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48 lines
1.0 KiB
Go
48 lines
1.0 KiB
Go
8 years ago
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package bit
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// ReadUbitVar reads a prefixed uint value. A prefix is 2 bits wide, followed
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// by the 4 least-significant bits, then a variable number of most-significant
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// bits based on the prefix.
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//
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// 00 - 4
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// 01 - 8
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// 10 - 12 (why 12? this really baffles me)
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// 11 - 32
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func ReadUBitVar(r Reader) uint64 {
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switch b := r.ReadBits(6); b >> 4 {
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case 0:
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return b & 0xf
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case 1:
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return b&0xf | r.ReadBits(4)<<4
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case 2:
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return b&0xf | r.ReadBits(8)<<4
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case 3:
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return b&0xf | r.ReadBits(28)<<4
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default:
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panic("not reached")
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}
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}
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// ReadVarInt reads a variable length int value as a uint64. This is the binary
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// representation used by Protobuf. Each byte contributes 7 bits to the value
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// in little-endian order. The most-significant bit of each byte represents a
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// continuation bit.
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func ReadVarInt(r Reader) uint64 {
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var (
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x uint64
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b uint64
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shift uint
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)
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for ; shift < 64; shift += 7 {
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b = r.ReadBits(8)
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if r.Err() != nil {
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return 0
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}
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x |= b & 0x7f << shift
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if b&0x80 == 0 {
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return x
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}
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}
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return x
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}
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