strmap: add a strset sub-type

Similar to adding strintmap for special-casing a string -> int mapping,
add a strset type for cases where we really are only interested in using
strmap for storing a set rather than a mapping.  In this case, we'll
always just store NULL for the value but the different struct type makes
it clearer than code comments how a variable is intended to be used.

The difference in usage also results in some differences in API: a few
things that aren't necessary or meaningful are dropped (namely, the
free_values argument to *_clear(), and the *_get() function), and
strset_add() is chosen as the API instead of strset_put().

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strmap.c

@@ -143,3 +143,20 @@ void strintmap_incr(struct strintmap *map, const char *str, intptr_t amt)
 	else
 		strintmap_set(map, str, map->default_value + amt);
 }
+
+int strset_add(struct strset *set, const char *str)
+{
+	/*
+	 * Cannot use strmap_put() because it'll return NULL in both cases:
+	 *   - cannot find str: NULL means "not found"
+	 *   - does find str: NULL is the value associated with str
+	 */
+	struct strmap_entry *entry = find_strmap_entry(&set->map, str);
+
+	if (entry)
+		return 0;
+
+	entry = create_entry(&set->map, str, NULL);
+	hashmap_add(&set->map.map, &entry->ent);
+	return 1;
+}

strmap.h

@@ -27,6 +27,7 @@ int cmp_strmap_entry(const void *hashmap_cmp_fn_data,
 			.map = STRMAP_INIT,   \
 			.default_value = 0,   \
 		       }
+#define STRSET_INIT { .map = STRMAP_INIT }
 
 /*
  * Initialize the members of the strmap.  Any keys added to the strmap will
@@ -196,4 +197,66 @@ static inline void strintmap_set(struct strintmap *map, const char *str,
  */
 void strintmap_incr(struct strintmap *map, const char *str, intptr_t amt);
 
+/*
+ * strset:
+ *    A set of strings.
+ *
+ * Primary differences with strmap:
+ *    1) The value is always NULL, and ignored.  As there is no value to free,
+ *       there is one fewer argument to strset_clear
+ *    2) No strset_get() because there is no value.
+ *    3) No strset_put(); use strset_add() instead.
+ */
+
+struct strset {
+	struct strmap map;
+};
+
+#define strset_for_each_entry(mystrset, iter, var)	\
+	strmap_for_each_entry(&(mystrset)->map, iter, var)
+
+static inline void strset_init(struct strset *set)
+{
+	strmap_init(&set->map);
+}
+
+static inline void strset_init_with_options(struct strset *set,
+					    int strdup_strings)
+{
+	strmap_init_with_options(&set->map, strdup_strings);
+}
+
+static inline void strset_clear(struct strset *set)
+{
+	strmap_clear(&set->map, 0);
+}
+
+static inline void strset_partial_clear(struct strset *set)
+{
+	strmap_partial_clear(&set->map, 0);
+}
+
+static inline int strset_contains(struct strset *set, const char *str)
+{
+	return strmap_contains(&set->map, str);
+}
+
+static inline void strset_remove(struct strset *set, const char *str)
+{
+	return strmap_remove(&set->map, str, 0);
+}
+
+static inline int strset_empty(struct strset *set)
+{
+	return strmap_empty(&set->map);
+}
+
+static inline unsigned int strset_get_size(struct strset *set)
+{
+	return strmap_get_size(&set->map);
+}
+
+/* Returns 1 if str is added to the set; returns 0 if str was already in set */
+int strset_add(struct strset *set, const char *str);
+
 #endif /* STRMAP_H */