## A.54 library(statistics): Get information about resource usage

This library provides predicates to obtain information about resource usage by your program. The predicates of this library are for human use at the toplevel: information is printed. All predicates obtain their information using public low-level primitives. These primitives can be use to obtain selective statistics during execution.

[det]statistics
  Print information about resource usage using print_message/2.
  See also
    All statistics printed are obtained through statistics/2.
[det]statistics(-Stats:dict)
  Stats is a dict representing the same information as statistics/0. This convience function is primarily intended to pass statistical information to e.g., a web client. Time critical code that wishes to collect statistics typically only need a small subset and should use statistics/2 to obtain exactly the data they need.
[nondet]thread_statistics(?Thread, -Stats:dict)
  Obtain statistical information about a single thread. Fails silently of the Thread is no longer alive.

  Stats | is a dict containing status, time and stack-size information about Thread.

[nondet]time(:Goal)
  Execute Goal, reporting statistics to the user. If Goal succeeds non-deterministically, retrying reports the statistics for providing the next answer.

  Note that is no portable way to get thread-specific CPU time. SWI-Prolog has implementations for Linux, Windows and MacOS. The automatic detection may work on some other operating systems.

  See also
    - statistics/2 for obtaining statistics in your program and understanding the reported values.
    - call_time/2, call_time/3 to obtain the timing in a dict.
  bug
    Inference statistics are often a few off.
call_time(:Goal, -Time:dict)
call_time(:Goal, -Time:dict, -Result)
  Call Goal as call/1, unifying Time with a dict that provides information on the resource usage. If Goal succeeds with a choice point, backtracking reports the time used to find the next answer, failure or exception. If Goal succeeds deterministically no choice point is left open. Currently Time contains the keys below. Future versions may provide additional keys.

  - wall:Seconds
  - cpu:Seconds
  - inferences:Count

  call_time/2 is defined as below. Note that for call_time/2 the time is only available if Goal succeeds.

      call_time(Goal, Time) :-
          call_time(Goal, Time, Result),
          call(Result).

  Result | is one of `true`, `false` or `throw(E)`, depending on whether or not the goal succeeded or raised an exception. Note that Result may be called using call/1 to propagate the failure or exception.