Blobmap is a specialized data structure for efficient storage and retrieval of Binary Large Objects (Blobs) within a continuous keyspace of 64-bit unsigned integers (uint64). The keyspace begins at a specified value n and spans m consecutive keys, covering the range from n to n + m - 1.

The data is stored in a read-only memory-mapped file to enable constant-time (O(1)) access, making it scalable for handling large datasets.

The file begins with a header containing metadata critical for blob management:

• Number of blobs: The total count of blobs in the file.
• Key offset: The starting key n for the keyspace.

An array of offset records follows the header. Each record stores the end offset of a blob, encoded as a 64-bit big-endian integer. The start of each blob is implicitly defined by the end offset of the preceding blob.

The blobs themselves are stored sequentially in the file. The data for each blob can be accessed by determining its byte range from the offset records.

The file concludes with an xxHash checksum, covering all preceding data. This can be used to verify the integrity of the blobmap during reads.

+----------------+----------------------+-------------------+-------------+
|     Header     |    Offset Table      |    Blob Data      |   xxHash    |
+----------------+----------------------+-------------------+-------------+
| Num of Blobs   | End Offset of Blob 1 | Blob 1 Data Bytes | Hash Value  |
| Key Offset (n) | End Offset of Blob 2 | Blob 2 Data Bytes |             |
|                | End Offset of Blob 3 | Blob 3 Data Bytes |             |
+----------------+----------------------+-------------------+-------------+

• Header: Stores the number of blobs and the starting key.
• Offset Table: Defines the end offsets of each blob.
• Blob Data: Contains the actual binary data of each blob, laid out sequentially.
• xxHash: Provides a checksum to ensure data integrity.

To access a specific blob, compute its byte range using the corresponding offsets in the table:

• The start of blob i is the end offset of blob i-1 (or immediately after the offset table for the first blob).
• The end of blob i is the offset at position i in the table.

This layout enables fast, direct access to any blob, minimizing overhead and maximizing scalability for large datasets.