33 lines
1.2 KiB
Nim
33 lines
1.2 KiB
Nim
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import stint
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const MID* = u256(2).pow(u256(255))
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const MAX* = high(Uint256)
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# Custom distance function described in: https://notes.ethereum.org/h58LZcqqRRuarxx4etOnGQ#Storage-Layout
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# The implementation looks different than in spec, due to the fact that in practice
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# we are operating on unsigned 256bit integers instead of signed big ints.
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# Thanks to this we do not need to use:
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# - modulo operations
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# - abs operation
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# and the results are eqivalent to function described in spec.
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#
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# The way it works is as follows. Let say we have integers modulo 8:
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# [0, 1, 2, 3, 4, 5, 6, 7]
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# and we want to calculate minimal distance between 0 and 5.
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# Raw difference is: 5 - 0 = 5, which is larger than mid point which is equal to 4.
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# From this we know that the shorter distance is the one wraping around 0, which
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# is equal to 3
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proc distance*(node_id: UInt256, content_id: UInt256): UInt256 =
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let rawDiff =
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if node_id > content_id:
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node_id - content_id
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else:
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content_id - node_id
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if rawDiff > MID:
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# If rawDiff is larger than mid this means that distance between node_id and
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# content_id is smaller when going from max side.
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MAX - rawDiff + UInt256.one
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else:
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rawDiff
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