package mapsize

import (
	"math"
	"unsafe"
)

type Mapsize struct {
	ptrSize  uint64
	intSize  uint64
	dataSize uint64
	len      uint64
}

func New() *Mapsize {
	return &Mapsize{
		ptrSize: uint64(unsafe.Sizeof(&[]int{0}[0])),
		intSize: uint64(unsafe.Sizeof(int(0))),
	}
}

func (m *Mapsize) Add(k, v int) {
	m.len++
	// string: ptr=psize, cap=isize, len=isize
	m.dataSize += uint64(k) + uint64(v) + 4*m.intSize + 2*m.ptrSize
}

func (m *Mapsize) Size() uint64 {
	hmapSize := m.intSize + 1 + 1 + 2 + 4 + m.ptrSize*4
	buckets := uint64(1 << uint(math.Ceil(math.Log2(float64(m.len)))))

	// entry overhead = key ptr, value ptr, hash, next
	// bucket overhead = 2 ptr (overflow)
	return hmapSize + m.dataSize + m.len*(4*m.ptrSize) + buckets*(2*m.ptrSize)
}