The hash.Hash provided by sha256.New is much more efficient
if we always provide it with data a multiple of the block size.
This avoids double-copying of data into the internal block
of sha256.digest.x. Effectively, we are managing a block ourselves
to ensure we only ever call hash.Hash.Write with full blocks.
Performance:
name old time/op new time/op delta
Hash 33.5µs ± 1% 20.6µs ± 1% -38.40% (p=0.000 n=10+9)
The logic has gone through CPU-hours of fuzzing.
Signed-off-by: Joe Tsai <joetsai@digital-static.net>
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