Datasets:
File size: 1,955 Bytes
4878bd3 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 | """Independently decodable, byte-exact Prithvi sample frames."""
import hashlib
import struct
import numpy as np
import zstandard as zstd
MAGIC = b'EOZ1'
FILENAMES = ('bands.npy', 'fmask.npy', 'lst.npy', 'metadata.json')
def _encode_words(raw):
if len(raw) % 2:
raise ValueError('Word transform requires even byte count')
words = np.frombuffer(raw, dtype='<u2')
delta = np.empty_like(words)
delta[:1] = words[:1]
np.subtract(words[1:], words[:-1], out=delta[1:])
return delta.view(np.uint8).reshape(-1, 2).T.copy().tobytes()
def _decode_words(raw):
delta = np.frombuffer(raw, dtype=np.uint8).reshape(2, -1).T.copy().view('<u2').reshape(-1)
return np.cumsum(delta, dtype=np.uint16).astype('<u2', copy=False).tobytes()
def original_digest(parts):
h = hashlib.sha256()
for part in parts:
h.update(struct.pack('<Q', len(part)))
h.update(part)
return h.hexdigest()
def encode(parts, level=3):
if len(parts) != 4:
raise ValueError('Expected four original files')
payload = MAGIC + struct.pack('<4I', *(len(p) for p in parts))
payload += _encode_words(parts[0]) + parts[1] + _encode_words(parts[2]) + parts[3]
return zstd.ZstdCompressor(level=level, write_checksum=True).compress(payload)
def decode(frame, expected_digest=None):
raw = zstd.ZstdDecompressor().decompress(frame, max_output_size=16 * 1024 * 1024)
if raw[:4] != MAGIC:
raise ValueError('Unsupported sample frame')
lengths = struct.unpack('<4I', raw[4:20])
if sum(lengths) + 20 != len(raw):
raise ValueError('Invalid sample lengths')
parts = []
offset = 20
for i, n in enumerate(lengths):
part = raw[offset:offset+n]
parts.append(_decode_words(part) if i in (0, 2) else part)
offset += n
if expected_digest and original_digest(parts) != expected_digest:
raise ValueError('Decoded content hash mismatch')
return parts
|