extend bs:pointer to allow indexing with integers

[mwette]

Consider extending the bs:pointer descriptor to allow integer indexes.
In C pointers are deferenced using array syntax as if they were arrays:

int *a;   => (define a (bytestructure (bs:pointer int) ...)
int x = a[3];  => (define x (bytestructure-ref a 3))

candidate patch to guile/pointer.scm:

--- bs-pointer.scm  2020-03-02 15:19:13.046101687 -0800
+++ fh-pointer.scm  2020-03-02 15:38:35.242496103 -0800
@@ -57,6 +57,13 @@
         (error "Tried to dereference null-pointer.")
         (ffi:pointer->bytevector (ffi:make-pointer address) content-size))))

+(define (pointer-idx-ref bytevector offset index content-size)
+  (let* ((base-address (bytevector-address-ref bytevector offset))
+    (address (+ base-address (* index content-size))))
+    (if (zero? base-address)
+   (error "Tried to dereference null-pointer.")
+   (ffi:pointer->bytevector (ffi:make-pointer address) content-size))))
+
 (define (pointer-set! bytevector offset value)
   (cond
    ((exact-integer? value)
@@ -88,17 +95,24 @@
       (when (eq? 'void descriptor)
         (error "Tried to follow void pointer."))
       (let* ((size (bytestructure-descriptor-size descriptor))
-             (bytevector* (if syntax?
-                              #`(pointer-ref #,bytevector #,offset #,size)
-                              (pointer-ref bytevector offset size)))
-             (index-datum (if syntax? (syntax->datum index) index)))
-        (if (eq? '* index-datum)
-            (values bytevector* 0 descriptor)
-            (if syntax?
-                (bytestructure-unwrap/syntax
-                 bytevector* 0 descriptor (syntax-list index index))
-                (bytestructure-unwrap*
-                 bytevector* 0 descriptor index))))))
+        (index-datum (if syntax? (syntax->datum index) index)))
+   (cond
+    ((eq? '* index-datum)
+     (if syntax?
+         (values #`(pointer-ref #,bytevector #,offset #,size) 0 descriptor)
+         (values (pointer-ref bytevector offset size) 0 descriptor)))
+    ((integer? index-datum)
+     (if syntax?
+         (values #`(pointer-idx-ref #,bytevector #,offset ,index #,size))
+         (values (pointer-idx-ref bytevector offset index-datum size))))
+    (else
+     (if syntax?
+         (let ((bytevector* #`(pointer-ref #,bytevector #,offset #,size)))
+       (bytestructure-unwrap/syntax
+        bytevector* 0 descriptor (syntax-list index index)))
+         (let ((bytevector* (pointer-ref bytevector offset size)))
+       (bytestructure-unwrap*
+        bytevector* 0 descriptor index))))))))
   (define (getter syntax? bytevector offset)
     (if syntax?
         #`(bytevector-address-ref #,bytevector #,offset)

[TaylanUB]

Thanks for the patch, I applied a modified version of it.

I decided to deprecate implicit dereferencing of pointers, so if you used to make use of that, you will now have to make sure that you always dereference pointers explicitly.

The reason is twofold:

• With the macro API, we can't know whether a provided argument will be an integer during runtime, unless it's an integer literal, but limiting the integer indexing feature to literals would be bad.

• Implementing this feature breaks backwards compatibility anyway, since an integer index was already accepted but caused an implicit dereference instead of this new behavior.

I think I've tested the change thoroughly enough so I'm closing this; please reopen if you face any issues.

[mwette]

got it -- I will be working on this next weekend