init

mapsize.go

@@ -0,0 +1,35 @@
+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)
+}