lib: Add generic binary search function to the kernel.

There a large number hand-coded binary searches in the kernel (run
"git grep search | grep binary" to find many of them). Since in my
experience, hand-coding binary searches can be error-prone, it seems
worth cleaning this up by providing a generic binary search function.

This generic binary search implementation comes from Ksplice. It has
the same basic API as the C library bsearch() function. Ksplice uses
it in half a dozen places with 4 different comparison functions, and I
think our code is substantially cleaner because of this.

Signed-off-by: Tim Abbott <[e-mail]>
Extra-bikeshedding-by: Alan Jenkins <[e-mail]>
Extra-bikeshedding-by: André Goddard Rosa <[e-mail]>
Extra-bikeshedding-by: Rusty Russell <[e-mail]>
Signed-off-by: Rusty Russell <[e-mail]>
Signed-off-by: Alessio Igor Bogani <[e-mail]>
Signed-off-by: Rusty Russell <[e-mail]>

include/linux/bsearch.h

···
+#ifndef _LINUX_BSEARCH_H
+#define _LINUX_BSEARCH_H
+
+#include <linux/types.h>
+
+void *bsearch(const void *key, const void *base, size_t num, size_t size,
+	      int (*cmp)(const void *key, const void *elt));
+
+#endif /* _LINUX_BSEARCH_H */

lib/Makefile

···
 
 obj-y += bcd.o div64.o sort.o parser.o halfmd4.o debug_locks.o random32.o \
 	 bust_spinlocks.o hexdump.o kasprintf.o bitmap.o scatterlist.o \
-	 string_helpers.o gcd.o lcm.o list_sort.o uuid.o flex_array.o
+	 string_helpers.o gcd.o lcm.o list_sort.o uuid.o flex_array.o \
+	 bsearch.o
 obj-y += kstrtox.o
 obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o
 

lib/bsearch.c

···
+/*
+ * A generic implementation of binary search for the Linux kernel
+ *
+ * Copyright (C) 2008-2009 Ksplice, Inc.
+ * Author: Tim Abbott <[e-mail]>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; version 2.
+ */
+
+#include <linux/module.h>
+#include <linux/bsearch.h>
+
+/*
+ * bsearch - binary search an array of elements
+ * @key: pointer to item being searched for
+ * @base: pointer to first element to search
+ * @num: number of elements
+ * @size: size of each element
+ * @cmp: pointer to comparison function
+ *
+ * This function does a binary search on the given array.  The
+ * contents of the array should already be in ascending sorted order
+ * under the provided comparison function.
+ *
+ * Note that the key need not have the same type as the elements in
+ * the array, e.g. key could be a string and the comparison function
+ * could compare the string with the struct's name field.  However, if
+ * the key and elements in the array are of the same type, you can use
+ * the same comparison function for both sort() and bsearch().
+ */
+void *bsearch(const void *key, const void *base, size_t num, size_t size,
+	      int (*cmp)(const void *key, const void *elt))
+{
+	size_t start = 0, end = num;
+	int result;
+
+	while (start < end) {
+		size_t mid = start + (end - start) / 2;
+
+		result = cmp(key, base + mid * size);
+		if (result < 0)
+			end = mid;
+		else if (result > 0)
+			start = mid + 1;
+		else
+			return (void *)base + mid * size;
+	}
+
+	return NULL;
+}
+EXPORT_SYMBOL(bsearch);