X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~mdw/git/mLib/blobdiff_plain/f8509853f72daec365373f9d8c18f63cf8bad54e..300a63912783f039c4df8be05542ce97786c3660:/darray.c

diff --git a/darray.c b/darray.c
index bee6955..83150f7 100644
--- a/darray.c
+++ b/darray.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: darray.c,v 1.2 1999/10/28 22:05:28 mdw Exp $
+ * $Id: darray.c,v 1.4 1999/11/06 12:40:45 mdw Exp $
  *
  * Dynamically growing dense arrays
  *
@@ -30,6 +30,12 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: darray.c,v $
+ * Revision 1.4  1999/11/06 12:40:45  mdw
+ * Minor changes to allocation strategy.
+ *
+ * Revision 1.3  1999/10/29 22:59:22  mdw
+ * New array adjustment macros for unsigned arguments.
+ *
  * Revision 1.2  1999/10/28 22:05:28  mdw
  * Modify and debug allocation routines.
  *
@@ -103,10 +109,11 @@ void *da_ensure(da_base *b, void *v, size_t sz, size_t n)
   /* --- Maybe just shunt data around a bit --- *
    *
    * If the vector is large enough, then theoretically we could cope by
-   * moving the objects about in their existing storage.
+   * moving the objects about in their existing storage.  It's not worth
+   * bothering if there's not actually double the amount of space I need.
    */
 
-  if (rq < b->sz + b->off) {
+  if (rq * 2 < b->sz + b->off) {
     q = p - (b->off - slots) * sz;
     memmove(q, p, b->len * sz);
     b->sz += b->off - slots;
@@ -115,15 +122,37 @@ void *da_ensure(da_base *b, void *v, size_t sz, size_t n)
     return (q);
   }
 
-  /* --- Reallocate the array --- */
+  /* --- Decide on a new size --- *
+   *
+   * There's a minimum possible size for the array which is used if it's
+   * currently completely empty.  Otherwise I choose the smallest power of
+   * two which is big enough, starting at double the current size.
+   */
 
   nsz = v ? b->sz + b->off : (DA_INITSZ >> 1);
   do nsz <<= 1; while (nsz < rq);
-  q = xmalloc(nsz * sz);
-  q += slots * sz;
-  memcpy(q, p, b->len * sz);
-  if (p)
-    free(p - b->off * sz);
+
+  /* --- Reallocate the block --- *
+   *
+   * If I'm not changing the base offset then it's worth using @realloc@;
+   * otherwise there'll probably be two calls to @memcpy@ to shunt the data
+   * around so it's not worth bothering.
+   */
+
+  if (p && slots == b->off) {
+    q = xrealloc(p - b->off * sz, nsz * sz);
+    q += slots * sz;
+  } else {
+    q = xmalloc(nsz * sz);
+    q += slots * sz;
+    if (p) {
+      memcpy(q, p, b->len * sz);
+      free(p - b->off * sz);
+    }
+  }
+
+  /* --- Fill in the other parts of the base structure --- */
+
   b->off = slots;
   b->sz = nsz - slots;
   b->unshift = b->push = 0;
@@ -182,10 +211,11 @@ void *da_shunt(da_base *b, void *v, size_t sz, size_t n)
   /* --- Maybe just shunt data around a bit --- *
    *
    * If the vector is large enough, then theoretically we could cope by
-   * moving the objects about in their existing storage.
+   * moving the objects about in their existing storage.  Again, if there's
+   * not actually twice the space needed, reallocate the array.
    */
 
-  if (rq < b->sz + b->off) {
+  if (rq * 2 < b->sz + b->off) {
     q = p + (b->sz - slots) * sz;
     memmove(q, p, b->len * sz);
     b->off += b->sz - slots;
@@ -194,15 +224,37 @@ void *da_shunt(da_base *b, void *v, size_t sz, size_t n)
     return (q);
   }
 
-  /* --- Reallocate the array --- */
+  /* --- Reallocate the array --- *
+   *
+   * The neat @realloc@ code doesn't need to be here: the offset changes
+   * almost all the time -- that's the whole point of this routine!
+   */
+
+  /* --- Decide on a new size --- *
+   *
+   * There's a minimum possible size for the array which is used if it's
+   * currently completely empty.  Otherwise I choose the smallest power of
+   * two which is big enough, starting at double the current size.
+   */
 
   nsz = v ? b->sz + b->off : (DA_INITSZ >> 1);
   do nsz <<= 1; while (nsz < rq);
+
+  /* --- Reallocate the block --- *
+   *
+   * The neat @realloc@ code doesn't need to be here: the offset changes
+   * almost all the time -- that's the whole point of this routine!
+   */
+
   q = xmalloc(nsz * sz);
   q += (nsz - slots) * sz;
-  memcpy(q, p, b->len * sz);
-  if (p)
+  if (p) {
+    memcpy(q, p, b->len * sz);
     free(p - b->off * sz);
+  }
+
+  /* --- Fill in the other parts of the base structure --- */
+  
   b->off = nsz - slots;
   b->sz = slots;
   b->unshift = b->push = 0;