Add docs

Author: martindurant

docs/checksum32.rst

@@ -22,3 +22,14 @@ Adler32
     .. automethod:: decode
     .. automethod:: get_config
     .. automethod:: from_config
+
+
+Fletcher32
+----------
+
+.. autoclass:: numcodecs.fletcher32.Fletcher32
+
+    .. autoattribute:: codec_id
+    .. automethod:: encode
+    .. automethod:: decode
+

docs/release.rst

@@ -15,7 +15,8 @@ Unreleased
 Enhancements
 ~~~~~~~~~~~~
 
-*
+* Add ``fletcher32`` checksum codec
+  By :user:`Martin Durant <martindurant>`, :issue:`410`.
 
 Fix
 ~~~

numcodecs/fletcher32.pyx

@@ -10,6 +10,8 @@ from libc.stdint cimport uint8_t, uint16_t, uint32_t
 
 
 cdef uint32_t _fletcher32(const uint8_t[::1] _data):
+    # converted from
+    # https://github.com/Unidata/netcdf-c/blob/main/plugins/H5checksum.c#L109
     cdef:
         const uint8_t *data = &_data[0]
         size_t _len = _data.shape[0]
@@ -46,6 +48,10 @@ cdef uint32_t _fletcher32(const uint8_t[::1] _data):
 class Fletcher32(Codec):
     """The fletcher checksum with 16-bit words and 32-bit output
 
+    This is the netCDF4/HED5 implementation, which is not equivalent
+    to the one in wikipedia
+    https://github.com/Unidata/netcdf-c/blob/main/plugins/H5checksum.c#L95
+
     With this codec, the checksum is concatenated on the end of the data
     bytes when encoded. At decode time, the checksum is performed on
     the data portion and compared with the four-byte checksum, raising