Fix const discard warnings in fletcher32 (#728)

When accessing a pointer from a `const` typed-memoryview, make sure the
pointer itself is also typed as `const` to avoid compiler warnings about
discarding `const`.

Author: jakirkham

docs/release.rst

@@ -14,6 +14,25 @@ Release notes
 
 .. _unreleased:
 
+Unreleased
+----------
+
+Enhancements
+~~~~~~~~~~~~
+
+Improvements
+~~~~~~~~~~~~
+
+Fixes
+~~~~~
+
+* Fix ``const`` discard warnings in ``fletcher32``.
+  By :user:`John Kirkham <jakirkham>`, :issue:`728`
+
+Maintenance
+~~~~~~~~~~~
+
+
 0.16.0
 ------
 

numcodecs/fletcher32.pyx

@@ -76,7 +76,7 @@ class Fletcher32(Codec):
         """Return buffer plus a footer with the fletcher checksum (4-bytes)"""
         buf = ensure_contiguous_ndarray(buf).ravel().view('uint8')
         cdef const uint8_t[::1] b_mv = buf
-        cdef uint8_t* b_ptr = &b_mv[0]
+        cdef const uint8_t* b_ptr = &b_mv[0]
         cdef Py_ssize_t b_len = len(b_mv)
 
         cdef Py_ssize_t out_len = b_len + FOOTER_LENGTH
@@ -92,7 +92,7 @@ class Fletcher32(Codec):
         """Check fletcher checksum, and return buffer without it"""
         b = ensure_contiguous_ndarray(buf).view('uint8')
         cdef const uint8_t[::1] b_mv = b
-        cdef uint8_t* b_ptr = &b_mv[0]
+        cdef const uint8_t* b_ptr = &b_mv[0]
         cdef Py_ssize_t b_len = len(b_mv)
 
         val = _fletcher32(b_mv[:-FOOTER_LENGTH])