1 files changed, 183 insertions, 0 deletions

diff --git a/mem/src/mem.c b/mem/src/mem.c
new file mode 100644
index 0000000..ff97f0f
--- /dev/null
+++ b/mem/src/mem.c
@@ -0,0 +1,183 @@
+#include "mem.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+bool mem_make_(
+    Memory* mem, Chunk* chunks, void* blocks, size_t num_blocks,
+    size_t block_size_bytes) {
+  assert(mem);
+  assert((chunks && blocks) || (!chunks && !blocks));
+  assert(num_blocks >= 1);
+
+  mem->block_size_bytes = block_size_bytes;
+  mem->num_blocks       = num_blocks;
+  mem->next_free_chunk  = 0;
+
+  // Allocate chunks and blocks if necessary and zero them out.
+  if (!chunks) {
+    chunks       = calloc(num_blocks, sizeof(Chunk));
+    blocks       = calloc(num_blocks, block_size_bytes);
+    mem->dynamic = true;
+    if (!chunks || !blocks) {
+      return false;
+    }
+  } else {
+    memset(blocks, 0, num_blocks * block_size_bytes);
+    memset(chunks, 0, num_blocks * sizeof(Chunk));
+    mem->dynamic = false;
+  }
+  mem->chunks = chunks;
+  mem->blocks = blocks;
+
+  // Initialize the head as one large free chunk.
+  Chunk* head      = &mem->chunks[0];
+  head->num_blocks = num_blocks;
+
+  return true;
+}
+
+void mem_del_(Memory* mem) {
+  assert(mem);
+  if (mem->dynamic) {
+    if (mem->chunks) {
+      free(mem->chunks);
+      mem->chunks = 0;
+    }
+    if (mem->blocks) {
+      free(mem->blocks);
+      mem->blocks = 0;
+    }
+  }
+}
+
+void mem_clear_(Memory* mem) {
+  assert(mem);
+  mem->next_free_chunk = 0;
+  memset(mem->blocks, 0, mem->num_blocks * mem->block_size_bytes);
+  memset(mem->chunks, 0, mem->num_blocks * sizeof(Chunk));
+
+  // Initialize the head as one large free chunk.
+  Chunk* head      = &mem->chunks[0];
+  head->num_blocks = mem->num_blocks;
+}
+
+void* mem_alloc_(Memory* mem, size_t num_blocks) {
+  assert(mem);
+  assert(num_blocks >= 1);
+
+  // Search for the first free chunk that can accommodate num_blocks.
+  const size_t start     = mem->next_free_chunk;
+  size_t       chunk_idx = start;
+  bool         found     = false;
+  do {
+    Chunk* chunk = &mem->chunks[chunk_idx];
+    if (!chunk->used) {
+      if (chunk->num_blocks > num_blocks) {
+        // Carve out a smaller chunk when the found chunk is larger than
+        // requested.
+        // [prev] <--> [chunk] <--> [new next] <--> [next]
+        const size_t new_next_idx = chunk_idx + num_blocks;
+        Chunk*       new_next     = &mem->chunks[new_next_idx];
+        if (chunk->next) {
+          mem->chunks[chunk->next].prev = new_next_idx;
+        }
+        new_next->prev = chunk_idx;
+        new_next->next = chunk->next;
+        chunk->next    = new_next_idx;
+
+        new_next->num_blocks = chunk->num_blocks - num_blocks;
+        chunk->num_blocks    = num_blocks;
+
+        chunk->used = true;
+        found       = true;
+        break;
+      } else if (chunk->num_blocks == num_blocks) {
+        chunk->used = true;
+        found       = true;
+        break;
+      }
+    }
+    chunk_idx = chunk->next; // Last chunk points back to 0, which is always the
+                             // start of some chunk. 'next' and 'prev' are
+                             // always valid pointers.
+  } while (chunk_idx != start);
+
+  if (found) {
+    mem->next_free_chunk = mem->chunks[chunk_idx].next;
+    return &mem->blocks[chunk_idx * mem->block_size_bytes];
+  } else {
+    return 0; // Large-enough free chunk not found.
+  }
+}
+
+// The given pointer is a pointer to this first block of the chunk.
+void mem_free_(Memory* mem, void** chunk_ptr) {
+  assert(mem);
+  assert(chunk_ptr);
+
+  const size_t chunk_idx =
+      ((uint8_t*)*chunk_ptr - mem->blocks) / mem->block_size_bytes;
+  assert(chunk_idx < mem->num_blocks);
+  Chunk* chunk = &mem->chunks[chunk_idx];
+
+  // Disallow double-frees.
+  assert(chunk->used);
+
+  // Zero out the chunk so that we don't get stray values the next time it is
+  // allocated.
+  memset(&mem->blocks[chunk_idx], 0, chunk->num_blocks * mem->block_size_bytes);
+
+  // Free the chunk. If it is contiguous with other free chunks, then merge.
+  // We only need to look at the chunk's immediate neighbours because no two
+  // free chunks are left contiguous after merging.
+  chunk->used = false;
+  if (chunk->next) {
+    Chunk* next = &mem->chunks[chunk->next];
+    if (!next->used) {
+      // Pre:  [chunk] <--> [next] <--> [next next]
+      // Post: [  chunk + next   ] <--> [next next]
+      chunk->num_blocks += mem->chunks[chunk->next].num_blocks;
+      chunk->next = next->next;
+      if (next->next) {
+        Chunk* next_next = &mem->chunks[next->next];
+        next_next->prev  = chunk_idx;
+      }
+      next->prev = next->next = next->num_blocks = 0;
+    }
+  }
+  if (chunk->prev) {
+    Chunk* prev = &mem->chunks[chunk->prev];
+    if (!prev->used) {
+      // Pre:  [prev] <--> [chunk] <--> [next]
+      // Post: [  prev + chunk   ] <--> [next]
+      prev->num_blocks += chunk->num_blocks;
+      prev->next = chunk->next;
+      if (chunk->next) {
+        Chunk* next = &mem->chunks[chunk->next];
+        next->prev  = chunk->prev;
+      }
+      chunk->prev = chunk->next = chunk->num_blocks = 0;
+    }
+  }
+
+  *chunk_ptr = 0;
+}
+
+// The handle is the chunk's index. We don't call it an index in the public API
+// because from the user's perspective, two chunks allocated back-to-back need
+// not be +1 away (the offset depends on how large the first chunk is).
+void* mem_get_chunk_(const Memory* mem, size_t chunk_handle) {
+  assert(mem);
+  assert(chunk_handle < mem->num_blocks);
+  assert(mem->chunks[chunk_handle].used);
+  return &mem->blocks[chunk_handle * mem->block_size_bytes];
+}
+
+// The given chunk pointer is a pointer to the blocks array.
+size_t mem_get_chunk_handle_(const Memory* mem, const void* chunk) {
+  assert(mem);
+  const size_t block_byte_index = (const uint8_t*)chunk - mem->blocks;
+  assert(block_byte_index % mem->block_size_bytes == 0);
+  return block_byte_index / mem->block_size_bytes;
+}