hdiff restart-support: add exponents encoding helpers

beacon-chain/db/kv/state_diff_helpers.go

@@ -21,9 +21,46 @@ import (
 
 var (
 	offsetKey           = []byte("offset")
+	exponentsKey        = []byte("exponents")
 	ErrSlotBeforeOffset = errors.New("slot is before state-diff root offset")
 )
 
+func encodeStateDiffExponents(exponents []int) ([]byte, error) {
+	if len(exponents) == 0 {
+		return nil, errors.New("state diff exponents cannot be empty")
+	}
+	if len(exponents) > 255 {
+		return nil, fmt.Errorf("state diff exponents length %d exceeds max 255", len(exponents))
+	}
+	encoded := make([]byte, len(exponents)+1)
+	encoded[0] = byte(len(exponents))
+	for i, exp := range exponents {
+		if exp < 2 || exp > flags.MaxStateDiffExponent {
+			return nil, fmt.Errorf("state diff exponent %d out of range for encoding", exp)
+		}
+		encoded[i+1] = byte(exp)
+	}
+	return encoded, nil
+}
+
+func decodeStateDiffExponents(encoded []byte) ([]int, error) {
+	if len(encoded) == 0 {
+		return nil, errors.New("state diff exponents missing length prefix")
+	}
+	count := int(encoded[0])
+	if count == 0 {
+		return nil, errors.New("state diff exponents length cannot be zero")
+	}
+	if len(encoded) != count+1 {
+		return nil, fmt.Errorf("state diff exponents length mismatch: expected %d got %d", count, len(encoded)-1)
+	}
+	exponents := make([]int, count)
+	for i := range count {
+		exponents[i] = int(encoded[i+1])
+	}
+	return exponents, nil
+}
+
 func makeKeyForStateDiffTree(level int, slot uint64) []byte {
 	buf := make([]byte, 16)
 	buf[0] = byte(level)

beacon-chain/db/kv/state_diff_test.go

@@ -34,6 +34,47 @@ func TestStateDiff_LoadOrInitOffset(t *testing.T) {
 	require.Equal(t, uint64(10), offset)
 }
 
+func TestStateDiff_EncodeDecodeExponents(t *testing.T) {
+	t.Run("roundtrip", func(t *testing.T) {
+		exponents := []int{21, 18, 16, 13}
+		encoded, err := encodeStateDiffExponents(exponents)
+		require.NoError(t, err)
+		decoded, err := decodeStateDiffExponents(encoded)
+		require.NoError(t, err)
+		require.DeepEqual(t, exponents, decoded)
+	})
+
+	t.Run("encode-empty", func(t *testing.T) {
+		_, err := encodeStateDiffExponents(nil)
+		require.ErrorContains(t, "cannot be empty", err)
+	})
+
+	t.Run("encode-negative", func(t *testing.T) {
+		_, err := encodeStateDiffExponents([]int{21, -1})
+		require.ErrorContains(t, "out of range", err)
+	})
+
+	t.Run("encode-too-large", func(t *testing.T) {
+		_, err := encodeStateDiffExponents([]int{flags.MaxStateDiffExponent + 1})
+		require.ErrorContains(t, "out of range", err)
+	})
+
+	t.Run("decode-empty", func(t *testing.T) {
+		_, err := decodeStateDiffExponents(nil)
+		require.ErrorContains(t, "missing length prefix", err)
+	})
+
+	t.Run("decode-zero-length", func(t *testing.T) {
+		_, err := decodeStateDiffExponents([]byte{0})
+		require.ErrorContains(t, "length cannot be zero", err)
+	})
+
+	t.Run("decode-length-mismatch", func(t *testing.T) {
+		_, err := decodeStateDiffExponents([]byte{2, 10})
+		require.ErrorContains(t, "length mismatch", err)
+	})
+}
+
 func TestStateDiff_ComputeLevel(t *testing.T) {
 	db := setupDB(t)
 	setDefaultStateDiffExponents()

cmd/beacon-chain/flags/config.go

@@ -9,7 +9,7 @@ import (
 	"github.com/urfave/cli/v2"
 )
 
-const maxStateDiffExponents = 30
+const MaxStateDiffExponent = 30
 
 // GlobalFlags specifies all the global flags for the
 // beacon node.
@@ -132,7 +132,7 @@ func validateStateDiffExponents(exponents []int) error {
 	if exponents[length-1] < 5 {
 		return errors.New("the last state diff exponent must be at least 5")
 	}
-	prev := maxStateDiffExponents + 1
+	prev := MaxStateDiffExponent + 1
 	for _, exp := range exponents {
 		if exp >= prev {
 			return errors.New("state diff exponents must be in strictly decreasing order, and each exponent must be <= 30")