feat: support for reading from Immutable DB

Fixes #12

database/immutable/chunk.go

@@ -0,0 +1,150 @@
+// Copyright 2024 Blink Labs Software
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package immutable
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/blinklabs-io/gouroboros/cbor"
+)
+
+const (
+	chunkFileExtension = ".chunk"
+)
+
+type chunk struct {
+	file         *os.File
+	fileSize     int64
+	secondary    *secondaryIndex
+	currentEntry *secondaryIndexEntry
+	nextEntry    *secondaryIndexEntry
+}
+
+func newChunk() *chunk {
+	return &chunk{}
+}
+
+func (c *chunk) Open(path string, secondary *secondaryIndex) error {
+	f, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	c.file = f
+	c.secondary = secondary
+	if stat, err := f.Stat(); err != nil {
+		return err
+	} else {
+		c.fileSize = stat.Size()
+	}
+	currentEntry, err := secondary.Next()
+	if err != nil {
+		return err
+	}
+	c.currentEntry = currentEntry
+	nextEntry, err := secondary.Next()
+	if err != nil {
+		return err
+	}
+	c.nextEntry = nextEntry
+	return nil
+}
+
+func (c *chunk) Close() error {
+	if err := c.secondary.Close(); err != nil {
+		return err
+	}
+	return c.file.Close()
+}
+
+func (c *chunk) Next() (*Block, error) {
+	if c.currentEntry == nil {
+		return nil, nil
+	}
+	if c.nextEntry == nil {
+		// Read last block
+		blockSize := c.fileSize - int64(c.currentEntry.BlockOffset)
+		blockData := make([]byte, blockSize)
+		// Seek to offset
+		if _, err := c.file.Seek(int64(c.currentEntry.BlockOffset), 0); err != nil {
+			return nil, err
+		}
+		n, err := c.file.Read(blockData)
+		if err != nil {
+			return nil, err
+		}
+		if int64(n) < blockSize {
+			return nil, fmt.Errorf("did not read expected amount of block data: expected %d, got %d", blockSize, n)
+		}
+		blkType, blkBytes, err := c.unwrapBlock(blockData)
+		if err != nil {
+			return nil, err
+		}
+		ret := &Block{
+			Type:  blkType,
+			Slot:  c.currentEntry.BlockOrEbb,
+			Hash:  c.currentEntry.HeaderHash[:],
+			IsEbb: c.currentEntry.IsEbb,
+			Cbor:  blkBytes[:],
+		}
+		c.currentEntry = nil
+		c.nextEntry = nil
+		return ret, nil
+	} else {
+		// Read middle block
+		blockSize := c.nextEntry.BlockOffset - c.currentEntry.BlockOffset
+		blockData := make([]byte, blockSize)
+		// Seek to offset
+		if _, err := c.file.Seek(int64(c.currentEntry.BlockOffset), 0); err != nil {
+			return nil, err
+		}
+		n, err := c.file.Read(blockData)
+		if err != nil {
+			return nil, err
+		}
+		if uint64(n) < blockSize {
+			return nil, fmt.Errorf("did not read expected amount of block data: expected %d, got %d", blockSize, n)
+		}
+		blkType, blkBytes, err := c.unwrapBlock(blockData)
+		if err != nil {
+			return nil, err
+		}
+		ret := &Block{
+			Type: blkType,
+			Slot: c.currentEntry.BlockOrEbb,
+			Hash: c.currentEntry.HeaderHash[:],
+			Cbor: blkBytes[:],
+		}
+		c.currentEntry = c.nextEntry
+		nextEntry, err := c.secondary.Next()
+		if err != nil {
+			return nil, err
+		}
+		c.nextEntry = nextEntry
+		return ret, nil
+	}
+}
+
+func (c *chunk) unwrapBlock(data []byte) (uint, []byte, error) {
+	tmpData := struct {
+		cbor.StructAsArray
+		BlockType uint
+		BlockCbor cbor.RawMessage
+	}{}
+	if _, err := cbor.Decode(data, &tmpData); err != nil {
+		return 0, nil, err
+	}
+	return tmpData.BlockType, []byte(tmpData.BlockCbor), nil
+}