Ledger Hashes
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(Not to be confused with the ["ledger hash" string data type][Hash], which uniquely identifies a ledger version. This section describes the LedgerHashes ledger object type.)
The LedgerHashes object type contains a history of prior ledgers that led up to this ledger version, in the form of their hashes. Objects of this ledger type are modified automatically when closing a ledger. (This is one of the only times a ledger's state data is modified without a transaction or pseudo-transaction.) The LedgerHashes objects exist to make it possible to look up a previous ledger's hash with only the current ledger version and at most one lookup of a previous ledger version.
There are two kinds of LedgerHashes object. Both types have the same fields. Each ledger version contains:
Exactly one "recent history" LedgerHashes object
A number of "previous history" LedgerHashes objects based on the current ledger index (that is, the length of the ledger history). Specifically, Xahau adds a new "previous history" object every 65536 ledger versions.
Note: As an exception, a new genesis ledger has no LedgerHashes objects at all, because it has no ledger history.
Example LedgerHashes object (trimmed for length):
A LedgerHashes object has the following fields:
FirstLedgerSequence
Number
UInt32
Yes
DEPRECATED Do not use. (The "recent hashes" object of the production Xahau has the value 2 in this field as a result of a previous rippled software. That value gets carried forward as the "recent hashes" object is updated. New "previous history" objects do not have this field, nor do "recent hashes" objects in parallel networks started with more recent versions of rippled.)
Flags
Number
UInt32
Yes
A bit-map of boolean flags enabled for this object. Currently, the protocol defines no flags for LedgerHashes objects. The value is always 0.
Hashes
Array of Strings
Vector256
Yes
An array of up to 256 ledger hashes. The contents depend on which sub-type of LedgerHashes object this is.
LastLedgerSequence
Number
UInt32
Yes
The [Ledger Index][] of the last entry in this object's Hashes array.
LedgerEntryType
String
UInt16
Yes
The value 0x0068, mapped to the string LedgerHashes, indicates that this object is a list of ledger hashes.
There is exactly one LedgerHashes object of the "recent history" sub-type in every ledger after the genesis ledger. This object contains the identifying hashes of the most recent 256 ledger versions (or fewer, if the ledger history has less than 256 ledgers total) in the Hashes array. Whenever a new ledger is closed, part of the process of closing it involves updating the "recent history" object with the hash of the previous ledger version this ledger version is derived from (also known as this ledger version's parent ledger). When there are more than 256 hashes, the oldest one is removed.
Using the "recent history" LedgerHashes object of a given ledger, you can get the hash of any ledger index within the 256 ledger versions before the given ledger version.
The "previous history" LedgerHashes entries collectively contain the hash of every 256th ledger version (also called "flag ledgers") in the full history of the ledger. When the child of a flag ledger closes, the flag ledger's hash is added to the Hashes array of the newest "previous history" LedgerHashes object. Every 65536 ledgers, rippled creates a new LedgerHashes object, so that each "previous history" object has the hashes of 256 flag ledgers.
Note: The oldest "previous history" LedgerHashes object contains only 255 entries because the genesis ledger has a ledger index of 1, not 0.
There are two formats for LedgerHashes object IDs, depending on whether the object is a "recent history" sub-type or a "previous history" sub-type.
The "recent history" LedgerHashes object has an ID that is the [SHA-512Half][] of the LedgerHashes space key (0x0073). In other words, the "recent history" always has the ID B4979A36CDC7F3D3D5C31A4EAE2AC7D7209DDA877588B9AFC66799692AB0D66B.
The "previous history" LedgerHashes objects have an ID that is the [SHA-512Half][] of the following values, concatenated in order:
The LedgerHashes space key (0x0073)
The 32-bit [Ledger Index][] of a flag ledger in the object's Hashes array, divided by 65536.
Tip: Dividing by 65536 keeps the most significant 16 bits, which are the same for all the flag ledgers listed in a "previous history" object, and only those ledgers. You can use this fact to look up the LedgerHashes object that contains the hash of any flag ledger.
The "previous history" LedgerHashes objects act as a so you can get the hash of any historical flag ledger from its index. From there, you can use that flag ledger's "recent history" object to get the hash of any other ledger.