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To tell whether we already have a block we find in a bootstrap file, we hash it and see if we have a block with that hash. Before block 203501 blocks were hashed using scrypt, which is designed to be slow. After block 203500 we switched to sha256 which is fast. Notice how blocks 0 and 10000 have hashes that start with a lot of zeroes - that's because they were proof-of-work blocks.
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Very interesting to see the efficiency gain of switching over to sha256 in practice.
The change made sense on it's face; but seeing it in action adds a certain pleasure to it.