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# frozen_string_literal: true
module Hitimes
# Internal: Internal setup that is done when the library is loaded
#
# We want to determine what clock to use for this machine. So we need to
# introspect the ruby environment and then setup some initial constants. These
# methods are used in lib/hitimes/instant.rb to help setup the CLOCK_ID
# constant at load time.
#
module Initialize
#
# After a fair bit of experimentaiton, it seems that the only clock_ids that
# are of any use are the following:
#
# POSIX:
#
# CLOCK_REALTIME A settable system-wide real-time clock. Measures
# wall-clock time. Affected by system jumps in time,
# adjtime(3), and NTP.
#
# CLOCK_MONOTONIC A nonsettable system-wide clock that represent
# monotomic time since some unspecified point in the
# past. Not affected by jumps in system time, but is
# affectd by adjtime(3) and NTP.
#
# Darwin:
#
# CLOCK_MONOTONIC_RAW clock that increments monotonically, tracking the
# time since an arbitrary point like CLOCK_MONOTONIC.
# However, this clock is unaffected by frequency or
# time adjustments.
#
# CLOCK_UPTIME_RAW clock that increments monotonically, in the same manner
# as CLOCK_MONOTONIC_RAW, but that does not increment
# while the system is asleep. The returned value is
# identical to the result of mach_absolute_time()
# after the appropriate mach_timebase conversion is applied.
#
# Linux:
#
# CLOCK_MONOTONIC_RAW Similar to CLOCK_MONOTONIC, but provides access to
# a raw hardware-based time that is not subject to NTP
# adjustments or the incremental adjustments performed
# by adjtime(3)
#
# CLOCK_BOOTTIME Identical to CLOCK_MONOTONIC, except it also includes any
# time that the system is suspended.
#
# *BSD:
#
# All the BSDs seem to have CLOCK_MONOTONIC and CLOCK_REALTIME although on
# NetBSD CLOCK_MONOTONIC is not affected by adjtime(2). It is unclear if
# they are affected by adjtime(2) on FreeBSD, OpenBSD, or DragonFlyBSD. -
# at least according to the man pages.
#
# What this boils down to as that pretty much all systems have CLOCK_REALTIME
# and CLOCK_MONOTONIC. The other clocks are system specific and may or may
# not exist. We'll try to use the most accurate clock available.
#
# So we'll try to use the following clocks in order of preference:
#
# On Linux and Darwin
# CLOCK_MONOTONIC_RAW, CLOCK_MONOTONIC, CLOCK_REALTIME
#
# Everyone else:
# CLOCK_MONOTONIC, CLOCK_REALTIME
#
# So in reality, well just test for constants on all of the above and use the
# first one that exists.
#
# If CLOCK_REALTIME is chose, we will dump a warning to the user.
# And if we can't finde one, which is really, really odd, we'll raise an exception.
POTENTIAL_CLOCK_IDS = %i[CLOCK_MONOTONIC_RAW CLOCK_MONOTONIC CLOCK_REALTIME].freeze
def determine_clock_id(potential_ids = POTENTIAL_CLOCK_IDS)
sym = potential_ids.find { |id| Process.const_defined?(id) }
unless sym
raise Hitimes::Error, <<~ERROR
Unable to find a high resolution clock at all. THIS IS A BUG!!
RUBY_DESCRIPTION: #{RUBY_DESCRIPTION}
Please report this bug to the hitimes issue tracker at
https://github.com/copiousfreetime/hitimes/issues
ERROR
end
if sym == :CLOCK_REALTIME
warn <<~TXT
Unable to find a high resolution clock. Using CLOCK_REALTIME for timing.
RUBY_DESCRIPTION: #{RUBY_DESCRIPTION}
Please report the above information to the hitimes issue tracker at
https://github.com/copiousfreetime/hitimes/issues
TXT
end
Process.const_get(sym)
end
module_function :determine_clock_id
end
end
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