add a simple set data structure

We work with a few sets, so add a data structure for it to avoid
ad-hoc implementations in other modules.  This is simplify a list
as the sets we're working with tends to be quite small: typically
less than 10.  As long as we don't need to support more, the perf
should be good enough.

src/module.mk

@@ -2,6 +2,7 @@ SRC-tmpfiles := \
 	main.c \
 
 SRC-libtmpfiles.so := \
+	set.c \
 
 all: tmpfiles
 tmpfiles: $(SRC-libtmpfiles.so:.c=.o) $(SRC-tmpfiles:.c=.o)

src/set.c

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2017-2019 Gentoo Foundation
+ * Copyright 2017-2019 The Chromium OS Authors
+ * Released under the 2-clause BSD license.
+ */
+
+#include "tmpfiles.h"
+
+void ordered_set_init(ordered_set_t *set)
+{
+	set->reserved = 16;
+	set->count = 0;
+	set->elements = xmalloc(sizeof(*set->elements) * set->reserved);
+}
+
+void ordered_set_add(ordered_set_t *set, const char *element)
+{
+	if (set->count == set->reserved) {
+		set->reserved *= 2;
+		set->elements = xrealloc(set->elements, sizeof(*set->elements) * set->reserved);
+	}
+
+	set->elements[set->count] = xstrdup(element);
+	set->count++;
+}
+
+bool ordered_set_exists(ordered_set_t *set, const char *element)
+{
+	for (size_t i = 0; i < set->count; ++i)
+		if (strcmp(set->elements[i], element) == 0)
+			return true;
+
+	return false;
+}
+
+void ordered_set_free(ordered_set_t *set)
+{
+	for (size_t i = 0; i < set->count; ++i)
+		free(set->elements[i]);
+
+	free(set->elements);
+	set->elements = NULL;
+}

src/set.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2017-2019 Gentoo Foundation
+ * Copyright 2017-2019 The Chromium OS Authors
+ * Released under the 2-clause BSD license.
+ */
+
+#ifndef TMPFILES_SET_H
+#define TMPFILES_SET_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+	char **elements;
+	size_t reserved;
+	size_t count;
+} ordered_set_t;
+
+/* Initialize a set.  Its storage should be declared already. */
+extern void ordered_set_init(ordered_set_t *set);
+
+/* Add a string to the set.  The string is duplicated. */
+extern void ordered_set_add(ordered_set_t *set, const char *element);
+
+/* Test whether the string is in the set. */
+extern bool ordered_set_exists(ordered_set_t *set, const char *element);
+
+/* Clear a set. Must call ordered_set_init before using again. */
+extern void ordered_set_free(ordered_set_t *set);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

src/tmpfiles.h

@@ -19,5 +19,6 @@
 #include "headers.h"
 
 #include "xfuncs.h"
+#include "set.h"
 
 #endif

test/module.mk

@@ -2,6 +2,7 @@
 LDFLAGS-tmpfiles_test = -pthread
 LDLIBS-tmpfiles_test = -lgtest
 SRC-tmpfiles_test := \
+	set_test.cc \
 	xfuncs_test.cc \
 
 OBJ-tmpfiles_test := $(SRC-tmpfiles_test:.cc=.o)

test/set_test.cc

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2017-2019 Gentoo Foundation
+ * Copyright 2017-2019 The Chromium OS Authors
+ * Released under the 2-clause BSD license.
+ */
+
+/* Tests for the set module. */
+
+#include "test.h"
+
+/* Make sure we can reinit things. */
+TEST(Set, Reinit) {
+	ordered_set_t set;
+	ordered_set_init(&set);
+	ordered_set_free(&set);
+	ordered_set_init(&set);
+	ordered_set_free(&set);
+	ordered_set_init(&set);
+	ordered_set_free(&set);
+}
+
+/* Smoke test for the API. */
+TEST(Set, Smoke) {
+	ordered_set_t set;
+	ordered_set_init(&set);
+	ASSERT_FALSE(ordered_set_exists(&set, "foo"));
+	ASSERT_FALSE(ordered_set_exists(&set, "bar"));
+	ordered_set_add(&set, "foo");
+	ASSERT_TRUE(ordered_set_exists(&set, "foo"));
+	ASSERT_FALSE(ordered_set_exists(&set, "bar"));
+	ordered_set_free(&set);
+}