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* [DEV-97] Moved github.com/daglabs/btcutil into github.com/daglabs/btcd/btcutil. * [DEV-97] Updated Gopkg.toml to no longer refer to btcutil. * [DEV-97] Renamed btcutil to util.
20 lines
588 B
Go
20 lines
588 B
Go
package util
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// log2FloorMasks defines the masks to use when quickly calculating
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// floor(log2(x)) in a constant log2(32) = 5 steps, where x is a uint32, using
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// shifts. They are derived from (2^(2^x) - 1) * (2^(2^x)), for x in 4..0.
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var log2FloorMasks = []uint32{0xffff0000, 0xff00, 0xf0, 0xc, 0x2}
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// FastLog2Floor calculates and returns floor(log2(x)) in a constant 5 steps.
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func FastLog2Floor(n uint32) uint8 {
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rv := uint8(0)
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exponent := uint8(16)
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for i := 0; i < 5; i++ {
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if n&log2FloorMasks[i] != 0 {
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rv += exponent
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n >>= exponent
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}
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exponent >>= 1
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}
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return rv
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} |