From c6e5bbae090d043b2e852bb8137bea8e5bca6a6f Mon Sep 17 00:00:00 2001
Message-Id: <c6e5bbae090d043b2e852bb8137bea8e5bca6a6f.1714964267.git.mdw@distorted.org.uk>
From: Mark Wooding <mdw@chiark.greenend.org.uk>
Date: Sat, 6 Nov 1999 12:40:45 +0000
Subject: [PATCH] Minor changes to allocation strategy.
Organization: Straylight/Edgeware

From: mdw <mdw>

---
 darray.c | 51 ++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 42 insertions(+), 9 deletions(-)

diff --git a/darray.c b/darray.c
index d7967c6..83150f7 100644
--- a/darray.c
+++ b/darray.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
- * $Id: darray.c,v 1.3 1999/10/29 22:59:22 mdw Exp $
+ * $Id: darray.c,v 1.4 1999/11/06 12:40:45 mdw Exp $
  *
  * Dynamically growing dense arrays
  *
@@ -30,6 +30,9 @@
 /*----- 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.
  *
@@ -106,10 +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;
@@ -118,15 +122,23 @@ void *da_ensure(da_base *b, void *v, size_t sz, size_t n)
     return (q);
   }
 
-  /* --- Reallocate the array --- *
+  /* --- Decide on a new size --- *
    *
-   * If the offset isn't changing, it's sensible to use @realloc@ if
-   * available.  Otherwise the overhead of copying all the data twice
-   * probably isn't worth it.
+   * 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 --- *
+   *
+   * 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;
@@ -138,6 +150,9 @@ void *da_ensure(da_base *b, void *v, size_t sz, size_t n)
       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;
@@ -196,10 +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;
@@ -214,14 +230,31 @@ void *da_shunt(da_base *b, void *v, size_t sz, size_t n)
    * 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;
   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;
-- 
[mdw]