internal/era: update to latest ere spec (#34896)

Update the EraE (ere) reader and builder to the latest e2store ere spec
  (https://github.com/eth-clients/e2store-format-specs/pull/16).

  The reader now derives the component layout from the on-disk e2store type
  tags via the dynamic block index, rather than assuming fixed slot positions.
  This makes the optional components (receipts, td, proof) resolvable in any
  supported subset.

---------

Co-authored-by: lightclient <lightclient@protonmail.com>

cmd/era/main.go

@@ -183,11 +183,11 @@ func open(ctx *cli.Context, epoch uint64) (era.Era, error) {
 	return openByPath(path)
 }
 
-// openByPath tries to open a single file as either eraE or era1 based on extension,
+// openByPath tries to open a single file as either Ere or Era1 based on extension,
 // falling back to the other reader if needed.
 func openByPath(path string) (era.Era, error) {
 	switch strings.ToLower(filepath.Ext(path)) {
-	case ".erae":
+	case ".ere":
 		if e, err := execdb.Open(path); err != nil {
 			return nil, err
 		} else {
@@ -229,7 +229,7 @@ func verify(ctx *cli.Context) error {
 
 	// Build the verification list respecting the rule:
 	// era1: must have accumulator, always verify
-	// erae: verify only if accumulator exists (pre-merge)
+	// ere:  verify only if accumulator exists (pre-merge / transition)
 
 	// Build list of files to verify.
 	verify := make([]string, 0, len(entries))
@@ -251,15 +251,15 @@ func verify(ctx *cli.Context) error {
 			}
 			verify = append(verify, path)
 
-		case ".erae":
+		case ".ere":
 			e, err := execdb.Open(path)
 			if err != nil {
-				return fmt.Errorf("error opening erae file %s: %w", name, err)
+				return fmt.Errorf("error opening ere file %s: %w", name, err)
 			}
 			_, accErr := e.Accumulator()
 			e.Close()
 			if accErr == nil {
-				verify = append(verify, path) // pre-merge only
+				verify = append(verify, path) // pre-merge / transition only
 			}
 		default:
 			return fmt.Errorf("unsupported era file: %s", name)

cmd/geth/chaincmd.go

@@ -526,15 +526,15 @@ func importHistory(ctx *cli.Context) error {
 
 	var (
 		format = ctx.String(utils.EraFormatFlag.Name)
-		from   func(era.ReadAtSeekCloser) (era.Era, error)
+		from   func(f era.ReadAtSeekCloser) (era.Era, error)
 	)
 	switch format {
 	case "era1", "era":
 		from = onedb.From
-	case "erae":
+	case "ere":
 		from = execdb.From
 	default:
-		return fmt.Errorf("unknown --era.format %q (expected 'era1' or 'erae')", format)
+		return fmt.Errorf("unknown --era.format %q (expected 'era1' or 'ere')", format)
 	}
 	if err := utils.ImportHistory(chain, dir, network, from); err != nil {
 		return err
@@ -580,11 +580,11 @@ func exportHistory(ctx *cli.Context) error {
 	case "era1", "era":
 		newBuilder = func(w io.Writer) era.Builder { return onedb.NewBuilder(w) }
 		filename = func(network string, epoch int, root common.Hash) string { return onedb.Filename(network, epoch, root) }
-	case "erae":
+	case "ere":
 		newBuilder = func(w io.Writer) era.Builder { return execdb.NewBuilder(w) }
 		filename = func(network string, epoch int, root common.Hash) string { return execdb.Filename(network, epoch, root) }
 	default:
-		return fmt.Errorf("unknown archive format %q (use 'era1' or 'erae')", format)
+		return fmt.Errorf("unknown archive format %q (use 'era1' or 'ere')", format)
 	}
 	if err := utils.ExportHistory(chain, dir, uint64(first), uint64(last), newBuilder, filename); err != nil {
 		utils.Fatalf("Export error: %v\n", err)

cmd/utils/flags.go

@@ -1110,7 +1110,7 @@ Please note that --` + MetricsHTTPFlag.Name + ` must be set to start the server.
 	// Era flags are a group of flags related to the era archive format.
 	EraFormatFlag = &cli.StringFlag{
 		Name:  "era.format",
-		Usage: "Archive format: 'era1' or 'erae'",
+		Usage: "Archive format: 'era1' or 'ere'",
 	}
 )
 

cmd/utils/history_test.go

@@ -53,7 +53,7 @@ func TestHistoryImportAndExport(t *testing.T) {
 		from     func(f era.ReadAtSeekCloser) (era.Era, error)
 	}{
 		{"era1", onedb.NewBuilder, onedb.Filename, onedb.From},
-		{"erae", execdb.NewBuilder, execdb.Filename, execdb.From},
+		{"ere", execdb.NewBuilder, execdb.Filename, execdb.From},
 	} {
 		t.Run(tt.name, func(t *testing.T) {
 			var (

internal/era/era.go

@@ -29,7 +29,7 @@ import (
 	"github.com/ethereum/go-ethereum/core/types"
 )
 
-// Type constants for the e2store entries in the Era1 and EraE formats.
+// Type constants for the e2store entries in the Era1 and Ere formats.
 var (
 	TypeVersion                uint16 = 0x3265
 	TypeCompressedHeader       uint16 = 0x03
@@ -40,10 +40,9 @@ var (
 	TypeCompressedSlimReceipts uint16 = 0x0a // uses eth/69 encoding
 	TypeProof                  uint16 = 0x0b
 	TypeBlockIndex             uint16 = 0x3266
-	TypeComponentIndex         uint16 = 0x3267
+	TypeDynamicBlockIndex      uint16 = 0x3267
 
 	MaxSize = 8192
-	// headerSize uint64 = 8
 )
 
 type ReadAtSeekCloser interface {
@@ -93,7 +92,7 @@ type Builder interface {
 
 	// Finalize writes all collected entries and returns the epoch identifier.
 	// For Era1 (onedb): returns the accumulator root.
-	// For EraE (execdb): returns the last block hash.
+	// For Ere (execdb): returns the last block hash.
 	Finalize() (common.Hash, error)
 
 	// Accumulator returns the accumulator root after Finalize has been called.
@@ -115,7 +114,7 @@ type Era interface {
 }
 
 // ReadDir reads all the era files in a directory for a given network.
-// Format: <network>-<epoch>-<hexroot>.erae or <network>-<epoch>-<hexroot>.era1
+// Format: <network>-<epoch>-<hexroot>(-<profile>)*.ere or <network>-<epoch>-<hexroot>.era1
 func ReadDir(dir, network string) ([]string, error) {
 	entries, err := os.ReadDir(dir)
 
@@ -129,14 +128,16 @@ func ReadDir(dir, network string) ([]string, error) {
 	)
 	for _, entry := range entries {
 		ext := path.Ext(entry.Name())
-		if ext != ".erae" && ext != ".era1" {
+		if ext != ".ere" && ext != ".era1" {
 			continue
 		}
 		if dirType == "" {
 			dirType = ext
 		}
 		parts := strings.Split(entry.Name(), "-")
-		if len(parts) != 3 || parts[0] != network {
+		// Ere files may carry an optional profile postfix (e.g. "-noproofs"),
+		// so the filename has at least 3 dash-separated parts.
+		if len(parts) < 3 || parts[0] != network {
 			// Invalid era filename, skip.
 			continue
 		}

internal/era/execdb/builder.go

@@ -16,40 +16,44 @@
 
 package execdb
 
-// EraE file format specification.
+// Ere file format specification.
+//
+// See https://github.com/eth-clients/e2store-format-specs/blob/main/formats/ere.md.
 //
 // The format can be summarized with the following expression:
 //
-//	eraE := Version | CompressedHeader* | CompressedBody* | CompressedSlimReceipts* | TotalDifficulty* | other-entries* | Accumulator? | ComponentIndex
+//	ere := Version | CompressedHeader+ | CompressedBody+ | CompressedSlimReceipts* | Proof* | TotalDifficulty* | other-entries* | Accumulator? | DynamicBlockIndex
 //
 // Each basic element is its own e2store entry:
 //
-//	Version              = { type: 0x3265, data: nil }
+//	Version                = { type: [0x65, 0x32], data: nil }
-//	CompressedHeader     = { type: 0x03,   data: snappyFramed(rlp(header)) }
+//	CompressedHeader       = { type: [0x03, 0x00], data: snappyFramed(rlp(header)) }
-//	CompressedBody       = { type: 0x04,   data: snappyFramed(rlp(body)) }
+//	CompressedBody         = { type: [0x04, 0x00], data: snappyFramed(rlp(body)) }
-//	CompressedSlimReceipts = { type: 0x0a, data: snappyFramed(rlp([tx-type, post-state-or-status, cumulative-gas, logs])) }
+//	CompressedSlimReceipts = { type: [0x0a, 0x00], data: snappyFramed(rlp([tx-type, post-state-or-status, cumulative-gas, logs])) }
-//	TotalDifficulty      = { type: 0x06,   data: uint256 (header.total_difficulty) }
+//	TotalDifficulty        = { type: [0x06, 0x00], data: uint256(header.total_difficulty) }
-//	AccumulatorRoot      = { type: 0x07,   data: hash_tree_root(List(HeaderRecord, 8192)) }
+//	Proof                  = { type: [0x0b, 0x00], data: snappyFramed(rlp([proof-type, ssz(proof)])) }
-//	ComponentIndex       = { type: 0x3267, data: component-index }
+//	AccumulatorRoot        = { type: [0x07, 0x00], data: hash_tree_root(List(HeaderRecord, 8192)) }
+//	DynamicBlockIndex      = { type: [0x67, 0x32], data: block-index }
 //
 // Notes:
 //   - TotalDifficulty is present for pre-merge and merge transition epochs.
 //     For pure post-merge epochs, TotalDifficulty is omitted entirely.
 //   - In merge transition epochs, post-merge blocks store the final total
 //     difficulty (the TD at which the merge occurred).
-//   - AccumulatorRoot is only written for pre-merge epochs.
+//   - AccumulatorRoot is only written for pre-merge or transition epochs.
 //   - HeaderRecord is defined in the Portal Network specification.
-//   - Proofs (type 0x09) are defined in the spec but not yet supported in this implementation.
+//   - Proof entries are recommended by the spec but not produced by this
+//     implementation; files written here use the "noproofs" profile postfix.
 //
-// ComponentIndex stores relative offsets to each block's components:
+// DynamicBlockIndex stores relative offsets to each block's components:
 //
-//	component-index := starting-number | indexes | indexes | ... | component-count | count
+//	block-index := starting-number | indexes | indexes | ... | component-count | count
-//	indexes := header-offset | body-offset | receipts-offset | td-offset?
+//	indexes     := header-index | body-index | receipts-index? | difficulty-index? | proof-index?
 //
 // All values are little-endian uint64.
 //
 // Due to the accumulator size limit of 8192, the maximum number of blocks in an
-// EraE file is also 8192.
+// Ere file is also 8192.
 
 import (
 	"bytes"
@@ -67,7 +71,7 @@ import (
 	"github.com/golang/snappy"
 )
 
-// Builder is used to build an EraE e2store file. It collects block entries and
+// Builder is used to build an Ere e2store file. It collects block entries and
 // writes them to the underlying e2store.Writer.
 type Builder struct {
 	w *e2store.Writer
@@ -326,7 +330,7 @@ func (b *Builder) writeIndex(o *offsets) error {
 	write(uint64(componentCount))
 	write(uint64(count))
 
-	n, err := b.w.Write(era.TypeComponentIndex, buf.Bytes())
+	n, err := b.w.Write(era.TypeDynamicBlockIndex, buf.Bytes())
 	b.written += uint64(n)
 	return err
 }

internal/era/execdb/era_test.go

@@ -18,19 +18,24 @@ package execdb
 
 import (
 	"bytes"
+	"encoding/binary"
 	"fmt"
 	"io"
 	"math/big"
 	"os"
+	"path/filepath"
 	"slices"
 	"testing"
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/internal/era"
+	"github.com/ethereum/go-ethereum/internal/era/e2store"
 	"github.com/ethereum/go-ethereum/rlp"
+	"github.com/golang/snappy"
 )
 
-func TestEraE(t *testing.T) {
+func TestEre(t *testing.T) {
 	t.Parallel()
 
 	tests := []struct {
@@ -74,7 +79,7 @@ func TestEraE(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			f, err := os.CreateTemp(t.TempDir(), "erae-test")
+			f, err := os.CreateTemp(t.TempDir(), "ere-test")
 			if err != nil {
 				t.Fatalf("error creating temp file: %v", err)
 			}
@@ -165,6 +170,18 @@ func TestEraE(t *testing.T) {
 			if e.Count() != uint64(totalBlocks) {
 				t.Fatalf("wrong block count: want %d, got %d", totalBlocks, e.Count())
 			}
+			// Verify the layout detected from on-disk type tags. Header,
+			// body, and receipts are always present; TD is only present
+			// when the epoch contains pre-merge blocks.
+			if !e.HasComponent(header) || !e.HasComponent(body) || !e.HasComponent(receipts) {
+				t.Fatalf("missing required component in layout %v", e.m.layout)
+			}
+			if got, want := e.HasComponent(td), tt.preMerge > 0; got != want {
+				t.Fatalf("td component presence mismatch: want %v, got %v", want, got)
+			}
+			if e.HasComponent(proof) {
+				t.Fatalf("proof component should not be present in layout %v", e.m.layout)
+			}
 
 			// Verify accumulator in file.
 			if tt.accumulator {
@@ -295,7 +312,7 @@ func TestEraE(t *testing.T) {
 func TestInitialTD(t *testing.T) {
 	t.Parallel()
 
-	f, err := os.CreateTemp(t.TempDir(), "erae-initial-td-test")
+	f, err := os.CreateTemp(t.TempDir(), "ere-initial-td-test")
 	if err != nil {
 		t.Fatalf("error creating temp file: %v", err)
 	}
@@ -339,6 +356,115 @@ func TestInitialTD(t *testing.T) {
 	}
 }
 
+// TestDetectLayoutNoReceipts builds an Ere file with no receipts component and
+// checks that block data can still be read, while receipt access fails clearly.
+func TestDetectLayoutNoReceipts(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, "synthetic.ere")
+	f, err := os.Create(path)
+	if err != nil {
+		t.Fatalf("create: %v", err)
+	}
+
+	w := e2store.NewWriter(f)
+	written := uint64(0)
+	writeEntry := func(typ uint16, data []byte) {
+		n, err := w.Write(typ, data)
+		if err != nil {
+			t.Fatalf("write type 0x%04x: %v", typ, err)
+		}
+		written += uint64(n)
+	}
+
+	var snappyBuf bytes.Buffer
+	writeSnappy := func(typ uint16, data []byte) {
+		snappyBuf.Reset()
+		sw := snappy.NewBufferedWriter(&snappyBuf)
+		if _, err := sw.Write(data); err != nil {
+			t.Fatalf("snappy write: %v", err)
+		}
+		if err := sw.Flush(); err != nil {
+			t.Fatalf("snappy flush: %v", err)
+		}
+		writeEntry(typ, snappyBuf.Bytes())
+	}
+
+	writeEntry(era.TypeVersion, nil)
+
+	// Block 0 components in order: header, body, td (no receipts).
+	headerBytes := mustEncode(&types.Header{Number: big.NewInt(0), Difficulty: big.NewInt(1)})
+	bodyBytes := mustEncode(&types.Body{})
+	tdLE := make([]byte, 32) // uint256(1) little-endian
+	tdLE[0] = 1
+
+	headerOff := written
+	writeSnappy(era.TypeCompressedHeader, headerBytes)
+	bodyOff := written
+	writeSnappy(era.TypeCompressedBody, bodyBytes)
+	tdOff := written
+	writeEntry(era.TypeTotalDifficulty, tdLE)
+
+	// Build the DynamicBlockIndex with 3 components per block, 1 block, and
+	// the third slot pointing at the TD entry rather than at receipts.
+	base := int64(written)
+	relative := func(absolute uint64) uint64 { return uint64(int64(absolute) - base) }
+
+	var indexBuf bytes.Buffer
+	writeU64 := func(v uint64) {
+		if err := binary.Write(&indexBuf, binary.LittleEndian, v); err != nil {
+			t.Fatalf("index write: %v", err)
+		}
+	}
+	writeU64(0) // starting block number
+	writeU64(relative(headerOff))
+	writeU64(relative(bodyOff))
+	writeU64(relative(tdOff))
+	writeU64(3) // component count
+	writeU64(1) // block count
+	writeEntry(era.TypeDynamicBlockIndex, indexBuf.Bytes())
+
+	if err := f.Close(); err != nil {
+		t.Fatalf("close: %v", err)
+	}
+
+	g, err := os.Open(path)
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	t.Cleanup(func() { g.Close() })
+	e, err := From(g)
+	if err != nil {
+		t.Fatalf("From rejected file with no receipts component: %v", err)
+	}
+
+	blk, err := e.GetBlockByNumber(0)
+	if err != nil {
+		t.Fatalf("GetBlockByNumber failed: %v", err)
+	}
+	if blk.NumberU64() != 0 {
+		t.Fatalf("wrong block number: got %d", blk.NumberU64())
+	}
+	if _, err := e.GetRawReceiptsByNumber(0); err == nil {
+		t.Fatal("expected GetRawReceiptsByNumber to fail")
+	}
+
+	it, err := e.Iterator()
+	if err != nil {
+		t.Fatalf("Iterator failed: %v", err)
+	}
+	if !it.Next() {
+		t.Fatalf("expected iterator to advance, err %v", it.Error())
+	}
+	if _, err := it.Block(); err != nil {
+		t.Fatalf("iterator Block failed: %v", err)
+	}
+	if _, err := it.Receipts(); err == nil {
+		t.Fatal("expected iterator Receipts to fail")
+	}
+}
+
 func mustEncode(obj any) []byte {
 	b, err := rlp.EncodeToBytes(obj)
 	if err != nil {

internal/era/execdb/iterator.go

@@ -86,7 +86,7 @@ func (it *Iterator) Receipts() (types.Receipts, error) {
 		return nil, err
 	}
 	if it.inner.Receipts == nil {
-		return nil, errors.New("receipts must be non‑nil")
+		return nil, errors.New("receipts not found")
 	}
 	var rs []*types.SlimReceipt
 	if err := rlp.Decode(it.inner.Receipts, &rs); err != nil {
@@ -180,19 +180,23 @@ func (it *RawIterator) Next() bool {
 		return false
 	}
 
-	receiptsOffset, err := it.e.receiptOff(it.next)
+	if it.e.HasComponent(receipts) {
-	if err != nil {
+		receiptsOffset, err := it.e.receiptOff(it.next)
-		it.setErr(err)
+		if err != nil {
-		return false
+			it.setErr(err)
-	}
+			return false
-	it.Receipts, _, err = newSnappyReader(it.e.s, era.TypeCompressedSlimReceipts, receiptsOffset)
+		}
-	if err != nil {
+		it.Receipts, _, err = newSnappyReader(it.e.s, era.TypeCompressedSlimReceipts, receiptsOffset)
-		it.setErr(err)
+		if err != nil {
-		return false
+			it.setErr(err)
+			return false
+		}
+	} else {
+		it.Receipts = nil
 	}
 
 	// Check if TD component is present in this file (pre-merge or merge-transition epoch).
-	if int(td) < int(it.e.m.components) {
+	if it.e.HasComponent(td) {
 		tdOffset, err := it.e.tdOff(it.next)
 		if err != nil {
 			it.setErr(err)

internal/era/execdb/reader.go

@@ -18,6 +18,7 @@ package execdb
 
 import (
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 	"math/big"
@@ -39,10 +40,13 @@ type Era struct {
 	m metadata // metadata for the Era file
 }
 
-// Filename returns a recognizable filename for an EraE file.
+// Filename returns a recognizable filename for an Ere file.
 // The filename uses the last block hash to uniquely identify the epoch's content.
+//
+// Files produced by this builder do not include Proof entries, so the
+// "noproofs" profile postfix is appended per the Ere spec.
 func Filename(network string, epoch int, lastBlockHash common.Hash) string {
-	return fmt.Sprintf("%s-%05d-%s.erae", network, epoch, lastBlockHash.Hex()[2:10])
+	return fmt.Sprintf("%s-%05d-%s-noproofs.ere", network, epoch, lastBlockHash.Hex()[2:10])
 }
 
 // Open accesses the era file.
@@ -51,8 +55,8 @@ func Open(path string) (*Era, error) {
 	if err != nil {
 		return nil, err
 	}
-	e := &Era{f: f, s: e2store.NewReader(f)}
+	e, err := from(f)
-	if err := e.loadIndex(); err != nil {
+	if err != nil {
 		f.Close()
 		return nil, err
 	}
@@ -71,9 +75,17 @@ func (e *Era) Close() error {
 
 // From returns an Era backed by f.
 func From(f era.ReadAtSeekCloser) (era.Era, error) {
+	e, err := from(f)
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	return e, nil
+}
+
+func from(f era.ReadAtSeekCloser) (*Era, error) {
 	e := &Era{f: f, s: e2store.NewReader(f)}
 	if err := e.loadIndex(); err != nil {
-		f.Close()
 		return nil, err
 	}
 	return e, nil
@@ -185,12 +197,19 @@ func (e *Era) GetRawReceiptsByNumber(blockNum uint64) ([]byte, error) {
 	return io.ReadAll(r)
 }
 
+// HasComponent reports whether the given component is recorded in the file's
+// index, as detected from the on-disk e2store type tags.
+func (e *Era) HasComponent(c componentType) bool {
+	_, ok := e.m.layout[c]
+	return ok
+}
+
 // InitialTD returns initial total difficulty before the difficulty of the
 // first block of the Era is applied. Returns an error if TD is not available
 // (e.g., post-merge epoch).
 func (e *Era) InitialTD() (*big.Int, error) {
 	// Check if TD component exists.
-	if int(td) >= int(e.m.components) {
+	if !e.HasComponent(td) {
 		return nil, fmt.Errorf("total difficulty not available in this epoch")
 	}
 
@@ -210,8 +229,8 @@ func (e *Era) InitialTD() (*big.Int, error) {
 	return new(big.Int).Sub(firstTD, header.Difficulty), nil
 }
 
-// Accumulator reads the accumulator entry in the EraE file if it exists.
+// Accumulator reads the accumulator entry if present. Only pre-merge and
-// Note that one premerge erae files will contain an accumulator entry.
+// merge-transition Ere files contain one.
 func (e *Era) Accumulator() (common.Hash, error) {
 	entry, err := e.s.Find(era.TypeAccumulator)
 	if err != nil {
@@ -220,7 +239,8 @@ func (e *Era) Accumulator() (common.Hash, error) {
 	return common.BytesToHash(entry.Value), nil
 }
 
-// loadIndex loads in the index table containing all offsets and caches it.
+// loadIndex loads in the index table trailer (start, count, component-count)
+// and then derives the component→slot layout from the on-disk type tags.
 func (e *Era) loadIndex() error {
 	var err error
 	e.m.length, err = e.f.Seek(0, io.SeekEnd)
@@ -241,30 +261,65 @@ func (e *Era) loadIndex() error {
 	if err != nil {
 		return err
 	}
-
 	e.m.start = binary.LittleEndian.Uint64(b[:8])
+
+	layout, err := e.detectLayout()
+	if err != nil {
+		return err
+	}
+	e.m.layout = layout
 	return nil
 }
 
-// headerOff, bodyOff, receiptOff, and tdOff return the offsets of the respective components for a given block number.
+// detectLayout reads the e2store type tag at each component slot of the first
-func (e *Era) headerOff(num uint64) (int64, error)  { return e.indexOffset(num, header) }
+// block and builds a componentType→slot map, so components are looked up by tag
-func (e *Era) bodyOff(num uint64) (int64, error)    { return e.indexOffset(num, body) }
+// rather than by a fixed position.
-func (e *Era) receiptOff(num uint64) (int64, error) { return e.indexOffset(num, receipts) }
+func (e *Era) detectLayout() (map[componentType]int, error) {
-func (e *Era) tdOff(num uint64) (int64, error)      { return e.indexOffset(num, td) }
+	if e.m.count == 0 {
-
+		return nil, errors.New("Ere file contains no blocks")
-// indexOffset calculates offset to a certain component for a block number within a file.
-func (e *Era) indexOffset(n uint64, component componentType) (int64, error) {
-	if n < e.m.start || n >= e.m.start+e.m.count {
-		return 0, fmt.Errorf("block %d out of range [%d,%d)", n, e.m.start, e.m.start+e.m.count)
 	}
-	if int(component) >= int(e.m.components) {
+	tagToComponent := map[uint16]componentType{
-		return 0, fmt.Errorf("component %d not present", component)
+		era.TypeCompressedHeader:       header,
+		era.TypeCompressedBody:         body,
+		era.TypeCompressedSlimReceipts: receipts,
+		era.TypeTotalDifficulty:        td,
+		era.TypeProof:                  proof,
 	}
+	layout := make(map[componentType]int, e.m.components)
+	for slot := 0; slot < int(e.m.components); slot++ {
+		off, err := e.slotOffset(0, slot)
+		if err != nil {
+			return nil, fmt.Errorf("read slot %d offset: %w", slot, err)
+		}
+		typ, _, err := e.s.ReadMetadataAt(off)
+		if err != nil {
+			return nil, fmt.Errorf("read slot %d type tag: %w", slot, err)
+		}
+		comp, ok := tagToComponent[typ]
+		if !ok {
+			return nil, fmt.Errorf("unknown e2store type 0x%04x at index slot %d", typ, slot)
+		}
+		if existing, dup := layout[comp]; dup {
+			return nil, fmt.Errorf("duplicate component %d at slots %d and %d", comp, existing, slot)
+		}
+		layout[comp] = slot
+	}
+	if _, ok := layout[header]; !ok {
+		return nil, errors.New("Ere index has no header component")
+	}
+	if _, ok := layout[body]; !ok {
+		return nil, errors.New("Ere index has no body component")
+	}
+	return layout, nil
+}
 
-	payloadlen := 8 + 8*e.m.count*e.m.components + 16 // 8 for start block, 8 per property per block, 16 for the number of properties and the number of blocks
+// slotOffset returns the absolute file offset of the entry at the given slot
+// of the given block index (0 = first block in file).
+func (e *Era) slotOffset(blockIdx uint64, slot int) (int64, error) {
+	payloadlen := 8 + 8*e.m.count*e.m.components + 16
 	indstart := e.m.length - int64(payloadlen) - 8
 
-	rec := (n-e.m.start)*e.m.components + uint64(component)
+	rec := blockIdx*e.m.components + uint64(slot)
 	pos := indstart + 8 + 8 + int64(rec*8)
 
 	var buf [8]byte
@@ -275,18 +330,38 @@ func (e *Era) indexOffset(n uint64, component componentType) (int64, error) {
 	return int64(rel) + indstart, nil
 }
 
-// metadata contains the information about the era file that is written into the file.
+// headerOff, bodyOff, receiptOff, and tdOff return the offsets of the respective components for a given block number.
-type metadata struct {
+func (e *Era) headerOff(num uint64) (int64, error)  { return e.indexOffset(num, header) }
-	start      uint64 // start block number
+func (e *Era) bodyOff(num uint64) (int64, error)    { return e.indexOffset(num, body) }
-	count      uint64 // number of blocks in the era
+func (e *Era) receiptOff(num uint64) (int64, error) { return e.indexOffset(num, receipts) }
-	components uint64 // number of properties
+func (e *Era) tdOff(num uint64) (int64, error)      { return e.indexOffset(num, td) }
-	length     int64  // length of the file in bytes
+
+// indexOffset calculates offset to a certain component for a block number
+// within a file.
+func (e *Era) indexOffset(n uint64, component componentType) (int64, error) {
+	if n < e.m.start || n >= e.m.start+e.m.count {
+		return 0, fmt.Errorf("block %d out of range [%d,%d)", n, e.m.start, e.m.start+e.m.count)
+	}
+	slot, ok := e.m.layout[component]
+	if !ok {
+		return 0, fmt.Errorf("component %d not present in this Ere file", component)
+	}
+	return e.slotOffset(n-e.m.start, slot)
 }
 
-// componentType represents the integer form of a specific type that can be present in the era file.
+// metadata contains the information about the era file that is written into the file.
+type metadata struct {
+	start      uint64                // start block number
+	count      uint64                // number of blocks in the era
+	components uint64                // number of slots per block in the index
+	layout     map[componentType]int // component → slot index, derived from on-disk type tags
+	length     int64                 // length of the file in bytes
+}
+
+// componentType identifies a kind of per-block entry (header, body, etc.).
 type componentType int
 
-// header, body, receipts, td, and proof are the different types of components that can be present in the era file.
+// td and proof are independently optional per the Ere spec.
 const (
 	header componentType = iota
 	body