < 8; j++) {
+            if (crc & 0x80000000) {
+                crc = (crc << 1) ^ poly;
+            } else {
+                crc <<= 1;
+            }
+        }
+    }
+
+    return crc ^ 0xFFFFFFFF;
+}