An Ordinal is an immutable object holding a run stamp and a sequence of stamps. The run stamp is an integer that identifies a particular run. All events reported during the same run should share the same run stamp. By contrast, each event reported during a particular run should have a different stamps sequence. One use case for the run stamp is that the initial run from ScalaTest's GUI will have run stamp 0. Subsequent reruns will have run stamps 1, 2, 3, etc., so that reports in the GUI can simply be sorted in "ordinal" order. Another use case is a set of servers used to run multiple tests simultaneously in a distributed fashion. The run stamp can be used to identify the run for which an event belongs.

The stamps sequence is designed to allow a sequential order of events to be specified during concurrent execution of ScalaTest suites. ScalaTest's model for concurrent execution is that the suites that make up a run may be executed concurrently, but the tests within a single suite will be executed sequentially. In addition to tests, suites may contain nested suites. The default implementation of execute in class Suite will first invoke runNestedSuites and then runTests. If no Distributor is passed to execute, the runNestedSuites method will execute the nested suites sequentially via the same thread that invoked runNestedSuites. As a result, suites will by default executed in depth first order when executed sequentially. If a Distributor is passed to execute, the runNestedSuites method will simply put its nested suites into the Distributor and return. Some other threads or processes must then execute those nested suites. Given the default implementations of execute and runNestedSuites described here, the Ordinal will allow the events from a concurrent run to be sorted in the same depth-first order that the events from a corresponding sequential run would arrive.

Each event reported during a run should be given a unique Ordinal. An Ordinal is required by all Event subclasses, instances of which are used to send information to the report function passed to a Suite's execute method. The first Ordinal for a run can be produced be passing a run stamp to Ordinal's lone public constructor:

 val firstOrdinal = new Ordinal(99)
 

The run stamp can be any integer. The Ordinal created in this way can be passed along with the first reported event of the run, such as a RunStarting event. Thereafter, new Ordinals for the same run can be obtained by calling either next or nextNewOldPair on the previously obtained Ordinal. In other words, given an Ordinal, you can obtain the next Ordinal by invoking one of these two "next" methods on the Ordinal you have in hand. Before executing a new Suite, the nextNewOldPair method should be invoked. This will return two new Ordinals, one for the new Suite about to be executed, and one for the currently executing entity (either a Suite or some sort of test runner). At any other time, the next Ordinal can be obtained by simply invoking next on the current Ordinal.

You can convert an Ordinal to a List by invoking toList on it. The resulting List will contain the run stamp as its first element, and the contents of its stamps sequence as the subsequent elements. The stamps sequence will initially be composed of a single element with the value 0. Thus, toList invoked on the firstOrdinal shown above will result in:

 firstOrdinal.toList // results in: List(99, 0)
 

Each time next is invoked, the rightmost integer returned by toList will increment:

 val secondOrdinal = firstOrdinal.next
 secondOrdinal.toList // results in: List(99, 1)
 
 val thirdOrdinal = secondOrdinal.next
 thirdOrdinal.toList  // result is : List(99, 2)
 

When nextNewOldPair is invoked the result will be a tuple whose first element is the first Ordinal for the new Suite about to be executed (for example, a nested Suite of the currently executing Suite). The second element is the next Ordinal for the currently executing Suite or other entity:

 val (nextForNewSuite, nextForThisRunner) = thirdOrdinal.nextNewOldPair
 nextForNewSuite.toList   // results in: (99, 2, 0)
 nextForThisRunner.toList // results in: (99, 3)
 

The toList method of the Ordinal for the new suite starts with the same sequence of elements as the Ordinal from which it was created, but has one more element, a 0, appended at the end. Subsequent invocations of next on this series of Ordinals will increment that last element:

 val newSuiteOrdinal2 = nextForNewSuite.next
 newSuiteOrdinal2.toList // results in: List(99, 2, 1)
 
 val newSuiteOrdinal3 = newSuiteOrdinal2.next
 newSuiteOrdinal3.toList  // result is : List(99, 2, 2)
 

This behavior allows events fired by Suite running concurrently to be reordered in a pre-determined sequence after all the events have been reported. The ordering of two Ordinals can be determined by first comparing the first element of the Lists obtained by invoking toList on both Ordinals. These values represent the runStamp. If one run stamp is a lower number than the other, that Ordinal comes first. For example, an Ordinal with a run stamp of 98 is ordered before an Ordinal with a run stamp of 99. If the run stamps are equal, the next number in the list is inspected. As with the run stamps, an Ordinal with a lower number is ordered before an Ordinal with a higher number. If two corresponding elements are equal, the next pair of elements will be inspected. This will continue no down the length of the Lists until a position is found where the element values are not equal, or the end of one or both of the Lists are reached. If the two Lists are identical all the way to the end, and both Lists have the same lengths, then the Ordinals are equal. (Equal Ordinals will not happen if correctly used by creating a new Ordinal for each fired event and each new Suite.). If the two Lists are identical all the way to the end of one, but the other List is longer (has more elements), then the shorter list is ordered before the longer one.

As an example, here are some Ordinal List forms in order:

 List(99, 0)
 List(99, 1)
 List(99, 2)
 List(99, 2, 0)
 List(99, 2, 1)
 List(99, 2, 2)
 List(99, 2, 2, 0)
 List(99, 2, 2, 1)
 List(99, 2, 2, 2)
 List(99, 2, 3)
 List(99, 2, 4)
 List(99, 2, 4, 0)
 List(99, 2, 4, 1)
 List(99, 2, 4, 2)
 List(99, 3)
 List(99, 4)
 List(99, 4, 0)
 List(99, 4, 1)
 List(99, 5)