common: Add tal_arr_remove helper

Suggested-by: Rusty Russell <@rustyrussell>
Signed-off-by: Christian Decker <decker.christian@gmail.com>

common/utils.c

@@ -49,3 +49,15 @@ void clean_tmpctx(void)
 	while ((p = tal_first(tmpctx)) != NULL)
 		tal_free(p);
 }
+
+void tal_arr_remove_(void *p, size_t elemsize, size_t n)
+{
+    // p is a pointer-to-pointer for tal_resize.
+    char *objp = *(char **)p;
+    size_t len = tal_bytelen(objp);
+    assert(len % elemsize == 0);
+    assert((n + 1) * elemsize <= len);
+    memmove(objp + elemsize * n, objp + elemsize * (n+1),
+                      len - (elemsize * (n+1)));
+    tal_resize((char **)p, len - elemsize);
+}

common/utils.h

@@ -29,6 +29,15 @@ u8 *tal_hexdata(const tal_t *ctx, const void *str, size_t len);
 	(tal_resize((p), tal_count(*(p))+1), (*p) + tal_count(*(p))-1)
 #endif
 
+/**
+ * Remove an element from an array
+ *
+ * This will shift the elements past the removed element, changing
+ * their position in memory, so only use this for arrays of pointers.
+ */
+#define tal_arr_remove(p, n) tal_arr_remove_((p), sizeof(**p), (n))
+void tal_arr_remove_(void *p, size_t elemsize, size_t n);
+
 /* Use the POSIX C locale. */
 void setup_locale(void);