restructed realloc and added program header

mm.c

@@ -1,3 +1,24 @@
+/*
+ * mm.c - Dynamic Memory Allocator
+ *
+ * This is a segregated free list implementation with multiple
+ * lists to hold different block sizes. The size ranges for our 8 lists * are powers of 2, with
+ * the smallest size on the lower bound being 8 words, 
+ * 
+ * A seperate list is used to store the sizes, and is checked when a 
+ * free block is added to a list.
+ * 
+ * Both allocated and free blocks are stored in the heap 
+ * with the pointer heap_listp.
+ * Each block has a header and a footer, which
+ * store the size in 31 bits, and 1 bit for determining whether it is 
+ * free.
+ *
+ * Blocks have a payload which holds hold necessary data and a list_elem struct so blocks can be stored in lists
+ */
+
+
+
 #include <string.h>
 #include <stdlib.h>
 #include <assert.h>
@@ -211,8 +232,9 @@ void mm_free(void *bp)
 //~ }
 
 void *mm_realloc(void *ptr, size_t size) {
-    void *new_ptr = NULL;
-
+    size_t bytesize;
+    void *new_ptr;
+    
     if (ptr == NULL) {
         return mm_malloc(size);
     }
@@ -223,20 +245,55 @@ void *mm_realloc(void *ptr, size_t size) {
     }
 
     struct block *oldblock = ptr - offsetof(struct block, payload);
-    size_t oldwords = blk_size(oldblock);
-    size_t oldsize = oldwords * WSIZE;
+    bool prev_alloc = prev_blk_footer(oldblock)->inuse;
+    bool next_alloc = !blk_free(next_blk(oldblock));
     
-    if (!(new_ptr = mm_malloc(size))) {
-        return NULL;
+    if (!next_alloc)
+    {
+	    struct block *right;
+	    right = next_blk(oldblock);
+	    list_remove(&right->elem);
+        set_block_used(oldblock, blk_size(oldblock) + blk_size(next_blk(oldblock)));
     }
-    if (size < oldsize) {
-        oldsize = size;
-    }
-    memcpy(new_ptr, ptr, oldsize);
-    mm_free(ptr);
+    
+    if (blk_size(oldblock) >= size) {
+        return oldblock->payload; 
+    }  
 
+    if (!prev_alloc)
+    {
+	    struct block *left;
+	    left = prev_blk(oldblock);
+	    list_remove(&left->elem);
+        set_block_used(left, blk_size(oldblock) + blk_size(left));
+	    oldblock = left;
+    }
+    
+  
+    if (blk_size(oldblock) >= size) 
+    {
+	    bytesize = blk_size(oldblock) * WSIZE; 
+	    if(size < bytesize) bytesize = size;
+	    memcpy(oldblock->payload, ptr, bytesize);
+	    return oldblock->payload;
+    }
+
+    
+    new_ptr = mm_malloc(size);
+    
+    
+    if(!new_ptr) return 0;
+
+    
+    struct block *copyblk = ptr - offsetof(struct block, payload);
+    bytesize = blk_size(copyblk) * WSIZE; //find number of BYTES, not words
+    if(size < bytesize) bytesize = size;
+    memcpy(new_ptr, ptr, bytesize);
+    
+ 
+    mm_free(oldblock->payload);
+    
     return new_ptr;
-
 }
 
 static struct block *coalesce(struct block *bp)