feat: unified writing/reading table

These changes table reading/writing to single read/write operation,
and layout to machine byteorder, so db file will be unsupported with
wrong byteorder.

internal/btree/btree.go

@@ -12,8 +12,8 @@ import (
 
 const (
 	sha1Size        = 20 // md5 sha1
-	tableSize       = (4096 - 1) / int(unsafe.Sizeof(item{}))
-	cacheSlots      = 23 // prime
+	tableSize       = (2048 - 1) / int(unsafe.Sizeof(item{}))
+	cacheSlots      = 13 // prime
 	superSize       = int(unsafe.Sizeof(super{}))
 	tableStructSize = int(unsafe.Sizeof(table{}))
 )

internal/btree/helper.go

@@ -1,8 +1,8 @@
 package btree
 
 import (
-	"encoding/binary"
 	"io"
+	"reflect"
 	"unsafe"
 )
 
@@ -47,119 +47,63 @@ func resetsha1(sha1 *byte) {
 
 // reading table
 
-func read64(r io.Reader) (int64, error) {
-	var b = make([]byte, 8)
-	_, err := r.Read(b)
-	if err != nil {
-		return 0, err
-	}
-	return int64(binary.LittleEndian.Uint64(b)), nil
-}
-
 func read32(r io.Reader) (int32, error) {
 	var b = make([]byte, 4)
 	_, err := r.Read(b)
 	if err != nil {
 		return 0, err
 	}
-	return int32(binary.LittleEndian.Uint32(b)), nil
+	return *(*int32)(unsafe.Pointer(&b[0])), nil
 }
 
 func readTable(r io.Reader, t *table) error {
-	for i := 0; i < tableSize; i++ {
-		err := readItem(r, &t.items[i])
-		if err != nil {
-			return err
-		}
-	}
-	switch unsafe.Sizeof(0) {
-	case 8:
-		i, err := read64(r)
-		t.size = int(i)
-		return err
-	case 4:
-		i, err := read32(r)
-		t.size = int(i)
-		return err
-	default:
-		panic("unreachable")
-	}
-}
-
-func readItem(r io.Reader, i *item) error {
-	_, err := r.Read(i.sha1[:])
+	buf := make([]byte, tableStructSize)
+	_, err := r.Read(buf)
 	if err != nil {
 		return err
 	}
-	i.offset, err = read64(r)
-	if err != nil {
-		return err
-	}
-	i.child, err = read64(r)
-	return err
+	*t = *(*table)(unsafe.Pointer(&buf[0]))
+	return nil
 }
 
 func readSuper(r io.Reader, s *super) error {
-	var err error
-	if s.top, err = read64(r); err != nil {
+	buf := make([]byte, superSize)
+	_, err := r.Read(buf)
+	if err != nil {
 		return err
 	}
-	if s.freeTop, err = read64(r); err != nil {
-		return err
-	}
-	s.alloc, err = read64(r)
-	return err
+	*s = *(*super)(unsafe.Pointer(&buf[0]))
+	return nil
 }
 
 // write table
 
-func write64(w io.Writer, i int64) error {
-	var b = make([]byte, 8)
-	binary.LittleEndian.PutUint64(b, uint64(i))
-	_, err := w.Write(b)
-	return err
-}
-
-func write32(w io.Writer, i int32) error {
-	var b = make([]byte, 4)
-	binary.LittleEndian.PutUint32(b, uint32(i))
-	_, err := w.Write(b)
+func write32(w io.Writer, t int32) error {
+	var p []byte
+	ph := (*reflect.SliceHeader)(unsafe.Pointer(&p))
+	ph.Data = uintptr(unsafe.Pointer(&t))
+	ph.Len = 4
+	ph.Cap = 4
+	_, err := w.Write(p)
 	return err
 }
 
 func writeTable(w io.Writer, t *table) error {
-	for i := 0; i < tableSize; i++ {
-		err := writeItem(w, &t.items[i])
-		if err != nil {
-			return err
-		}
-	}
-	switch unsafe.Sizeof(0) {
-	case 8:
-		return write64(w, int64(t.size))
-	case 4:
-		return write32(w, int32(t.size))
-	default:
-		panic("unreachable")
-	}
-}
-
-func writeItem(w io.Writer, i *item) error {
-	if _, err := w.Write(i.sha1[:]); err != nil {
-		return err
-	}
-	if err := write64(w, i.offset); err != nil {
-		return err
-	}
-	return write64(w, i.child)
+	var p []byte
+	ph := (*reflect.SliceHeader)(unsafe.Pointer(&p))
+	ph.Data = uintptr(unsafe.Pointer(t))
+	ph.Len = tableStructSize
+	ph.Cap = tableStructSize
+	_, err := w.Write(p)
+	return err
 }
 
 func writeSuper(w io.Writer, s *super) error {
-	if err := write64(w, s.top); err != nil {
-		return err
-	}
-	if err := write64(w, s.freeTop); err != nil {
-		return err
-	}
-	return write64(w, s.alloc)
+	var p []byte
+	ph := (*reflect.SliceHeader)(unsafe.Pointer(&p))
+	ph.Data = uintptr(unsafe.Pointer(s))
+	ph.Len = superSize
+	ph.Cap = superSize
+	_, err := w.Write(p)
+	return err
 }