53 lines
1.4 KiB
Go
53 lines
1.4 KiB
Go
package varint
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import (
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"encoding/binary"
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)
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// Decode reads the first varint encoded number from the given buffer.
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//
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// On success, the function returns the varint as an int64, and the number of
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// bytes read (0 if there was an error).
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func Decode(b []byte) (int64, int) {
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if len(b) == 0 {
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return 0, 0
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}
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// 0xxxxxxx 7-bit positive number
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if (b[0] & 0x80) == 0 {
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return int64(b[0]), 1
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}
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// 10xxxxxx + 1 byte 14-bit positive number
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if (b[0]&0xC0) == 0x80 && len(b) >= 2 {
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return int64(b[0]&0x3F)<<8 | int64(b[1]), 2
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}
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// 110xxxxx + 2 bytes 21-bit positive number
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if (b[0]&0xE0) == 0xC0 && len(b) >= 3 {
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return int64(b[0]&0x1F)<<16 | int64(b[1])<<8 | int64(b[2]), 3
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}
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// 1110xxxx + 3 bytes 28-bit positive number
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if (b[0]&0xF0) == 0xE0 && len(b) >= 4 {
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return int64(b[0]&0xF)<<24 | int64(b[1])<<16 | int64(b[2])<<8 | int64(b[3]), 4
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}
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// 111100__ + int (32-bit) 32-bit positive number
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if (b[0]&0xFC) == 0xF0 && len(b) >= 5 {
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return int64(binary.BigEndian.Uint32(b[1:])), 5
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}
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// 111101__ + long (64-bit) 64-bit number
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if (b[0]&0xFC) == 0xF4 && len(b) >= 9 {
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return int64(binary.BigEndian.Uint64(b[1:])), 9
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}
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// 111110__ + varint Negative recursive varint
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if b[0]&0xFC == 0xF8 {
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if v, n := Decode(b[1:]); n > 0 {
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return -v, n + 1
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}
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return 0, 0
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}
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// 111111xx Byte-inverted negative two bit number (~xx)
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if b[0]&0xFC == 0xFC {
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return ^int64(b[0] & 0x03), 1
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}
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return 0, 0
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}
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