From ec8e6ce0a114a35f13c019b4fcc6a22d5b429a95 Mon Sep 17 00:00:00 2001
From: Peter Palfrader <peter@palfrader.org>
Date: Fri, 6 Jun 2003 11:27:59 +0000
Subject: We no longer assume a normal distribution of latencies but instead
 use percentiles when calculating a life probability of an outstanding ping. 
 Also we do not show the mean of latency but the median as this seems to be
 'more correct'. This also means we no longer need the normal distribution
 libraries from lib/

---
 Echolot/Stats.pm | 89 +++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 52 insertions(+), 37 deletions(-)

(limited to 'Echolot')

diff --git a/Echolot/Stats.pm b/Echolot/Stats.pm
index 905f210..c7255c7 100644
--- a/Echolot/Stats.pm
+++ b/Echolot/Stats.pm
@@ -1,7 +1,7 @@
 package Echolot::Stats;
 
 # (c) 2002 Peter Palfrader <peter@palfrader.org>
-# $Id: Stats.pm,v 1.56 2003/06/06 09:32:37 weasel Exp $
+# $Id: Stats.pm,v 1.57 2003/06/06 11:27:59 weasel Exp $
 #
 
 =pod
@@ -21,12 +21,6 @@ use strict;
 use English;
 use Echolot::Log;
 
-use Statistics::Distrib::Normal qw{};
-
-my $NORMAL = new Statistics::Distrib::Normal;
-$NORMAL->mu(0);
-$NORMAL->sigma(1);
-
 my $STATS_DAYS;
 my $SECONDS_PER_DAY;
 
@@ -190,11 +184,36 @@ sub build_list2_capsstr($) {
 	return $str;
 }
 
+sub median($) {
+	my ($arr) = @_;
+	my $cnt = scalar @$arr;
+	if ($cnt == 0) {
+		return undef;
+	} elsif ($cnt % 2 == 0) {
+		return ($arr->[ int(($cnt - 1 ) / 2) ] + $arr->[ int($cnt / 2) ] ) / 2;
+	} else {
+		return $arr->[ int(($cnt - 1 ) / 2) ];
+	};
+};
+
+sub percentile($$) {
+	my ($lat, $lats) = @_;
+
+	my $num = scalar @$lats;
+	my $i;
+	for (my $i=0; $i < $num; $i++) {
+		last if $lat < $lats->[$i];
+	}
+	return ($num - $i) / $num;
+}
 
 sub calculate($$) {
 	my ($addr, $types) = @_;
 	my $now = time();
 
+	my $SKEW_ABS = 15*60;
+	my $SKEW_PERCENT = 0.80;
+
 	my @out;
 	my @done;
 	
@@ -207,51 +226,47 @@ sub calculate($$) {
 		};
 	};
 
-	my $latency = 0;
-	my $received = 0;
-	my $sent = 0;
-	my @latency;
-	my @received;
-	my @sent;
+	my @latency_total = map { $_->[1] } @done;
+	my @latency_day;
+	my $sent_total;
+	my $received_total;
+	my @sent_day;
+	my @received_day;
 	for my $done (@done) {
-		$latency += $done->[1];   $latency [int(($now - $done->[0]) / $SECONDS_PER_DAY)] += $done->[1];
-		$sent ++;                 $sent    [int(($now - $done->[0]) / $SECONDS_PER_DAY)] ++;
-		$received ++;             $received[int(($now - $done->[0]) / $SECONDS_PER_DAY)] ++;
-	};
-	$latency /= (scalar @done) if (scalar @done);
-	$latency = undef unless (scalar @done);
-	for ( 0 .. $STATS_DAYS - 1 ) {
-		$latency[$_] /= $received[$_] if ($received[$_]);
+		push @{ $latency_day [int(($now - $done->[0]) / $SECONDS_PER_DAY)] }, $done->[1];
+		$sent_total ++;     $sent_day    [int(($now - $done->[0]) / $SECONDS_PER_DAY)] ++;
+		$received_total ++; $received_day[int(($now - $done->[0]) / $SECONDS_PER_DAY)] ++;
 	};
 
-	my $variance = 0;
-	$variance += ($latency - $_->[1]) ** 2 for (@done);
-	$variance /= (scalar @done) if (scalar @done);
-
-	my $deviation = sqrt($variance);
+	@latency_total = sort { $a <=> $b } @latency_total;
+	my $latency_median = median (\@latency_total);
+	my @latency_median_day;
+	for ( 0 .. $STATS_DAYS - 1 ) {
+		@{$latency_day[$_]} = sort { $a <=> $b } $latency_day[$_];
+		$latency_median_day[$_] = median ( $latency_day[$_] );
+	}
 
 	if (scalar @out) {
-		my @p = 
-			($deviation != 0) ?
-				$NORMAL->utp( map { ($now - $_ - $latency) / $deviation } @out ) :
+		my @p = ( scalar @latency_total ) ?
+				map { percentile( ($now - $_ - $SKEW_ABS)/$SKEW_PERCENT , \@latency_total ) } @out :
 				map { 0 } @out;
 		for (my $i=0; $i < scalar @out; $i++) {
-			$sent ++;            $sent    [int(($now - $out[$i]) / $SECONDS_PER_DAY)] ++;
-			$received += $p[$i]; $received[int(($now - $out[$i]) / $SECONDS_PER_DAY)] += $p[$i];
+			$sent_total ++;            $sent_day    [int(($now - $out[$i]) / $SECONDS_PER_DAY)] ++;
+			$received_total += $p[$i]; $received_day[int(($now - $out[$i]) / $SECONDS_PER_DAY)] += $p[$i];
 		};
 	};
-	$received /= $sent if ($sent);
+	$received_total /= $sent_total if ($sent_total);
 	for ( 0 .. $STATS_DAYS - 1 ) {
-		$received[$_] /= $sent[$_] if ($sent[$_]);
+		$received_day[$_] /= $sent_day[$_] if ($sent_day[$_]);
 	};
 
 
 
 	return {
-		avr_latency     => $latency,
-		avr_reliability => $received,
-		latency_day     => \@latency,
-		reliability_day => \@received
+		avr_latency     => $latency_median,
+		avr_reliability => $received_total,
+		latency_day     => \@latency_day,
+		reliability_day => \@received_day
 	};
 };
 
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