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// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package triegen implements a code generator for a trie for associating
// unsigned integer values with UTF-8 encoded runes.
// Many of the go.text packages use tries for storing per-rune information. A
// trie is especially useful if many of the runes have the same value. If this
// is the case, many blocks can be expected to be shared allowing for
// information on many runes to be stored in little space.
// As most of the lookups are done directly on []byte slices, the tries use the
// UTF-8 bytes directly for the lookup. This saves a conversion from UTF-8 to
// runes and contributes a little bit to better performance. It also naturally
// provides a fast path for ASCII.
// Space is also an issue. There are many code points defined in Unicode and as
// a result tables can get quite large. So every byte counts. The triegen
// package automatically chooses the smallest integer values to represent the
// tables. Compacters allow further compression of the trie by allowing for
// alternative representations of individual trie blocks.
// triegen allows generating multiple tries as a single structure. This is
// useful when, for example, one wants to generate tries for several languages
// that have a lot of values in common. Some existing libraries for
// internationalization store all per-language data as a dynamically loadable
// chunk. The go.text packages are designed with the assumption that the user
// typically wants to compile in support for all supported languages, in line
// with the approach common to Go to create a single standalone binary. The
// multi-root trie approach can give significant storage savings in this
// scenario.
// triegen generates both tables and code. The code is optimized to use the
// automatically chosen data types. The following code is generated for a Trie
// or multiple Tries named "foo":
// - type fooTrie
// The trie type.
// - func newFooTrie(x int) *fooTrie
// Trie constructor, where x is the index of the trie passed to Gen.
// - func (t *fooTrie) lookup(s []byte) (v uintX, sz int)
// The lookup method, where uintX is automatically chosen.
// - func lookupString, lookupUnsafe and lookupStringUnsafe
// Variants of the above.
// - var fooValues and fooIndex and any tables generated by Compacters.
// The core trie data.
// - var fooTrieHandles
// Indexes of starter blocks in case of multiple trie roots.
// It is recommended that users test the generated trie by checking the returned
// value for every rune. Such exhaustive tests are possible as the number of
// runes in Unicode is limited.
package triegen // import ""
// TODO: Arguably, the internally optimized data types would not have to be
// exposed in the generated API. We could also investigate not generating the
// code, but using it through a package. We would have to investigate the impact
// on performance of making such change, though. For packages like unicode/norm,
// small changes like this could tank performance.
import (
// builder builds a set of tries for associating values with runes. The set of
// tries can share common index and value blocks.
type builder struct {
Name string
// ValueType is the type of the trie values looked up.
ValueType string
// ValueSize is the byte size of the ValueType.
ValueSize int
// IndexType is the type of trie index values used for all UTF-8 bytes of
// a rune except the last one.
IndexType string
// IndexSize is the byte size of the IndexType.
IndexSize int
// SourceType is used when generating the lookup functions. If the user
// requests StringSupport, all lookup functions will be generated for
// string input as well.
SourceType string
Trie []*Trie
IndexBlocks []*node
ValueBlocks [][]uint64
Compactions []compaction
Checksum uint64
ASCIIBlock string
StarterBlock string
indexBlockIdx map[uint64]int
valueBlockIdx map[uint64]nodeIndex
asciiBlockIdx map[uint64]int
// Stats are used to fill out the template.
Stats struct {
NValueEntries int
NValueBytes int
NIndexEntries int
NIndexBytes int
NHandleBytes int
err error
// A nodeIndex encodes the index of a node, which is defined by the compaction
// which stores it and an index within the compaction. For internal nodes, the
// compaction is always 0.
type nodeIndex struct {
compaction int
index int
// compaction keeps track of stats used for the compaction.
type compaction struct {
c Compacter
blocks []*node
maxHandle uint32
totalSize int
// Used by template-based generator and thus exported.
Cutoff uint32
Offset uint32
Handler string
func (b *builder) setError(err error) {
if b.err == nil {
b.err = err
// An Option can be passed to Gen.
type Option func(b *builder) error
// Compact configures the trie generator to use the given Compacter.
func Compact(c Compacter) Option {
return func(b *builder) error {
b.Compactions = append(b.Compactions, compaction{
c: c,
Handler: c.Handler() + "(n, b)"})
return nil
// Gen writes Go code for a shared trie lookup structure to w for the given
// Tries. The generated trie type will be called nameTrie. newNameTrie(x) will
// return the *nameTrie for tries[x]. A value can be looked up by using one of
// the various lookup methods defined on nameTrie. It returns the table size of
// the generated trie.
func Gen(w io.Writer, name string, tries []*Trie, opts ...Option) (sz int, err error) {
// The index contains two dummy blocks, followed by the zero block. The zero
// block is at offset 0x80, so that the offset for the zero block for
// continuation bytes is 0.
b := &builder{
Name: name,
Trie: tries,
IndexBlocks: []*node{{}, {}, {}},
Compactions: []compaction{{
Handler: name + "Values[n<<6+uint32(b)]",
// The 0 key in indexBlockIdx and valueBlockIdx is the hash of the zero
// block.
indexBlockIdx: map[uint64]int{0: 0},
valueBlockIdx: map[uint64]nodeIndex{0: {}},
asciiBlockIdx: map[uint64]int{},
b.Compactions[0].c = (*simpleCompacter)(b)
for _, f := range opts {
if err := f(b); err != nil {
return 0, err
if b.err != nil {
return 0, b.err
if err = b.print(w); err != nil {
return 0, err
return b.Size(), nil
// A Trie represents a single root node of a trie. A builder may build several
// overlapping tries at once.
type Trie struct {
root *node
// hiddenTrie contains values we want to be visible to the template generator,
// but hidden from the API documentation.
type hiddenTrie struct {
Name string
Checksum uint64
ASCIIIndex int
StarterIndex int
// NewTrie returns a new trie root.
func NewTrie(name string) *Trie {
return &Trie{
children: make([]*node, blockSize),
values: make([]uint64, utf8.RuneSelf),
hiddenTrie{Name: name},
// Gen is a convenience wrapper around the Gen func passing t as the only trie
// and uses the name passed to NewTrie. It returns the size of the generated
// tables.
func (t *Trie) Gen(w io.Writer, opts ...Option) (sz int, err error) {
return Gen(w, t.Name, []*Trie{t}, opts...)
// node is a node of the intermediate trie structure.
type node struct {
// children holds this node's children. It is always of length 64.
// A child node may be nil.
children []*node
// values contains the values of this node. If it is non-nil, this node is
// either a root or leaf node:
// For root nodes, len(values) == 128 and it maps the bytes in [0x00, 0x7F].
// For leaf nodes, len(values) == 64 and it maps the bytes in [0x80, 0xBF].
values []uint64
index nodeIndex
// Insert associates value with the given rune. Insert will panic if a non-zero
// value is passed for an invalid rune.
func (t *Trie) Insert(r rune, value uint64) {
if value == 0 {
s := string(r)
if []rune(s)[0] != r && value != 0 {
// Note: The UCD tables will always assign what amounts to a zero value
// to a surrogate. Allowing a zero value for an illegal rune allows
// users to iterate over [0..MaxRune] without having to explicitly
// exclude surrogates, which would be tedious.
panic(fmt.Sprintf("triegen: non-zero value for invalid rune %U", r))
if len(s) == 1 {
// It is a root node value (ASCII).
t.root.values[s[0]] = value
n := t.root
for ; len(s) > 1; s = s[1:] {
if n.children == nil {
n.children = make([]*node, blockSize)
p := s[0] % blockSize
c := n.children[p]
if c == nil {
c = &node{}
n.children[p] = c
if len(s) > 2 && c.values != nil {
log.Fatalf("triegen: insert(%U): found internal node with values", r)
n = c
if n.values == nil {
n.values = make([]uint64, blockSize)
if n.children != nil {
log.Fatalf("triegen: insert(%U): found leaf node that also has child nodes", r)
n.values[s[0]-0x80] = value
// Size returns the number of bytes the generated trie will take to store. It
// needs to be exported as it is used in the templates.
func (b *builder) Size() int {
// Index blocks.
sz := len(b.IndexBlocks) * blockSize * b.IndexSize
// Skip the first compaction, which represents the normal value blocks, as
// its totalSize does not account for the ASCII blocks, which are managed
// separately.
sz += len(b.ValueBlocks) * blockSize * b.ValueSize
for _, c := range b.Compactions[1:] {
sz += c.totalSize
// TODO: this computation does not account for the fixed overhead of a using
// a compaction, either code or data. As for data, though, the typical
// overhead of data is in the order of bytes (2 bytes for cases). Further,
// the savings of using a compaction should anyway be substantial for it to
// be worth it.
// For multi-root tries, we also need to account for the handles.
if len(b.Trie) > 1 {
sz += 2 * b.IndexSize * len(b.Trie)
return sz
func (b *builder) build() {
// Compute the sizes of the values.
var vmax uint64
for _, t := range b.Trie {
vmax = maxValue(t.root, vmax)
b.ValueType, b.ValueSize = getIntType(vmax)
// Compute all block allocations.
// TODO: first compute the ASCII blocks for all tries and then the other
// nodes. ASCII blocks are more restricted in placement, as they require two
// blocks to be placed consecutively. Processing them first may improve
// sharing (at least one zero block can be expected to be saved.)
for _, t := range b.Trie {
b.Checksum += b.buildTrie(t)
// Compute the offsets for all the Compacters.
offset := uint32(0)
for i := range b.Compactions {
c := &b.Compactions[i]
c.Offset = offset
offset += c.maxHandle + 1
c.Cutoff = offset
// Compute the sizes of indexes.
// TODO: different byte positions could have different sizes. So far we have
// not found a case where this is beneficial.
imax := uint64(b.Compactions[len(b.Compactions)-1].Cutoff)
for _, ib := range b.IndexBlocks {
if x := uint64(ib.index.index); x > imax {
imax = x
b.IndexType, b.IndexSize = getIntType(imax)
func maxValue(n *node, max uint64) uint64 {
if n == nil {
return max
for _, c := range n.children {
max = maxValue(c, max)
for _, v := range n.values {
if max < v {
max = v
return max
func getIntType(v uint64) (string, int) {
switch {
case v < 1<<8:
return "uint8", 1
case v < 1<<16:
return "uint16", 2
case v < 1<<32:
return "uint32", 4
return "uint64", 8
const (
blockSize = 64
// Subtract two blocks to offset 0x80, the first continuation byte.
blockOffset = 2
// Subtract three blocks to offset 0xC0, the first non-ASCII starter.
rootBlockOffset = 3
var crcTable = crc64.MakeTable(crc64.ISO)
func (b *builder) buildTrie(t *Trie) uint64 {
n := t.root
// Get the ASCII offset. For the first trie, the ASCII block will be at
// position 0.
hasher := crc64.New(crcTable)
binary.Write(hasher, binary.BigEndian, n.values)
hash := hasher.Sum64()
v, ok := b.asciiBlockIdx[hash]
if !ok {
v = len(b.ValueBlocks)
b.asciiBlockIdx[hash] = v
b.ValueBlocks = append(b.ValueBlocks, n.values[:blockSize], n.values[blockSize:])
if v == 0 {
// Add the zero block at position 2 so that it will be assigned a
// zero reference in the lookup blocks.
// TODO: always do this? This would allow us to remove a check from
// the trie lookup, but at the expense of extra space. Analyze
// performance for unicode/norm.
b.ValueBlocks = append(b.ValueBlocks, make([]uint64, blockSize))
t.ASCIIIndex = v
// Compute remaining offsets.
t.Checksum = b.computeOffsets(n, true)
// We already subtracted the normal blockOffset from the index. Subtract the
// difference for starter bytes.
t.StarterIndex = n.index.index - (rootBlockOffset - blockOffset)
return t.Checksum
func (b *builder) computeOffsets(n *node, root bool) uint64 {
// For the first trie, the root lookup block will be at position 3, which is
// the offset for UTF-8 non-ASCII starter bytes.
first := len(b.IndexBlocks) == rootBlockOffset
if first {
b.IndexBlocks = append(b.IndexBlocks, n)
// We special-case the cases where all values recursively are 0. This allows
// for the use of a zero block to which all such values can be directed.
hash := uint64(0)
if n.children != nil || n.values != nil {
hasher := crc64.New(crcTable)
for _, c := range n.children {
var v uint64
if c != nil {
v = b.computeOffsets(c, false)
binary.Write(hasher, binary.BigEndian, v)
binary.Write(hasher, binary.BigEndian, n.values)
hash = hasher.Sum64()
if first {
b.indexBlockIdx[hash] = rootBlockOffset - blockOffset
// Compacters don't apply to internal nodes.
if n.children != nil {
v, ok := b.indexBlockIdx[hash]
if !ok {
v = len(b.IndexBlocks) - blockOffset
b.IndexBlocks = append(b.IndexBlocks, n)
b.indexBlockIdx[hash] = v
n.index = nodeIndex{0, v}
} else {
h, ok := b.valueBlockIdx[hash]
if !ok {
bestI, bestSize := 0, blockSize*b.ValueSize
for i, c := range b.Compactions[1:] {
if sz, ok := c.c.Size(n.values); ok && bestSize > sz {
bestI, bestSize = i+1, sz
c := &b.Compactions[bestI]
c.totalSize += bestSize
v := c.c.Store(n.values)
if c.maxHandle < v {
c.maxHandle = v
h = nodeIndex{bestI, int(v)}
b.valueBlockIdx[hash] = h
n.index = h
return hash