init

cmd/estimate/main.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"os"
+	"strconv"
+
+	"git.intruders.space/bot/mapsize"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		panic("need argument")
+	}
+
+	n, err := strconv.Atoi(os.Args[1])
+	if err != nil {
+		panic("bad argument")
+	}
+
+	m := make(map[string]string)
+	ms := mapsize.New()
+
+	for i := 0; i < n*1_000_000; i++ {
+		k := strconv.FormatUint(rand.Uint64(), 10)
+		v := "abc"
+
+		m[k] = v
+		ms.Add(len(k), len(v))
+	}
+
+	fmt.Println("do you want an estimate? (y/y)")
+	var c byte
+	fmt.Scan(&c)
+
+	fmt.Printf("\nestimate: %d kB\n", ms.Size()/1024)
+
+}

go.mod

@@ -0,0 +1,3 @@
+module git.intruders.space/bot/mapsize
+
+go 1.20

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)
+}