#!/usr/bin/env ruby
require 'net/imap'

# Source server connection info.
SOURCE_HOST = 'mail.example.com'
SOURCE_PORT = 143
SOURCE_SSL  = false
SOURCE_USER = 'username'
SOURCE_PASS = 'password'

# Destination server connection info.
DEST_HOST = 'imap.gmail.com'
DEST_PORT = 993
DEST_SSL  = true
DEST_USER = 'username@gmail.com'
DEST_PASS = 'password'

# Mapping of source folders to destination folders. The key is the name of the
# folder on the source server, the value is the name on the destination server.
# Any folder not specified here will be ignored. If a destination folder does
# not exist, it will be created.
FOLDERS = {
  'INBOX' => 'INBOX',
  'sourcefolder' => 'gmailfolder'
}

# Utility methods.
def dd(message)
   puts "[#{DEST_HOST}] #{message}"
end

def ds(message)
   puts "[#{SOURCE_HOST}] #{message}"
end

# Connect and log into both servers.
ds 'connecting...'
source = Net::IMAP.new(SOURCE_HOST, SOURCE_PORT, SOURCE_SSL)

ds 'logging in...'
source.login(SOURCE_USER, SOURCE_PASS)

dd 'connecting...'
dest = Net::IMAP.new(DEST_HOST, DEST_PORT, DEST_SSL)

dd 'logging in...'
dest.login(DEST_USER, DEST_PASS)

# Loop through folders and copy messages.
FOLDERS.each do |source_folder, dest_folder|
  # Open source folder in read-only mode.
  begin
	ds "selecting folder '#{source_folder}'..."
	source.examine(source_folder)
  rescue => e
	ds "error: select failed: #{e}"
	next
  end

  # Open (or create) destination folder in read-write mode.
  begin
	dd "selecting folder '#{dest_folder}'..."
	dest.select(dest_folder)
  rescue => e
	begin
	  dd "folder not found; creating..."
	  dest.create(dest_folder)
	  dest.select(dest_folder)
	rescue => ee
	  dd "error: could not create folder: #{e}"
	  next
	end
  end

  # Build a lookup hash of all message ids present in the destination folder.
  dest_info = {}

  dd 'analyzing existing messages...'
  uids = dest.uid_search(['ALL'])
  if uids.length > 0
	dest.uid_fetch(uids, ['ENVELOPE']).each do |data|
	  dest_info[data.attr['ENVELOPE'].message_id] = true
	end
  end

  # Loop through all messages in the source folder.
  uids = source.uid_search(['ALL'])
  if uids.length > 0
	source.uid_fetch(uids, ['ENVELOPE']).each do |data|
	  mid = data.attr['ENVELOPE'].message_id

	  # If this message is already in the destination folder, skip it.
	  next if dest_info[mid]

	  # Download the full message body from the source folder.
	  ds "downloading message #{mid}..."
	  msg = source.uid_fetch(data.attr['UID'], ['RFC822', 'FLAGS',
		  'INTERNALDATE']).first

	  # Append the message to the destination folder, preserving flags and
	  # internal timestamp.
	  dd "storing message #{mid}..."
	  dest.append(dest_folder, msg.attr['RFC822'], msg.attr['FLAGS'],
		  msg.attr['INTERNALDATE'])
	end
  end

  source.close
  dest.close
end

puts 'done'