=== Index Block version 2 (`IDX2`)

The index block stores references to all blocks present in the file.
It is composed of a header, followed by a sequence of entries, the count of which is defined within the header.

Multiple index blocks may exist within a file to represent previous states or historical versions; however, only the final index block must be referenced by the main file header.

==== Structure Definition

[source,c]
#define INDEX2_MAGIC 0x32584449
/**Header for the index, followed by entries */
typedef struct IndexHeader2
{
    /**Identifier, <see cref="BlockType.Index" /> */
    uint32_t identifier;
    /**How many entries follow this header */
    uint64_t entries;
    /**CRC64-ECMA of the index */
    uint64_t crc64;
} IndexHeader;

==== Field Descriptions

[cols="2,2,2,6",options="header"]
|===
|Type
|Size
|Name
|Description

|uint32_t
|4 bytes
|identifier
|The index block identifier, always `IDX2`

|uint64_t
|8 bytes
|entries
|The number of entries following this header

|uint64_t
|8 bytes
|crc64
|CRC64-ECMA checksum of the entries following this header
|===

==== Index entries

[source,c]
/**Index entry */
typedef struct IndexEntry
{
    /**Type of item pointed by this entry */
    uint32_t blockType;
    /**Type of data contained by the block pointed by this entry */
    uint16_t dataType;
    /**Offset in file where item is stored */
    uint64_t offset;
} IndexEntry;

==== Field Descriptions

[cols="2,2,2,6",options="header"]
|===
|Type
|Size
|Name
|Description

|uint32_t
|4 bytes
|blockType
|The type of block this entry points to.

|uint32_t
|2 bytes
|dataType
|The type of data the block pointed by this entry contains.

|uint64_t
|8 bytes
|offset
|The offset in bytes from the start of the file where the block pointed by this entry starts.
|===