1 files changed, 86 insertions, 0 deletions
diff --git a/internal/store/header.go b/internal/store/header.go
new file mode 100644
index 00000000..a6e756f9
--- /dev/null
+++ b/internal/store/header.go
@@ -0,0 +1,86 @@
+package store
+
+import (
+	"encoding/hex"
+	"errors"
+	"io"
+	"os"
+	"strconv"
+	"syscall"
+
+	"hakurei.app/hst"
+)
+
+const (
+	// entryHeaderMagic are magic bytes at the beginning of the state entry file.
+	entryHeaderMagic = "\x00\xff\xca\xfe"
+	// entryHeaderRevision follows entryHeaderMagic and is incremented for revisions of the format.
+	entryHeaderRevision = "\x00\x00"
+	// entryHeaderSize is the fixed size of the header in bytes, including the enablement byte and its complement.
+	entryHeaderSize = len(entryHeaderMagic+entryHeaderRevision) + 2
+)
+
+// entryHeaderEncode encodes a state entry header for a [hst.Enablement] byte.
+func entryHeaderEncode(et hst.Enablement) *[entryHeaderSize]byte {
+	data := [entryHeaderSize]byte([]byte(
+		entryHeaderMagic + entryHeaderRevision + string([]hst.Enablement{et, ^et}),
+	))
+	return &data
+}
+
+// entryHeaderDecode validates a state entry header and returns the [hst.Enablement] byte.
+func entryHeaderDecode(data *[entryHeaderSize]byte) (hst.Enablement, error) {
+	if magic := data[:len(entryHeaderMagic)]; string(magic) != entryHeaderMagic {
+		return 0, errors.New("invalid header " + hex.EncodeToString(magic))
+	}
+	if revision := data[len(entryHeaderMagic):len(entryHeaderMagic+entryHeaderRevision)]; string(revision) != entryHeaderRevision {
+		return 0, errors.New("unexpected revision " + hex.EncodeToString(revision))
+	}
+
+	et := data[len(entryHeaderMagic+entryHeaderRevision)]
+	if et != ^data[len(entryHeaderMagic+entryHeaderRevision)+1] {
+		return 0, errors.New("header enablement value is inconsistent")
+	}
+	return hst.Enablement(et), nil
+}
+
+// EntrySizeError is returned for a file too small to hold a state entry header.
+type EntrySizeError struct {
+	Name string
+	Size int64
+}
+
+func (e *EntrySizeError) Error() string {
+	if e.Name == "" {
+		return "state entry file is too short"
+	}
+	return "state entry file " + strconv.Quote(e.Name) + " is too short"
+}
+
+// entryCheckFile checks whether [os.FileInfo] refers to a file that might hold [hst.State].
+func entryCheckFile(fi os.FileInfo) error {
+	if fi.IsDir() {
+		return syscall.EISDIR
+	}
+	if s := fi.Size(); s <= int64(entryHeaderSize) {
+		return &EntrySizeError{Name: fi.Name(), Size: s}
+	}
+	return nil
+}
+
+// entryReadHeader reads [hst.Enablement] from an [io.Reader].
+func entryReadHeader(r io.Reader) (hst.Enablement, error) {
+	var data [entryHeaderSize]byte
+	if n, err := r.Read(data[:]); err != nil {
+		return 0, err
+	} else if n != entryHeaderSize {
+		return 0, &EntrySizeError{Size: int64(n)}
+	}
+	return entryHeaderDecode(&data)
+}
+
+// entryWriteHeader writes [hst.Enablement] header to an [io.Writer].
+func entryWriteHeader(w io.Writer, et hst.Enablement) error {
+	_, err := w.Write(entryHeaderEncode(et)[:])
+	return err
+}