#!/usr/bin/env ruby
require "shellwords"

def error(msg, code) $stderr.puts msg; exit code end
def git(*args)
  cmd = ["git", *args].map{|s| s.to_s}.shelljoin
  out = IO.popen(cmd, err: File::NULL, &:read)
  raise unless $?.success?
  out
rescue
  ""
end

def resolve_merge(x)
  ps = git("rev-list","--parents","-n1",x).split
  return nil if ps.empty? || ps.size < 3
  ps[0]
end

def find_by_ancestry(arg_commit, revset)
  git("rev-list","--merges",*revset).split.each do |m|
    line = git("rev-list","--parents","-n1",m).split
    next if line.size < 3
    line[2..-1].each do |p|
      return m if system("git","merge-base","--is-ancestor",p,arg_commit)
    end
  end
  nil
end

def find_by_grep(needle, revset)
  pats = [
    "Merge branch '#{needle}' ",
    "Merge branch '#{needle}'",
    "Merge remote-tracking branch 'origin/#{needle}'",
    "Merge branch \"#{needle}\"",
    "Merge branch #{needle}"
  ]
  pats.each do |pat|
    h = git("log",*revset,"--merges","--pretty=%H","--fixed-strings","--grep",pat,"-n1").lines.first&.strip
    return h unless h.to_s.empty?
  end
  nil
end

arg = ARGV[0] || ""
tgt = ARGV[1]
error("usage: #{$0} <branch-or-merge-commit-or-fragment> [target-branch]", 1) if arg.empty?

merge = resolve_merge(arg)

revset =
  if tgt && !tgt.empty?
    error("no such target: #{tgt}", 2) if git("rev-parse","--verify","#{tgt}^{commit}").empty?
    [tgt]
  else
    ["--all"]
  end

if !merge
  arg_commit = git("rev-parse","--verify","#{arg}^{commit}").strip
  if !arg_commit.empty?
    merge = find_by_ancestry(arg_commit, revset)
  end
  merge ||= find_by_grep(arg, revset)
end

error("no merge commit found for '#{arg}' in #{revset.join(",")}", 3) if !merge || merge.empty?

p1 = git("rev-parse","#{merge}^1").strip
error("could not resolve first parent of #{merge}", 4) if p1.empty?

ok = system("git","diff","--binary",p1,merge)
exit(5) unless ok