4 files changed, 31 insertions, 33 deletions

diff --git a/mempool/README.md b/mempool/README.md
index ed2935e..7eb950e 100644
--- a/mempool/README.md
+++ b/mempool/README.md
@@ -1,20 +1,15 @@
 # Mempool
 
-A memory pool implementation.
+A memory pool of fixed-sized blocks with O(1) allocation and deallocation.
 
 Each block in the pool is tagged with a boolean value that indicates whether the
-block is free or in use. The allocator otherwise maintains little additional
-information. Therefore, some operations have linear-time complexity.
-Specifically:
+block is free or in use, as well as a pointer to the next free/used block.
+Blocks thus form two lists, a free list and a used list. The allocator
+maintains the two lists when allocating/deallocating blocks.
 
-- Allocating a block scans the pool for the next free block in linear time.
-- Freeing a block runs in constant time.
-- Iterating over the pool's used blocks is linear over the number of blocks in
-  the pool, as opposed to just the number of used blocks.
+The allocator's internal data is stored separately from the block data so that
+the data remains tightly packed.
 
-The allocator's internal data is also stored separately from the main array of
-blocks so that the block data remains tightly packed.
-
-For convenience of programming and debugging, free blocks in the mempool always
-remain zeroed out. If your data structures are valid when zeroed out, then you
-do not need to explicitly initialize them.
+Free blocks in the mempool always remain zeroed out for convenience of
+programming and debugging. If the application's data structures are valid when
+zeroed out, then they do not need to be explicitly initialized.
diff --git a/mempool/include/mempool.h b/mempool/include/mempool.h
index 245173b..04d0313 100644
--- a/mempool/include/mempool.h
+++ b/mempool/include/mempool.h
@@ -106,8 +106,8 @@
 /// It is valid to mempool_free() the object at each step of the iteration.
 #define mempool_foreach(POOL, ITER, BODY)                                  \
   {                                                                        \
-    size_t i = (POOL)->pool.used;                                          \
     if ((POOL)->pool.num_used_blocks > 0) {                                \
+      size_t i = (POOL)->pool.used;                                        \
       do {                                                                 \
         if ((POOL)->pool.block_info[i].used) {                             \
           __typeof__((POOL)->object[0])* ITER =                            \
diff --git a/mempool/src/mempool.c b/mempool/src/mempool.c
index 46f1053..c398c4f 100644
--- a/mempool/src/mempool.c
+++ b/mempool/src/mempool.c
@@ -34,7 +34,7 @@ bool mempool_make_(
     block_info    = calloc(num_blocks, sizeof(BlockInfo));
     blocks        = calloc(num_blocks, block_size_bytes);
     pool->dynamic = true;
-    if ((block_info == 0) || (blocks == 0)) {
+    if ((block_info == nullptr) || (blocks == nullptr)) {
       return false;
     }
   } else {
@@ -55,19 +55,20 @@ void mempool_del_(mempool* pool) {
   if (pool->dynamic) {
     if (pool->block_info) {
       free(pool->block_info);
-      pool->block_info = 0;
+      pool->block_info = nullptr;
     }
     if (pool->blocks) {
       free(pool->blocks);
-      pool->blocks = 0;
+      pool->blocks = nullptr;
     }
   }
 }
 
 void mempool_clear_(mempool* pool) {
   assert(pool);
-  pool->head = 0;
-  pool->used = 0;
+  pool->head            = 0;
+  pool->used            = 0;
+  pool->num_used_blocks = 0;
   memset(pool->blocks, 0, pool->num_blocks * pool->block_size_bytes);
   memset(pool->block_info, 0, pool->num_blocks * sizeof(BlockInfo));
   init_free_list(pool);
@@ -81,7 +82,7 @@ void* mempool_alloc_(mempool* pool) {
     if (pool->trap) {
       FAIL("mempool allocation failed, increase the pool's capacity.");
     }
-    return 0; // Pool is full.
+    return nullptr; // Pool is full.
   }
 
   // Allocate the block.
@@ -124,7 +125,7 @@ void mempool_free_(mempool* pool, void** block_ptr) {
 
   pool->num_used_blocks--;
 
-  *block_ptr = 0;
+  *block_ptr = nullptr;
 }
 
 void* mempool_get_block_(const mempool* pool, size_t block_index) {
diff --git a/mempool/test/mempool_test.c b/mempool/test/mempool_test.c
index 843f7e7..6d904bc 100644
--- a/mempool/test/mempool_test.c
+++ b/mempool/test/mempool_test.c
@@ -39,7 +39,7 @@ TEST_CASE(mempool_allocate_until_full) {
 
   for (int i = 0; i < NUM_BLOCKS; ++i) {
     const int* block = mempool_alloc(&pool);
-    TEST_TRUE(block != 0);
+    TEST_TRUE(block != nullptr);
   }
 
   TEST_TRUE(mempool_size(&pool) == NUM_BLOCKS);
@@ -50,10 +50,10 @@ TEST_CASE(mempool_fill_then_free) {
   test_pool pool;
   mempool_make(&pool);
 
-  int* blocks[NUM_BLOCKS] = {0};
+  int* blocks[NUM_BLOCKS] = {nullptr};
   for (int i = 0; i < NUM_BLOCKS; ++i) {
     blocks[i] = mempool_alloc(&pool);
-    TEST_TRUE(blocks[i] != 0);
+    TEST_TRUE(blocks[i] != nullptr);
   }
 
   for (int i = 0; i < NUM_BLOCKS; ++i) {
@@ -74,7 +74,7 @@ TEST_CASE(mempool_allocate_beyond_max_size) {
 
   // Fully allocate the pool.
   for (int i = 0; i < NUM_BLOCKS; ++i) {
-    TEST_TRUE(mempool_alloc(&pool) != 0);
+    TEST_TRUE(mempool_alloc(&pool) != nullptr);
   }
 
   // Past the end.
@@ -105,7 +105,7 @@ TEST_CASE(mempool_zero_free_block_after_free) {
   mempool_make(&pool);
 
   int* val = mempool_alloc(&pool);
-  TEST_TRUE(val != 0);
+  TEST_TRUE(val != nullptr);
   *val = 177;
 
   int* old_val = val;
@@ -131,7 +131,7 @@ TEST_CASE(mempool_traverse_partially_full) {
 
   for (int i = 0; i < N; ++i) {
     int* val = mempool_alloc(&pool);
-    TEST_TRUE(val != 0);
+    TEST_TRUE(val != nullptr);
     *val = i + 1;
   }
 
@@ -146,7 +146,7 @@ TEST_CASE(mempool_traverse_full) {
 
   for (int i = 0; i < NUM_BLOCKS; ++i) {
     int* val = mempool_alloc(&pool);
-    TEST_TRUE(val != 0);
+    TEST_TRUE(val != nullptr);
     *val = i + 1;
   }
 
@@ -161,7 +161,7 @@ TEST_CASE(mempool_get_block) {
 
   for (int i = 0; i < NUM_BLOCKS; ++i) {
     int* block = mempool_alloc(&pool);
-    TEST_TRUE(block != 0);
+    TEST_TRUE(block != nullptr);
     *block = i;
     TEST_EQUAL(mempool_get_block_index(&pool, block), (size_t)i);
   }
@@ -178,16 +178,18 @@ TEST_CASE(mem_clear_then_reuse) {
 
   // Allocate chunks, contents not important.
   for (int i = 0; i < NUM_BLOCKS; ++i) {
-    int* chunk = mempool_alloc(&mem);
-    TEST_TRUE(chunk != 0);
+    const int* chunk = mempool_alloc(&mem);
+    TEST_TRUE(chunk != nullptr);
   }
 
   mempool_clear(&mem);
+  TEST_EQUAL(mempool_size(&mem), 0);
+  TEST_EQUAL(mempool_capacity(&mem), NUM_BLOCKS);
 
   // Allocate chunks and assign values 0..N.
   for (int i = 0; i < NUM_BLOCKS; ++i) {
     int* chunk = mempool_alloc(&mem);
-    TEST_TRUE(chunk != 0);
+    TEST_TRUE(chunk != nullptr);
     *chunk = i + 1;
   }