This is a handy helper for creating one vector from another,
mapping each element of the origin vector to an element of
the target vector with a given conversion function. It can
also be used to "concatenate" vectors by passing in a non-NULL first
argument.

Signed-off-by: Martin Wilck <mwi...@suse.com>
---
 libmultipath/vector.h | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/libmultipath/vector.h b/libmultipath/vector.h
index 3f6e579ae19a..6018b57525bf 100644
--- a/libmultipath/vector.h
+++ b/libmultipath/vector.h
@@ -42,6 +42,35 @@ typedef struct _vector *vector;
 #define vector_foreach_slot_backwards(v,p,i) \
        for (i = VECTOR_SIZE(v); i > 0 && ((p) = (v)->slot[i-1]); i--)
 
+#define identity(x) (x)
+/*
+ * Given a vector vec with elements of given type,
+ * return a newly allocated vector with elements conv(e) for each element
+ * e in vec. "conv" may be a macro or a function.
+ * Use "identity" for a simple copy.
+ */
+#define vector_convert(new, vec, type, conv)                           \
+       ({                                                              \
+               const struct _vector *__v = (vec);                      \
+               vector __t = (new);                                     \
+               type *__j;                                              \
+               int __i;                                                \
+                                                                       \
+               if (__t == NULL)                                        \
+                       __t = vector_alloc();                           \
+               if (__t != NULL) {                                      \
+                       vector_foreach_slot(__v, __j, __i) {            \
+                               if (vector_alloc_slot(__t) == NULL) {   \
+                                       vector_free(__t);               \
+                                       __t = NULL;                     \
+                                       break;                          \
+                               }                                       \
+                               vector_set_slot(__t, conv(__j));        \
+                       }                                               \
+               }                                                       \
+               __t;                                                    \
+       })
+
 /* Prototypes */
 extern vector vector_alloc(void);
 extern void *vector_alloc_slot(vector v);
-- 
2.16.1

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