b2/param.c: new function merge_vars to merge prioritized lists of variables

b2/param.c

@@ -144,6 +144,38 @@ const char *var_lookup(const struct param *vars, const char *name)
 }
 
 
+/*
+ * The variables in list "a" have lower priority than those in list "b",
+ * i.e., a variable in "a" is only picked if there is no variable with the
+ * same name in "b".
+ */
+
+struct param *merge_vars(const struct param *a, const struct param *b)
+{
+	const struct param *v;
+	struct param *res, **last = &res;
+
+	while (a) {
+		for (v = b; v; v = v->next)
+			if (a->u.name == v->u.name)
+				break;
+		if (!v) {
+			*last = alloc_type(struct param);
+			**last = *a;
+			last = &(*last)->next;
+		}
+		a = a->next;
+	}
+	for (v = b; v; v = v->next) {
+		*last = alloc_type(struct param);
+		**last = *v;
+		last = &(*last)->next;
+	}
+	*last = NULL;
+	return res;
+}
+
+
 void free_vars(struct param *vars)
 {
 	struct param *next;

b2/param.h

@@ -120,6 +120,7 @@ extern const struct param_ops param_ops_rel;
 
 struct param *make_var(const char *name, enum relop op, const char *val);
 const char *var_lookup(const struct param *vars, const char *name);
+struct param *merge_vars(const struct param *a, const struct param *b);
 void free_vars(struct param *vars);
 
 #endif /* !PARAM_H */