Use 0 as empty metadata to allocate faster

Current value has to be written for all metadata values when we're
instantiating a new map or growing.

This changes empty to be the zero byte, and tombstone to be byte 1, by
adding 2 to all hashes.
Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

bits.go

@@ -37,7 +37,7 @@ func metaMatchH2(m *metadata, h h2) bitset {
 }
 
 func metaMatchEmpty(m *metadata) bitset {
-	return hasZeroByte(castUint64(m) ^ hiBits)
+	return hasZeroByte(castUint64(m))
 }
 
 func nextMatch(b *bitset) uint32 {

bits_test.go

@@ -26,24 +26,25 @@ import (
 func TestMatchMetadata(t *testing.T) {
 	var meta metadata
 	for i := range meta {
-		meta[i] = h2(i)
+		meta[i] = h2(i) + 2
 	}
 	t.Run("metaMatchH2", func(t *testing.T) {
-		for _, x := range meta {
-			mask := metaMatchH2(&meta, h2(x))
+		for i, m := range meta {
+			mask := metaMatchH2(&meta, m)
 			assert.NotZero(t, mask)
-			assert.Equal(t, uint32(x), nextMatch(&mask))
+			assert.Equal(t, uint32(i), nextMatch(&mask))
 		}
 	})
 	t.Run("metaMatchEmpty", func(t *testing.T) {
 		mask := metaMatchEmpty(&meta)
 		assert.Equal(t, mask, bitset(0))
+
 		for i := range meta {
 			meta[i] = empty
 			mask = metaMatchEmpty(&meta)
 			assert.NotZero(t, mask)
 			assert.Equal(t, uint32(i), nextMatch(&mask))
-			meta[i] = h2(i)
+			meta[i] = h2(i) + 2
 		}
 	})
 	t.Run("nextMatch", func(t *testing.T) {
@@ -90,11 +91,10 @@ func nextPow2(x uint32) uint32 {
 }
 
 func TestConstants(t *testing.T) {
-	c1, c2 := empty, tombstone
-	assert.Equal(t, byte(0b1000_0000), byte(c1))
-	assert.Equal(t, byte(0b1000_0000), reinterpretCast(c1))
-	assert.Equal(t, byte(0b1111_1110), byte(c2))
-	assert.Equal(t, byte(0b1111_1110), reinterpretCast(c2))
+	assert.Equal(t, byte(0b0000_0000), byte(empty))
+	assert.Equal(t, byte(0b0000_0000), reinterpretCast(empty))
+	assert.Equal(t, byte(0b0000_0001), byte(tombstone))
+	assert.Equal(t, byte(0b0000_0001), reinterpretCast(tombstone))
 }
 
 func reinterpretCast(i h2) byte {

map.go

@@ -47,8 +47,8 @@ type group[K comparable, V any] struct {
 const (
 	h1Mask    uint64 = 0xffff_ffff_ffff_ff80
 	h2Mask    uint64 = 0x0000_0000_0000_007f
-	empty     h2     = 0b1000_0000
-	tombstone h2     = 0b1111_1110
+	empty     h2     = 0b0000_0000
+	tombstone h2     = 0b0000_0001
 )
 
 // h1 is a 57 bit hash prefix
@@ -236,10 +236,8 @@ func (m *Map[K, V]) Iter(cb func(k K, v V) (stop bool)) {
 
 // Clear removes all elements from the Map.
 func (m *Map[K, V]) Clear() {
-	for i, c := range m.ctrl {
-		for j := range c {
-			m.ctrl[i][j] = empty
-		}
+	for i := range m.ctrl {
+		m.ctrl[i] = metadata{}
 	}
 	var k K
 	var v V
@@ -341,7 +339,7 @@ func newEmptyMetadata() (meta metadata) {
 }
 
 func splitHash(h uint64) (h1, h2) {
-	return h1((h & h1Mask) >> 7), h2(h & h2Mask)
+	return h1((h & h1Mask) >> 7), h2(h&h2Mask) + 2
 }
 
 func probeStart(hi h1, groups int) uint32 {