From e544d86c797edec613673c7272f8d4f8b05d87f8 Mon Sep 17 00:00:00 2001
From: Nir Soffer <nsoffer@redhat.com>
Date: Fri, 5 Nov 2021 22:16:26 +0200
Subject: [PATCH] common/urils/vector.c: Optimize vector append

Minimize reallocs by growing the backing array by factor of 1.5.

Testing show that now append() is fast without calling reserve()
upfront, simplifying code using vector.

    NBDKIT_BENCH=1 ./test-vector
    bench_reserve: 1000000 appends in 0.004496 s
    bench_append: 1000000 appends in 0.004180 s

This can make a difference in code appending millions of items.

Ported from libnbd commit 985dfa72ae2e41901f0af21e7205ef85428cd4bd.

(cherry picked from commit 12356fa97a840de19bb61e0abedd6e7c7e578e5a)
---
 common/utils/vector.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/common/utils/vector.c b/common/utils/vector.c
index 00cd2546..7df17e1b 100644
--- a/common/utils/vector.c
+++ b/common/utils/vector.c
@@ -41,11 +41,21 @@ int
 generic_vector_reserve (struct generic_vector *v, size_t n, size_t itemsize)
 {
   void *newptr;
+  size_t reqalloc, newalloc;
 
-  newptr = realloc (v->ptr, (n + v->alloc) * itemsize);
+  reqalloc = v->alloc + n;
+  if (reqalloc < v->alloc)
+    return -1; /* overflow */
+
+  newalloc = (v->alloc * 3 + 1) / 2;
+
+  if (newalloc < reqalloc)
+    newalloc = reqalloc;
+
+  newptr = realloc (v->ptr, newalloc * itemsize);
   if (newptr == NULL)
     return -1;
   v->ptr = newptr;
-  v->alloc += n;
+  v->alloc = newalloc;
   return 0;
 }
-- 
2.31.1