git-author-stats.rb

def authors_from_commit(commit)
  authors = []
  commit.chomp.split(' and ').map{|a| authors.push a}
  authors
end

def authors_from_commits(commits)
  authors = []
  commits.each do |commit|
    authors += authors_from_commit(commit)
  end
  return authors
end

def group_by_author(authors)
  stats = Hash.new(0)
  authors.map do |author|
    stats[author] += 1
  end
  stats
end

def by_value(hash)
  out = ""
  hash.sort{|a,b| b[1]<=>a[1]}.each do |elem|
    out += "#{elem[0]} (#{elem[1]})\n"
  end
  out
end

def assertEqual(expected, actual)
  if expected == actual
    print '.'
  else
    puts "\nFAIL: #{expected} expected, but got #{actual}" 
  end
end

def test
  # Split paired authors into names
  authors = authors_from_commit("Bob Test and Charles Code")
  assertEqual("Bob Test", authors[0])
  assertEqual("Charles Code", authors[1])
  # Respect single authors too
  assertEqual("Derek Driven", authors_from_commit("Derek Driven")[0])
  # Count authors up
  commits = ['Bob Test', 'Charles and Bob Test']
  stats = group_by_author(authors_from_commits(commits)) 
  assertEqual(2, stats['Bob Test'])
  assertEqual(1, stats['Charles'])
  # Print out by descending commit order
  output = by_value(group_by_author(authors_from_commits(commits)))
  assertEqual(output, "Bob Test (2)\nCharles (1)\n")
  puts "\nTest Run Finished"
end

def run
  committers = `git log --pretty=format:%an`
  puts by_value(group_by_author(authors_from_commits(committers)))
end
ARGV[0] == '-t' ? test : run

Fully tested now with a made up test 'framework'. Run with -t to run tests.