How to efficiently find last key and value in GTree - c

Question

I need to develop a set of functions to extend glib2 GTree with:
find first element
find last
find nearest (floor, ceil, greatest less than, least greater than)
Finding first is easy. You simply stop the g_tree_foreach() calback after first. But how to find the last element without traversing the whole tree?
I thought I could use g_tree_search() with a callback that keeps returning a positive value until found, but how do I know I'm currently on the last element?
#include <stdio.h>
#include <sys/types.h>
#include <string.h>
#include <glib.h>
static
gint compare_int(gconstpointer p1, gconstpointer p2) {
int i1 = GPOINTER_TO_INT(p1);
int i2 = GPOINTER_TO_INT(p2);
//printf("%d %d\n", i1, i2);
return i1 == i2 ? 0 : i1 > i2 ? 1 : -1;
}
static
gboolean traverse(gpointer key, gpointer value, gpointer data) {
//int ikey = GPOINTER_TO_INT(key);
const char *sval = (const char *)value;
printf("%s\n", sval);
return FALSE;
}
static
gint find_last(gconstpointer p, gpointer user_data) {
return 1;
}
static inline const char *NULS(const char *s) {
return s ? s : "NULL";
}
int main(int argc, char *argv[]) {
GTree *tree = g_tree_new(compare_int);
g_tree_insert(tree, GINT_TO_POINTER(10), "ten");
g_tree_insert(tree, GINT_TO_POINTER(-99), "minus ninety-nine");
g_tree_insert(tree, GINT_TO_POINTER(8), "eight");
g_tree_foreach(tree, traverse, NULL);
printf("=======\n%s\n", NULS((const char*)g_tree_search(tree, (GCompareFunc)find_last, NULL)));
return 0;
}

Answer 1

I didn't want to fully implement my own tree, because I wanted to perform advanced search on GTree instances received from 3rd-party code.
Instead I thought that Glib authors would hardly change their internal structures these days and that I could use their fields directly.
The result is the extended version of the internal function g_tree_find_node() from gtree.c. I added two parameters to control whether I want first, last or nearest node. The algorithm for nearest nodes differs from java's TreeMap, because our node doesn't have a pointer to its parent. Full code with the unit test is here: gtreeex.c.
typedef enum {
FIND_EXACT = 0,
FIND_FLOOR = 0x2,
FIND_CEIL = 0x20,
FIND_LOWER = (FIND_FLOOR + 1),
FIND_HIGHER = (FIND_CEIL + 1)
} find_mode;
static GTreeNode *
g_tree_find_node_ex (GTree *tree,
gconstpointer key,
GCompareDataFunc key_compare,
find_mode mode
)
{
GTreeNode *node;
gint cmp;
GTreeNode *last_lesser_node = NULL;
GTreeNode *last_greater_node = NULL;
node = tree->root;
if (!node)
return NULL;
while (1)
{
cmp = key_compare (key, node->key, tree->key_compare_data);
if (cmp == 0) {
if (mode == FIND_LOWER) {
cmp = -1;
} else if (mode == FIND_HIGHER) {
cmp = 1;
} else {
return node;
}
}
if (cmp < 0)
{
if (!node->left_child) {
if ( (mode & FIND_FLOOR) ) {
return last_lesser_node; /* can be null */
}
if ( (mode & FIND_CEIL) ) {
return node;
}
return NULL;
}
last_greater_node = node;
node = node->left;
}
else
{
if (!node->right_child) {
if ( (mode & FIND_CEIL) ) {
return last_greater_node; /* can be null */
}
if ( (mode & FIND_FLOOR) ) {
return node;
}
return NULL;
}
last_lesser_node = node;
node = node->right;
}
}
}
For better performance it's possible to use preprocessor macros instead of the two new parameters, replace if with #if and include the bits header multiple times.