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// Copyright 2025-2026 肖其顿 (XIAO QI DUN)
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package threed
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import (
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"encoding/binary"
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"fmt"
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"io"
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"math"
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"math/bits"
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"unicode/utf8"
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)
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// u3dValues 读取U3D未压缩字段，不允许越过当前块
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type u3dValues struct {
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data []byte
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pos int
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err error
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}
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// u3dBits 按ECMA-363第10章逆解16位算术区间，字节内按低位优先读取
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type u3dBits struct {
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data []byte
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pos uint64
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low, high, code uint32
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err error
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reserve func(uint64, uint64) error
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}
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// u3dHistogram 保存一个动态压缩上下文的频次及前缀和
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type u3dHistogram struct {
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counts []uint32
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tree []uint32
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total uint32
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}
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// finish 校验压缩块的冲刷字段及结束位置
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// 返回: error 截断、冲刷或多余数据错误
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func (r *u3dBits) finish() error {
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if r.err != nil {
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return r.err
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}
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if r.pos > uint64(len(r.data))*8 {
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return io.ErrUnexpectedEOF
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}
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if r.u32() != 0 {
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return fmt.Errorf("invalid U3D compression flush")
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}
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if r.err != nil {
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return r.err
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}
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if (r.pos-16+7)/8 != uint64(len(r.data)) {
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return fmt.Errorf("unexpected U3D compressed block data")
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}
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return nil
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}
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// take 读取定长字段，失败后不继续推进位置
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// 入参: size 字节数
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// 返回: []byte 字段数据，失败时为空
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func (r *u3dValues) take(size int) []byte {
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if r.err != nil {
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return nil
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}
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if size < 0 || size > len(r.data)-r.pos {
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r.err = io.ErrUnexpectedEOF
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return nil
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}
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value := r.data[r.pos : r.pos+size]
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r.pos += size
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return value
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}
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// u8 读取一个未压缩字节
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// 返回: byte 数值
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func (r *u3dValues) u8() byte {
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value := r.take(1)
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if value == nil {
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return 0
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}
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return value[0]
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}
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// u32 读取小端32位整数
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// 返回: uint32 数值
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func (r *u3dValues) u32() uint32 {
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value := r.take(4)
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if value == nil {
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return 0
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}
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return binary.LittleEndian.Uint32(value)
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}
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// f32 读取有限单精度数值
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// 返回: float32 数值
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func (r *u3dValues) f32() float32 {
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value := math.Float32frombits(r.u32())
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if !finite(float64(value)) && r.err == nil {
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r.err = fmt.Errorf("invalid U3D floating-point value")
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}
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return value
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}
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// text 读取带16位字节长度的UTF-8字符串
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// 返回: string 文本
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func (r *u3dValues) text() string {
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length := r.take(2)
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if length == nil {
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return ""
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}
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data := r.take(int(binary.LittleEndian.Uint16(length)))
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if r.err == nil && !utf8.Valid(data) {
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r.err = fmt.Errorf("invalid U3D UTF-8 string")
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}
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return string(data)
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}
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// done 检查字段是否恰好覆盖当前块
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// 返回: error 读取或多余字段错误
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func (r *u3dValues) done() error {
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if r.err != nil {
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return r.err
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}
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if r.pos != len(r.data) {
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return fmt.Errorf("unexpected U3D block data")
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}
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return nil
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}
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// newU3DBits 初始化当前块的独立算术解码状态
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// 入参: data 块数据
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// 返回: *u3dBits 解码器
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func newU3DBits(data []byte) *u3dBits {
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r := &u3dBits{data: data, high: 0xffff}
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for range 16 {
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r.code = r.code<<1 | r.bit()
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}
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return r
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}
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// bit 读取低位优先比特，区间预读最多允许16个末尾零位
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// 返回: uint32 比特
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func (r *u3dBits) bit() uint32 {
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if r.err != nil {
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return 0
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}
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limit := uint64(len(r.data)) * 8
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var value uint32
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if r.pos < limit {
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value = uint32(r.data[r.pos/8]>>(r.pos%8)) & 1
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} else if r.pos >= limit+16 {
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r.err = io.ErrUnexpectedEOF
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return 0
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}
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r.pos++
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return value
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}
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// cumulative 求当前编码值在上下文累计频次中的位置
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// 入参: total 总频次
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// 返回: uint32 累计频次位置
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func (r *u3dBits) cumulative(total uint32) uint32 {
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if r.err != nil {
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return 0
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}
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if total == 0 || r.code < r.low || r.code > r.high {
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r.err = fmt.Errorf("invalid U3D arithmetic interval")
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return 0
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}
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return uint32((uint64(r.code-r.low+1)*uint64(total) - 1) / uint64(r.high-r.low+1))
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}
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// advance 收缩符号区间并执行同位及下溢归一化
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// 入参: start 累计频次, count 符号频次, total 总频次
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func (r *u3dBits) advance(start, count, total uint32) {
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if r.err != nil {
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return
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}
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if count == 0 || start >= total || count > total-start {
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r.err = fmt.Errorf("invalid U3D symbol frequency")
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return
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}
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width := uint64(r.high-r.low) + 1
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r.high = r.low + uint32(width*uint64(start+count)/uint64(total)) - 1
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r.low += uint32(width * uint64(start) / uint64(total))
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for r.err == nil {
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var offset uint32
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switch {
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case r.high < 0x8000:
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case r.low >= 0x8000:
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offset = 0x8000
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case r.low >= 0x4000 && r.high < 0xc000:
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offset = 0x4000
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default:
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return
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}
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r.low = (r.low - offset) << 1
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r.high = (r.high-offset)<<1 | 1
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r.code = (r.code-offset)<<1 | r.bit()
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}
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}
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// byteValue 逆解静态256符号上下文中的反序字节
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// 返回: byte 原始字节
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func (r *u3dBits) byteValue() byte {
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value := r.cumulative(256)
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r.advance(value, 1, 256)
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return bits.Reverse8(byte(value))
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}
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// u32 逆解四个小端字节，不重置压缩区间
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// 返回: uint32 原始整数
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func (r *u3dBits) u32() uint32 {
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var value uint32
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for shift := range 4 {
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value |= uint32(r.byteValue()) << (8 * shift)
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}
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return value
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}
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// index 读取静态上下文的网格索引，大范围按标准使用未压缩整数
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// 入参: count 索引范围
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// 返回: uint32 索引
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func (r *u3dBits) index(count uint32) uint32 {
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if count > 0x3ffe {
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return r.u32()
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}
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value := r.cumulative(count)
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r.advance(value, 1, count)
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return value
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}
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// dynamic 读取动态上下文数值，逃逸后的字面量仍使用当前算术区间
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// 入参: h 频次上下文
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// 返回: uint32 原始整数
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func (r *u3dBits) dynamic(h *u3dHistogram) uint32 {
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return r.dynamicValue(h, false)
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}
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// dynamicValue 按字段位宽处理动态上下文的逃逸字面量，分配前检查可选预算
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// 入参: h 频次上下文, small 是否为8位字段
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// 返回: uint32 解码值
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func (r *u3dBits) dynamicValue(h *u3dHistogram, small bool) uint32 {
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if r.err != nil {
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return 0
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}
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if h.total == 0 {
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if r.reserve != nil {
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if err := r.reserve(1, 32); err != nil {
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r.err = err
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return 0
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}
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}
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h.counts = make([]uint32, 1)
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h.tree = make([]uint32, 2)
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h.add(0)
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}
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index := r.cumulative(h.total)
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symbol, start := h.find(index)
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if r.err != nil {
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return 0
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}
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r.advance(start, h.counts[symbol], h.total)
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r.histogramAdd(h, symbol)
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if r.err != nil {
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return 0
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}
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if symbol != 0 {
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if small && symbol > 256 {
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r.err = fmt.Errorf("invalid U3D compressed byte")
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}
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return symbol - 1
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}
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var value uint32
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if small {
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value = uint32(r.byteValue())
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} else {
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value = r.u32()
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}
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if value < 0xffff && r.err == nil {
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r.histogramAdd(h, value+1)
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}
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return value
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}
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// histogramAdd 在频次数组扩展前计入解码预算
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// 入参: h 频次上下文, symbol 待更新符号
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func (r *u3dBits) histogramAdd(h *u3dHistogram, symbol uint32) {
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if r.err != nil {
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return
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}
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if r.reserve != nil && symbol <= 0xffff && int(symbol) >= len(h.counts) {
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length := min(65536, max(int(symbol)+1, len(h.counts)*2))
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if err := r.reserve(uint64(length-len(h.counts)), 32); err != nil {
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r.err = err
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return
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}
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}
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h.add(symbol)
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}
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// find 以树状前缀和定位符号，避免按符号数量线性扫描
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// 入参: position 累计频次位置
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// 返回: uint32 符号, uint32 符号前缀频次
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func (h *u3dHistogram) find(position uint32) (uint32, uint32) {
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index, prefix := 0, uint32(0)
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for step := 1 << (bits.Len(uint(len(h.tree))) - 1); step != 0; step >>= 1 {
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next := index + step
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if next < len(h.tree) && h.tree[next] <= position-prefix {
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prefix += h.tree[next]
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index = next
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}
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}
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return uint32(index), prefix
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}
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// add 更新动态频次，并按ECMA-363阈值缩减历史频次
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// 入参: symbol 符号，超过16位的值不加入上下文
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func (h *u3dHistogram) add(symbol uint32) {
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if symbol > 0xffff {
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return
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}
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if h.total >= 0x1fff {
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for index := range h.counts {
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h.counts[index] >>= 1
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}
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h.counts[0]++
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h.rebuild()
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}
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if int(symbol) >= len(h.counts) {
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length := min(65536, max(int(symbol)+1, len(h.counts)*2))
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h.counts = append(h.counts, make([]uint32, length-len(h.counts))...)
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h.rebuild()
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}
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h.counts[symbol]++
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h.total++
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for index := int(symbol) + 1; index < len(h.tree); index += index & -index {
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h.tree[index]++
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}
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}
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// rebuild 在线性时间内重建动态频次索引
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func (h *u3dHistogram) rebuild() {
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h.tree = make([]uint32, len(h.counts)+1)
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h.total = 0
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for index, count := range h.counts {
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h.total += count
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i := index + 1
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h.tree[i] += count
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if next := i + (i & -i); next < len(h.tree) {
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h.tree[next] += h.tree[i]
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}
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}
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}
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