shared: add NM_UTILS_GET_NEXT_REALLOC_SIZE_1000 define

When we have a buffer that we want to grow exponentially with
nm_utils_get_next_realloc_size(), then there are certain buffer
sizes that are better suited.

For example, if you have an empty NMStrBuf (len == 0), and you
want to allocate roughly one kilobyte, then 1024 is a bad choice,
because nm_utils_get_next_realloc_size() will give you 2024 bytes.

NM_UTILS_GET_NEXT_REALLOC_SIZE_1000 might be better in this case.

2 files changed, 25 insertions, 0 deletions

diff --git a/shared/nm-glib-aux/nm-shared-utils.h b/shared/nm-glib-aux/nm-shared-utils.h
index 57a26487b8..8f65f5806f 100644
--- a/shared/nm-glib-aux/nm-shared-utils.h
+++ b/shared/nm-glib-aux/nm-shared-utils.h
@@ -1835,6 +1835,26 @@ nm_utils_strdup_reset (char **dst, const char *src)
 
 /*****************************************************************************/
 
+/* nm_utils_get_next_realloc_size() is used to grow buffers exponentially, when
+ * the final size is unknown. As such, it has borders for which it allocates
+ * certain buffer sizes.
+ *
+ * The use of these defines is to get favorable allocation sequences.
+ * For example, nm_str_buf_init() asks for an initial allocation size. Note that
+ * it reserves the exactly requested amount, under the assumption that the
+ * user may know how many bytes will be required. However, often the caller
+ * doesn't know in advance, and NMStrBuf grows exponentially by calling
+ * nm_utils_get_next_realloc_size().
+ * Imagine you call nm_str_buf_init() with an initial buffer size 100, and you
+ * add one character at a time. Then the first reallocation will increase the
+ * buffer size only from 100 to 104.
+ * If you however start with an initial buffer size of 104, then the next reallocation
+ * via nm_utils_get_next_realloc_size() gives you 232, and so on. By using
+ * these sizes, it results in fewer reallocations, if you anyway don't now the exact
+ * size in advance. */
+#define NM_UTILS_GET_NEXT_REALLOC_SIZE_104     ((gsize) 104)
+#define NM_UTILS_GET_NEXT_REALLOC_SIZE_1000    ((gsize) 1000)
+
 gsize nm_utils_get_next_realloc_size (gboolean true_realloc, gsize requested);
 
 /*****************************************************************************/
diff --git a/shared/nm-glib-aux/tests/test-shared-general.c b/shared/nm-glib-aux/tests/test-shared-general.c
index 2c2aa3fcfd..61c6c4a4c7 100644
--- a/shared/nm-glib-aux/tests/test-shared-general.c
+++ b/shared/nm-glib-aux/tests/test-shared-general.c
@@ -635,9 +635,14 @@ test_nm_utils_get_next_realloc_size (void)
 		{  G_MAXSIZE - 24u,      G_MAXSIZE, G_MAXSIZE },
 		{  G_MAXSIZE - 1u,       G_MAXSIZE, G_MAXSIZE },
 		{  G_MAXSIZE,            G_MAXSIZE, G_MAXSIZE },
+		{  NM_UTILS_GET_NEXT_REALLOC_SIZE_104,  NM_UTILS_GET_NEXT_REALLOC_SIZE_104,  NM_UTILS_GET_NEXT_REALLOC_SIZE_104 },
+		{  NM_UTILS_GET_NEXT_REALLOC_SIZE_1000, NM_UTILS_GET_NEXT_REALLOC_SIZE_1000, NM_UTILS_GET_NEXT_REALLOC_SIZE_1000 },
 	};
 	guint i;
 
+	G_STATIC_ASSERT_EXPR (NM_UTILS_GET_NEXT_REALLOC_SIZE_104  == 104u);
+	G_STATIC_ASSERT_EXPR (NM_UTILS_GET_NEXT_REALLOC_SIZE_1000 == 1000u);
+
 	for (i = 0; i < G_N_ELEMENTS (test_data) + 5000u; i++) {
 		gsize requested0;