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170 lines
3.6 KiB
Go

package bit
import (
"bytes"
)
// normalized values are represented with 11 significant bits. we pre-compute a
// divisor so that we can use a multiply instruction and avoid using
// floating-point division during the lifecycle of the program.
8 years ago
const normal_divisor = float32(1.0) / float32(2047)
const (
coord_ibits = 14 // number of integer bits in a coord value
coord_fbits = 5 // number of fractional bits in a coord value
coord_res = 1.0 / 1 << coord_fbits
)
// ReadUbitVar reads a prefixed uint value. A prefix is 2 bits wide, followed
// by the 4 least-significant bits, then a variable number of most-significant
// bits based on the prefix.
//
// 00 - 4
// 01 - 8
// 10 - 12 (why 12? this really baffles me)
// 11 - 32
func ReadUBitVar(r Reader) uint64 {
switch b := r.ReadBits(6); b >> 4 {
case 0:
return b & 0xf
case 1:
return b&0xf | r.ReadBits(4)<<4
case 2:
return b&0xf | r.ReadBits(8)<<4
case 3:
return b&0xf | r.ReadBits(28)<<4
default:
panic("not reached")
}
}
// reads some sort of uint in a variable length encoding that appears in
// fieldpath. this encoding is deeply baffling.
func ReadUBitVarFP(r Reader) uint64 {
if ReadBool(r) {
return r.ReadBits(2)
}
if ReadBool(r) {
return r.ReadBits(4)
}
if ReadBool(r) {
return r.ReadBits(10)
}
if ReadBool(r) {
return r.ReadBits(17)
}
return r.ReadBits(31)
}
// ReadVarInt reads a variable length int value as a uint64. This is the binary
// representation used by Protobuf. Each byte contributes 7 bits to the value
// in little-endian order. The most-significant bit of each byte represents a
// continuation bit.
func ReadVarInt(r Reader) uint64 {
var (
x uint64
b uint64
shift uint
)
for ; shift < 64; shift += 7 {
b = r.ReadBits(8)
if r.Err() != nil {
return 0
}
x |= b & 0x7f << shift
if b&0x80 == 0 {
return x
}
}
return x
}
// reads a 32bit varint
func ReadVarInt32(r Reader) uint32 {
var (
x uint64
b uint64
shift uint
)
for ; shift < 32; shift += 7 {
b = r.ReadBits(8)
if r.Err() != nil {
return 0
}
x |= b & 0x7f << shift
if b&0x80 == 0 {
return uint32(x)
}
}
return uint32(x)
}
func ReadBool(r Reader) bool {
return r.ReadBits(1) != 0
}
// reads a null-terminated string
func ReadString(r Reader) string {
var buf bytes.Buffer
for b := r.ReadByte(); b != 0; b = r.ReadByte() {
buf.WriteByte(b)
}
return buf.String()
}
8 years ago
func ReadZigZag(r Reader) int64 {
u := ReadVarInt(r)
if u&1 > 0 {
return ^int64(u >> 1)
}
return int64(u >> 1)
}
// reads a ZigZag-encoded 32 bit signed integer
func ReadZigZag32(r Reader) int32 {
u := ReadVarInt32(r)
if u&1 > 0 {
return ^int32(u >> 1)
}
return int32(u >> 1)
}
// reads a 12-bit normalized float. The first bit represents a sign bit, the
// next sequence of 11 bits represents some normalized value between 0 and
// 2047, allowing up to 4096 positions between -1.0 and 1.0. the resulting
// float will always be between -1.0 and 1.0 (that's why it's normal)
func ReadNormal(r Reader) float32 {
// sign bit
if ReadBool(r) {
return float32(r.ReadBits(11)) * normal_divisor
} else {
return -float32(r.ReadBits(11)) * normal_divisor
}
}
8 years ago
// an angle is just a quantized float between 0 and 360
func ReadAngle(r Reader, bits uint) float32 {
return float32(r.ReadBits(bits)) * 360.0 / float32(uint(1)<<bits-1)
}
func ReadCoord(r Reader) float32 {
i := ReadBool(r)
f := ReadBool(r)
if !(i || f) {
return 0
}
neg := ReadBool(r)
var v float32
if i {
v = float32(r.ReadBits(coord_ibits) + 1)
}
if f {
v += float32(r.ReadBits(coord_fbits)) * coord_res
}
if neg {
return -v
}
return v
}