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For Fletcher32, define a constant FOOTER_LENGTH #718

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Mar 31, 2025
Merged

For Fletcher32, define a constant FOOTER_LENGTH #718

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19 changes: 13 additions & 6 deletions numcodecs/fletcher32.pyx
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,13 @@ from numcodecs.abc import Codec
from numcodecs.compat import ensure_contiguous_ndarray


cdef extern from *:
"""
const Py_ssize_t FOOTER_LENGTH = sizeof(uint32_t);
"""
const Py_ssize_t FOOTER_LENGTH


cdef uint32_t _fletcher32(const uint8_t[::1] _data):
# converted from
# https://github.com/Unidata/netcdf-c/blob/main/plugins/H5checksum.c#L109
Expand Down Expand Up @@ -66,13 +73,13 @@ class Fletcher32(Codec):
codec_id = "fletcher32"

def encode(self, buf):
"""Return buffer plus 4-byte fletcher checksum"""
"""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 Py_ssize_t b_len = len(b_mv)

cdef Py_ssize_t out_len = b_len + 4
cdef Py_ssize_t out_len = b_len + FOOTER_LENGTH
cdef bytes out = PyBytes_FromStringAndSize(NULL, out_len)
cdef uint8_t* out_ptr = <uint8_t*>out

Expand All @@ -88,8 +95,8 @@ class Fletcher32(Codec):
cdef uint8_t* b_ptr = &b_mv[0]
cdef Py_ssize_t b_len = len(b_mv)

val = _fletcher32(b_mv[:-4])
found = load_le32(&b_mv[-4])
val = _fletcher32(b_mv[:-FOOTER_LENGTH])
found = load_le32(&b_mv[-FOOTER_LENGTH])
if val != found:
raise RuntimeError(
f"The fletcher32 checksum of the data ({val}) did not"
Expand All @@ -102,7 +109,7 @@ class Fletcher32(Codec):
if out is not None:
out_mv = ensure_contiguous_ndarray(out).view("uint8")
out_ptr = &out_mv[0]
memcpy(out_ptr, b_ptr, b_len - 4)
memcpy(out_ptr, b_ptr, b_len - FOOTER_LENGTH)
else:
out = b_mv[:-4]
out = b_mv[:-FOOTER_LENGTH]
return out
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