Ruby program to simulate coin tosses until a selected pattern is matched. Two distinct patterns are examined and compared statistically.

coins.rb

#!/usr/bin/env ruby
# coins.rb
# count average flips until HTH is reached
#

def flip ; rand(2) == 1 ? 'H' : 'T' ; end

# 'HTH'.count_flips

class String
  def count_flips
    target = self.to_s
    src = ''
    count = 0
    while (target != src)   # loop until we match the target
      src += flip           # append new toss
      count += 1            # count flips
      src.slice!(0,1) if src.size > target.size
    end
    count                   # return flip count
  end
end

# Enumerable didn't include this -- why?
class Array
  def sum 
    self.reduce(:+)
  end
  def average
    Float(self.sum) / self.nitems
  end
end

# average_flips
# 
# count coin flips until a target is reached
# do this repeated for a day, then average those counts.
# 
# Simulate human-speed coin flips by estimating time for each flip, and
# assuming constant flipping for 24 hours, without a break.

$flip_time = 4    # 4 seconds per flip
$max_flips_per_day = 24 * 60 * 60 / $flip_time

class String
  def flip_stats
    target = self

    counts = []
    (1..$max_flips_per_day).each{|i| counts += [target.count_flips]}
    min_count, max_count = counts.minmax
    average_count = counts.average
    [min_count, max_count, average_count]
  end
end

['HTH', 'HTT'].each {|target|
  puts "Computing #{target} stats.." if $DEBUG
  (min, max, avg) = target.flip_stats
  printf "Target: %s  Coin flip stats: min: %4.1f  max: %5.1f  avg: %5.1f\n", target, min, max, avg
}
exit

Sample output:

$ ./coins.rb 
Target: HTH  Coin flip stats: min:  3.0  max:  69.0  avg:  10.0
Target: HTT  Coin flip stats: min:  3.0  max:  53.0  avg:   8.0