module Mailmate::EmlLookup

def self.eml_id_for_message_id(message_id)

repeatedly.
index — fine for one-shot CLI lookups; cache the result if you need it
return the local eml-id (integer) or nil. O(n) scan of the message-id
Reverse-lookup: given an RFC Message-ID (with or without angle brackets), 
def self.eml_id_for_message_id(message_id)
  needle = message_id.to_s.strip
  return nil if needle.empty?
  candidates = needle.start_with?("<") && needle.end_with?(">") ?
                 [needle, needle[1..-2]] :
                 [needle, "<#{needle}>"]
  Mailmate::IndexReader.for("message-id").each_record do |eml_id, value|
    return eml_id if candidates.include?(value)
  end
  nil
rescue ArgumentError
  nil
end

def self.path_for(eml_id)

Returns an absolute path string, or nil if not found. 
def self.path_for(eml_id)
  via_index(eml_id) || via_glob(eml_id)
end

def self.resolve_id(input)

RFC Message-ID (anything else) to a local eml-id.
Resolve an identifier that may be either an eml-id (all digits) or an 
def self.resolve_id(input)
  s = input.to_s.strip
  return s.to_i if s =~ /\A\d+\z/
  eml_id_for_message_id(s)
end

def self.source_url_for(eml_id)

Returns nil if the index doesn't have a record for it.
Lookup the `imap://...` URL recorded in `#source` for this eml_id. 
def self.source_url_for(eml_id)
  Mailmate::IndexReader.for("#source").value_for(eml_id)
rescue ArgumentError
  # #source index missing (non-default MailMate install? fresh sync?).
  nil
end

def self.url_to_path(url, eml_id)

absolute path of the .eml file. Internal but exposed for tests.
Convert an `imap://account@host/mailbox/path` URL to the on-disk 
def self.url_to_path(url, eml_id)
  stripped = url.sub(%r{\Aimap://}, "")
  account, mailbox_path = stripped.split("/", 2)
  return nil if account.nil? || mailbox_path.nil? || mailbox_path.empty?
  mailbox_dirs = mailbox_path.split("/").map { |seg| "#{seg}.mailbox" }.join("/")
  File.join(Mailmate.config.imap_root, account, mailbox_dirs, "Messages", "#{eml_id}.eml")
end

def self.via_glob(eml_id)

Force the glob fallback. 
def self.via_glob(eml_id)
  matches = Dir.glob("#{Mailmate.config.imap_root}/*/**/Messages/#{eml_id}.eml")
  matches.first
end

def self.via_index(eml_id)

Force the index path (useful for tests and benchmarking). 
def self.via_index(eml_id)
  url = source_url_for(eml_id)
  return nil if url.nil?
  url_to_path(url, eml_id)
end