kconfig: convert linked list of files to hash table

Currently, a linked list is used to keep track of all the Kconfig
files that have ever been parsed. Every time the "source" statement
is encountered, the linked list is traversed to check if the file has
been opened before. This prevents the same file from being recorded
in include/config/auto.conf.cmd again.

Given 1500+ Kconfig files parsed, a hashtable is now a more optimal
data structure.

By the way, you may wonder why we check this in the first place.
It matters only when the same file is included multiple times.
In old days, such a use case was forbidden, but commit f094f8a1
("kconfig: allow multiple inclusion of the same file") provided a bit
more flexibility. Of course, it is almost hypothetical...
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/util.c

@@ -7,6 +7,8 @@
 #include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
+
+#include "hashtable.h"
 #include "lkc.h"
 
 unsigned int strhash(const char *s)
@@ -19,32 +21,32 @@ unsigned int strhash(const char *s)
 	return hash;
 }
 
+/* hash table of all parsed Kconfig files */
+static HASHTABLE_DEFINE(file_hashtable, 1U << 11);
+
 struct file {
-	struct file *next;
+	struct hlist_node node;
 	char name[];
 };
 
-static struct file *file_list;
-
 /* file already present in list? If not add it */
 const char *file_lookup(const char *name)
 {
 	struct file *file;
 	size_t len;
+	int hash = strhash(name);
 
-	for (file = file_list; file; file = file->next) {
-		if (!strcmp(name, file->name)) {
+	hash_for_each_possible(file_hashtable, file, node, hash)
+		if (!strcmp(name, file->name))
 			return file->name;
-		}
-	}
 
 	len = strlen(name);
 	file = xmalloc(sizeof(*file) + len + 1);
 	memset(file, 0, sizeof(*file));
 	memcpy(file->name, name, len);
 	file->name[len] = '\0';
-	file->next = file_list;
-	file_list = file;
+
+	hash_add(file_hashtable, &file->node, hash);
 
 	str_printf(&autoconf_cmd, "\t%s \\\n", name);