Source File
	pool.go

Belonging Package
	github.com/tetratelabs/wazero/internal/engine/wazevo/wazevoapi

package wazevoapi

const poolPageSize = 128

// Pool is a pool of T that can be allocated and reset.
// This is useful to avoid unnecessary allocations.
type Pool[T any] struct {
	pages            []*[poolPageSize]T
	resetFn          func(*T)
	allocated, index int
}

// NewPool returns a new Pool.
// resetFn is called when a new T is allocated in Pool.Allocate.
func NewPool[T any](resetFn func(*T)) Pool[T] {
	var ret Pool[T]
	ret.resetFn = resetFn
	ret.Reset()
	return ret
}

// Allocated returns the number of allocated T currently in the pool.
func (p *Pool[T]) Allocated() int {
	return p.allocated
}

// Allocate allocates a new T from the pool.
func (p *Pool[T]) Allocate() *T {
	if p.index == poolPageSize {
		if len(p.pages) == cap(p.pages) {
			p.pages = append(p.pages, new([poolPageSize]T))
		} else {
			i := len(p.pages)
			p.pages = p.pages[:i+1]
			if p.pages[i] == nil {
				p.pages[i] = new([poolPageSize]T)
			}
		}
		p.index = 0
	}
	ret := &p.pages[len(p.pages)-1][p.index]
	if p.resetFn != nil {
		p.resetFn(ret)
	}
	p.index++
	p.allocated++
	return ret
}

// View returns the pointer to i-th item from the pool.
func (p *Pool[T]) View(i int) *T {
	page, index := i/poolPageSize, i%poolPageSize
	return &p.pages[page][index]
}

// Reset resets the pool.
func (p *Pool[T]) Reset() {
	p.pages = p.pages[:0]
	p.index = poolPageSize
	p.allocated = 0
}

// IDedPool is a pool of T that can be allocated and reset, with a way to get T by an ID.
type IDedPool[T any] struct {
	pool             Pool[T]
	idToItems        []*T
	maxIDEncountered int
}

// NewIDedPool returns a new IDedPool.
func NewIDedPool[T any](resetFn func(*T)) IDedPool[T] {
	return IDedPool[T]{pool: NewPool[T](resetFn), maxIDEncountered: -1}
}

// GetOrAllocate returns the T with the given id.
func (p *IDedPool[T]) GetOrAllocate(id int) *T {
	if p.maxIDEncountered < id {
		p.maxIDEncountered = id
	}
	if id >= len(p.idToItems) {
		p.idToItems = append(p.idToItems, make([]*T, id-len(p.idToItems)+1)...)
	}
	if p.idToItems[id] == nil {
		p.idToItems[id] = p.pool.Allocate()
	}
	return p.idToItems[id]
}

// Get returns the T with the given id, or nil if it's not allocated.
func (p *IDedPool[T]) Get(id int) *T {
	if id >= len(p.idToItems) {
		return nil
	}
	return p.idToItems[id]
}

// Reset resets the pool.
func (p *IDedPool[T]) Reset() {
	p.pool.Reset()
	for i := 0; i <= p.maxIDEncountered; i++ {
		p.idToItems[i] = nil
	}
	p.maxIDEncountered = -1
}

// MaxIDEncountered returns the maximum id encountered so far.
func (p *IDedPool[T]) MaxIDEncountered() int {
	return p.maxIDEncountered
}

// arraySize is the size of the array used in VarLengthPool's arrayPool.
// This is chosen to be 8, which is empirically a good number among 8, 12, 16 and 20.
const arraySize = 8

// VarLengthPool is a pool of VarLength[T] that can be allocated and reset.
type (
	VarLengthPool[T any] struct {
		arrayPool Pool[varLengthPoolArray[T]]
		slicePool Pool[[]T]
	}
	// varLengthPoolArray wraps an array and keeps track of the next index to be used to avoid the heap allocation.
	varLengthPoolArray[T any] struct {
		arr  [arraySize]T
		next int
	}
)

// VarLength is a variable length array that can be reused via a pool.
type VarLength[T any] struct {
	arr *varLengthPoolArray[T]
	slc *[]T
}

// NewVarLengthPool returns a new VarLengthPool.
func NewVarLengthPool[T any]() VarLengthPool[T] {
	return VarLengthPool[T]{
		arrayPool: NewPool[varLengthPoolArray[T]](func(v *varLengthPoolArray[T]) {
			v.next = 0
		}),
		slicePool: NewPool[[]T](func(i *[]T) {
			*i = (*i)[:0]
		}),
	}
}

// NewNilVarLength returns a new VarLength[T] with a nil backing.
func NewNilVarLength[T any]() VarLength[T] {
	return VarLength[T]{}
}

// Allocate allocates a new VarLength[T] from the pool.
func (p *VarLengthPool[T]) Allocate(knownMin int) VarLength[T] {
	if knownMin <= arraySize {
		arr := p.arrayPool.Allocate()
		return VarLength[T]{arr: arr}
	}
	slc := p.slicePool.Allocate()
	return VarLength[T]{slc: slc}
}

// Reset resets the pool.
func (p *VarLengthPool[T]) Reset() {
	p.arrayPool.Reset()
	p.slicePool.Reset()
}

// Append appends items to the backing slice just like the `append` builtin function in Go.
func (i VarLength[T]) Append(p *VarLengthPool[T], items ...T) VarLength[T] {
	if i.slc != nil {
		*i.slc = append(*i.slc, items...)
		return i
	}

	if i.arr == nil {
		i.arr = p.arrayPool.Allocate()
	}

	arr := i.arr
	if arr.next+len(items) <= arraySize {
		for _, item := range items {
			arr.arr[arr.next] = item
			arr.next++
		}
	} else {
		slc := p.slicePool.Allocate()
		// Copy the array to the slice.
		for ptr := 0; ptr < arr.next; ptr++ {
			*slc = append(*slc, arr.arr[ptr])
		}
		i.slc = slc
		*i.slc = append(*i.slc, items...)
	}
	return i
}

// View returns the backing slice.
func (i VarLength[T]) View() []T {
	if i.slc != nil {
		return *i.slc
	} else if i.arr != nil {
		arr := i.arr
		return arr.arr[:arr.next]
	}
	return nil
}

// Cut cuts the backing slice to the given length.
// Precondition: n <= len(i.backing).
func (i VarLength[T]) Cut(n int) {
	if i.slc != nil {
		*i.slc = (*i.slc)[:n]
	} else if i.arr != nil {
		i.arr.next = n
	}
}

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