gotosocial/vendor/github.com/golang/geo/s2/lexicon.go

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// Copyright 2020 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package s2
import (
"encoding/binary"
"hash/adler32"
"math"
"sort"
)
// TODO(roberts): If any of these are worth making public, change the
// method signatures and type names.
// emptySetID represents the last ID that will ever be generated.
// (Non-negative IDs are reserved for singleton sets.)
var emptySetID = int32(math.MinInt32)
// idSetLexicon compactly represents a set of non-negative
// integers such as array indices ("ID sets"). It is especially suitable when
// either (1) there are many duplicate sets, or (2) there are many singleton
// or empty sets. See also sequenceLexicon.
//
// Each distinct ID set is mapped to a 32-bit integer. Empty and singleton
// sets take up no additional space; the set itself is represented
// by the unique ID assigned to the set. Duplicate sets are automatically
// eliminated. Note also that ID sets are referred to using 32-bit integers
// rather than pointers.
type idSetLexicon struct {
idSets *sequenceLexicon
}
func newIDSetLexicon() *idSetLexicon {
return &idSetLexicon{
idSets: newSequenceLexicon(),
}
}
// add adds the given set of integers to the lexicon if it is not already
// present, and return the unique ID for this set. The values are automatically
// sorted and duplicates are removed.
//
// The primary difference between this and sequenceLexicon are:
// 1. Empty and singleton sets are represented implicitly; they use no space.
// 2. Sets are represented rather than sequences; the ordering of values is
// not important and duplicates are removed.
// 3. The values must be 32-bit non-negative integers only.
func (l *idSetLexicon) add(ids ...int32) int32 {
// Empty sets have a special ID chosen not to conflict with other IDs.
if len(ids) == 0 {
return emptySetID
}
// Singleton sets are represented by their element.
if len(ids) == 1 {
return ids[0]
}
// Canonicalize the set by sorting and removing duplicates.
//
// Creates a new slice in order to not alter the supplied values.
set := uniqueInt32s(ids)
// Non-singleton sets are represented by the bitwise complement of the ID
// returned by the sequenceLexicon
return ^l.idSets.add(set)
}
// idSet returns the set of integers corresponding to an ID returned by add.
func (l *idSetLexicon) idSet(setID int32) []int32 {
if setID >= 0 {
return []int32{setID}
}
if setID == emptySetID {
return []int32{}
}
return l.idSets.sequence(^setID)
}
func (l *idSetLexicon) clear() {
l.idSets.clear()
}
// sequenceLexicon compactly represents a sequence of values (e.g., tuples).
// It automatically eliminates duplicates slices, and maps the remaining
// sequences to sequentially increasing integer IDs. See also idSetLexicon.
//
// Each distinct sequence is mapped to a 32-bit integer.
type sequenceLexicon struct {
values []int32
begins []uint32
// idSet is a mapping of a sequence hash to sequence index in the lexicon.
idSet map[uint32]int32
}
func newSequenceLexicon() *sequenceLexicon {
return &sequenceLexicon{
begins: []uint32{0},
idSet: make(map[uint32]int32),
}
}
// clears all data from the lexicon.
func (l *sequenceLexicon) clear() {
l.values = nil
l.begins = []uint32{0}
l.idSet = make(map[uint32]int32)
}
// add adds the given value to the lexicon if it is not already present, and
// returns its ID. IDs are assigned sequentially starting from zero.
func (l *sequenceLexicon) add(ids []int32) int32 {
if id, ok := l.idSet[hashSet(ids)]; ok {
return id
}
for _, v := range ids {
l.values = append(l.values, v)
}
l.begins = append(l.begins, uint32(len(l.values)))
id := int32(len(l.begins)) - 2
l.idSet[hashSet(ids)] = id
return id
}
// sequence returns the original sequence of values for the given ID.
func (l *sequenceLexicon) sequence(id int32) []int32 {
return l.values[l.begins[id]:l.begins[id+1]]
}
// size reports the number of value sequences in the lexicon.
func (l *sequenceLexicon) size() int {
// Subtract one because the list of begins starts out with the first element set to 0.
return len(l.begins) - 1
}
// hash returns a hash of this sequence of int32s.
func hashSet(s []int32) uint32 {
// TODO(roberts): We just need a way to nicely hash all the values down to
// a 32-bit value. To ensure no unnecessary dependencies we use the core
// library types available to do this. Is there a better option?
a := adler32.New()
binary.Write(a, binary.LittleEndian, s)
return a.Sum32()
}
// uniqueInt32s returns the sorted and uniqued set of int32s from the input.
func uniqueInt32s(in []int32) []int32 {
var vals []int32
m := make(map[int32]bool)
for _, i := range in {
if m[i] {
continue
}
m[i] = true
vals = append(vals, i)
}
sort.Slice(vals, func(i, j int) bool { return vals[i] < vals[j] })
return vals
}