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I am trying to understand how the integer operation ID returned by horizon REST API is calculated.

I found this answer which mentions the inputs of the calculation but does not show how to perform the calculation:

  • Ledger sequence
  • Transaction application order within ledger tx set
  • Operation application order within tx
2

Found it documented in Horizon source code:

// Operations within the stellar network have a total order, expressed by three
// pieces of information:  the ledger sequence the operation was validated in,
// the order which the operation's containing transaction was applied in
// that ledger, and the index of the operation within that parent transaction.
//
// We express this order by packing those three pieces of information into a
// single signed 64-bit number (we used a signed number for SQL compatibility).
//
// The follow diagram shows this format:
//
//    0                   1                   2                   3
//    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//   |                    Ledger Sequence Number                     |
//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//   |     Transaction Application Order     |       Op Index        |
//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

You can find a the Go code in the same link.

Here is a sample Python example:

LEDGER_MASK = (1 << 32) - 1
TRANSACTION_MASK = (1 << 20) - 1
OPERATION_MASK = (1 << 12) - 1

LEDGER_SHIFT = 32
TRANSACTION_SHIFT = 12
OPERATION_SHIFT = 0

def calculate_operation_id(
        ledger_sequence: int,
        transaction_order_in_ledger: int,  # 1-based
        operation_order_in_transaction: int,  # 1-based
):
    operation_id = (ledger_sequence & LEDGER_MASK) << \
        LEDGER_SHIFT
    operation_id = operation_id | (transaction_order_in_ledger & TRANSACTION_MASK) << \
        TRANSACTION_SHIFT
    operation_id = operation_id | (operation_order_in_transaction & OPERATION_MASK) << \
        OPERATION_SHIFT
    return operation_id

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