TTClock

In TTPython, all data values are represented by tokens, and all tokens are timestamped (see TTTime). Timestamps derive from clocks. TTClocks represent timelines that are relatve to another timeline or clock domain. Each TTClock has a parent clock from which it derives its notion of time. Of these TTClocks, one must have a direct relation to real-time (or some time-keeping hardware); this is the ‘root’ clock, which has no parent. In this way, TTClocks within a TTPython program form a tree, which is conceptually similar to ‘Stratum’ in clock synchronization frameworks like the Network Time Protocol.

Currently, the clock tree is used as a specification for these timelines, but was originally conceived as a generalization of clock synchronization trees in which derived clocks operated at lower precision than their parent. This would define how finely clocks should be synchronized to meet the application requirements. Constructing trees in this way has a strong relation to the mapping of the program and how well ensembles can synchronize their clocks with each other. For now, we rely on the underlying system to synchronize the root clock to Universal Coordinated Time to the best of its abilities (this is generally on the order of 5 ms).

Every clock in the tree counts in integer ticks. This is an important concept. In our system, we do not have a way to express a point in time that is infinitessimally narrow. Instead, we mark time in intervals. These may be quite small, but nevertheless, they span some finite segment of the real time line. This also has implications for the underlying clock hardware. Just as a real clock counts in ticks (even mechanisms like the old escape wheel), we also make this fundamental assumption. It is ultimately a choice for the programmer to define the smallest time interval (including time offsets) in a TTPython program because the underlying hardware can only count in discrete steps.

Time values for a given clock form an integer sequence that is monotonic increasing. This sequence can be thought of as a namespace. For now, we only need to state that when a derived clock is established, we not only can set the relative tick rate (period), we also establish the point on its parent’s time line at which the derived clock is at t=0. We call this the epoch.

period:

for a given derived clock, a positive integer that expresses the number of ticks of the parent clock per tick of the derived clock

epoch:

for a given derived clock, an integer that expresses the smallest tick of the parent clock corresponding to the derived clock’s t=0

ancestor clock:

or a given derived clock, any of the clocks along the tree starting from and including the parent clock to the root clock.

Note

For any two clocks in the tree, there will always be a common ancestor clock (possibly the root clock).

The TTClock class

The TTClock class implements the above. Every TTPython program needs at least one clock (the root clock).

class ticktalkpython.Clock.TTClock(name, parent_clock, period, epoch)

Create a TTClock by specifying its name (for documentation’s sake), parent, period and epoch

Parameters:

• name (string) – a descriptor for this clock; used solely for documentation

• parent_clock (TTClock) – This clock’s parent in the clock tree

• period (int) – the number of parent clock ticks corresponding to ONE tick of this clock

• epoch (int) – the lowest-numbered tick of the parent corresponding to t=0 of this clock

static common_ancestor(clock_a, clock_b)

Walk the path toward the root clock from two different clocks until we find the common ancestor clock

Parameters:

• clock_a (TTClock) – one of the two clocks (this method is commutative)

• clock_b (TTClock) – the other clock

Returns:

the common ancestor

Return type:

TTClock

is_root()

Is this clock the root?

Returns:

True iff this is the root clock

Return type:

bool

json()

Convert the Clock into a TTClockSpec and into a json

now()

Report the current time as read from the root clock, translated into this clock’s equivalent tick count

Returns:

the current tick of this clock

Return type:

int

parent()

Return the parent of this clock (or itself if this clock is root)

Returns:

the parent

Return type:

TTClock

static root()

Return the self-defined root clock.

Note that a root clock has two special instance variables: a function returning the current time (in ticks of the root clock), and a mapping between ticks and real-time (in seconds); these must be functions and integers, respectively. These can be accessed via root_clock.root_now() and root_clock.root_ticks_per_second

Returns:

root clock

Return type:

TTClock

root_ticks_per_tick()

In terms of the root clock, return many ticks are there are for a tick of the child

Returns:

The number of ticks in the root domain per tick of this clock

Return type:

int

ticks_per_second()

In terms of the real timeline, return how many ticks of this clock there are in a second

Returns:

The number of ticks per second in this clock domain (truncated if there is a remainder)

Return type:

int

trace_to_root()

Trace through the clock tree to the root clock

Returns:

The root clock

Return type:

TTClock

class ticktalkpython.Clock.TTClockSpec(name, parent_clock=None, period=1, epoch=0)

TTClockSpec is a specification of a clock, and describes its relation to the parent clock without carrying a direct memory reference to said clock.

In practice, this is often used to prevent the entire clock tree from being copied or serialized when TTTime and TTToken objects are shared between processes or ensembles. TTTimeSpec uses TTClockSpec in place of TTClock

Parameters:

• name (string) – A name used to refer to the clock, matching the TTClock exactly

• parent_clock (string) – A name referring to a parent clock (rather than a direct reference as used in TTClock)

• period (int) – The period of this clock specfication; period ticks of the parent clock equate to 1 tick of this

• epoch (int) – The offset of this clock with respect to the parent clock. This clock would be epoch ticks ahead of the parent clock, where epoch ticks are in the domain of the parent

classmethod from_clock(clock)

Create a TTClockSpec from a TTClock object. Useful for removing references to make Clocks easily serializable. TTClock` references are converted to clock names.

Parameters:

clock (TTClock) – The clock to create a specification for

Returns:

Clock Specificaton matching the formulation of the clock itself

Return type:

TTClockSpec

is_root()

Determine if this TTClockSpec is for a root clock, based on whether there is a parent clock defined or not

Returns:

True if this TTClockSpec is a root clock; False otherwise

Return type:

bool

json()

Convert the TTClockSpec into an equivalent JSON representation for serialization

to_clock(available_clocks)

Retrieve the reference to the correct clock from a set of the available ones. This is determined the TTClockSpec’s recorded name and parent’s name