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// Copyright 2025 CUE Authors
//
// 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 intset provides an allocation-efficient hash set for unsigned
// integer types. It does not provide a way to delete keys, and will not
// be efficient when most of the variation in keys is in the higher bits,
// because it uses masking rather than modulus to calculate hash slots.
//
// It's efficient to clear, working around https://github.com/golang/go/issues/70617
//
// Usage example:
//
// type ID uint32
// s := intset.New[ID](128)
// s.Add(ID(42))
// fmt.Println(s.Has(ID(42))) // true
package intset
import (
"fmt"
"math/bits"
)
// Int holds the possible integer types that the set can hold.
type Int interface {
~uint8 | ~uint16 | ~uint32 | ~uint64
}
// Set is a high-performance hash set for unsigned integer types.
type Set[T Int] struct {
// keys holds the members of the set. If two keys hash
// to the same slot, linear probing is used to find the next
// available slot. An entry at index i is considered "present"
// if generations[i] == gen.
keys []T
generations []uint32 // generation number for each member of keys.
gen uint32 // current generation (never zero)
size int // live keys in current generation
maxLoad int // threshold size for rehashing
}
const maxLoadFactor = 0.75 // 75% load factor before resizing
// New returns a pointer to an initialized Set with at least capacity slots.
// Capacity is rounded up to the next power of two and min 8.
func New[T Int](capacity int) *Set[T] {
capacity = nextPow2(max(capacity, 8))
return &Set[T]{
keys: make([]T, capacity),
generations: make([]uint32, capacity),
gen: 1,
maxLoad: int(float64(capacity) * maxLoadFactor),
}
}
// Clear discards all keys in O(1) without allocating.
func (s *Set[T]) Clear() {
if len(s.keys) == 0 {
return
}
s.gen++
s.size = 0
if s.gen == 0 { // wrapped – zero gens slice
s.gen = 1
for i := range s.generations {
s.generations[i] = 0
}
}
}
// Len returns the number of keys currently in the set.
func (s *Set[T]) Len() int { return s.size }
// Has reports whether x is present.
func (s *Set[T]) Has(x T) bool {
if len(s.keys) == 0 {
return false
}
mask := len(s.keys) - 1
i := int(x & T(mask))
for {
if s.generations[i] != s.gen {
return false
}
if s.keys[i] == x {
return true
}
i = (i + 1) & mask
}
}
// Add inserts x and returns true if it was newly added.
func (s *Set[T]) Add(x T) bool {
if s.size+1 >= s.maxLoad {
s.rehash(len(s.keys) * 2)
}
mask := len(s.keys) - 1
i := int(x & T(mask))
for {
if s.generations[i] != s.gen {
s.keys[i] = x
s.generations[i] = s.gen
s.size++
return true
}
if s.keys[i] == x {
return false
}
i = (i + 1) & mask
}
}
// rehash grows the table to newCap (must be a power of two) and reinserts live keys.
func (s *Set[T]) rehash(newCap int) {
oldKeys := s.keys
oldGens := s.generations
oldGen := s.gen
s.keys = make([]T, newCap)
s.generations = make([]uint32, newCap)
s.gen = 1
s.size = 0
s.maxLoad = int(float64(newCap) * maxLoadFactor)
mask := newCap - 1
for idx, g := range oldGens {
if g != oldGen {
continue
}
k := oldKeys[idx]
j := int(k & T(mask))
for {
if s.generations[j] != s.gen {
s.keys[j] = k
s.generations[j] = s.gen
s.size++
break
}
j = (j + 1) & mask
}
}
}
// nextPow2 returns the next power of two ≥ v.
func nextPow2(v int) int {
if v == 0 {
return 1
}
if bits.OnesCount(uint(v)) == 1 {
return v
}
n := bits.UintSize - bits.LeadingZeros(uint(v))
if n < 0 {
panic(fmt.Errorf("negative shift on nextPow2(%d): %d", v, n))
}
return 1 << (bits.UintSize - bits.LeadingZeros(uint(v)))
}
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