Tweak Ansible's host inventory.

ansible-inventory.el

(defun expand-character (expansion c)
  (if (null expansion)
      (list (list c))
    (mapcar (lambda (item)
              (cons c item)) ;; This reverses the string.
            expansion)))

(defun expand-sequence (expansion p q)
  (apply #'append
         (mapcar (lambda (c)
                   (expand-character expansion c))
                 (number-sequence p q))))

(defun parse-ansible-bracket (input expansion)
  (let ((q (car input)) ;; reverse input
        (input (cdr input)))
    (let ((colon (car input))
          (input (cdr input)))
      (if (= colon 58) ;; #\:
          (let ((p (car input))
                (input (cdr input)))
            (let ((close (car input))
                  (input (cdr input)))
              (if (= close 91) ;; #\[
                  (parse-ansible-pattern input (expand-sequence expansion p q))
                (error "no closing bracket"))))
        (error "no colon")))))

(defun parse-ansible-pattern (input expansion)
  (if (null input)
      expansion
    (let ((c (car input))
          (input (cdr input)))
      (if (= c 93) ;; #\]
          (parse-ansible-bracket input expansion)
        (parse-ansible-pattern input (expand-character expansion c))))))

(defun ansible-expand-host-pattern ()
  "Expand Ansible host patterns like 'host-[a:c][1:3]' into nine
host names."
  (interactive)
  (beginning-of-line)
  (let ((bol (point)))
    (forward-line)
    (let ((eol (point)))
      (let ((line (reverse ;; Reversing the input is cheaper than
                           ;; reversing the expansion and the sort
                           ;; order of the expansion is correct.
                   (string-to-list
                    (buffer-substring-no-properties bol eol)))))
        (let ((expansion (mapcar (lambda (item)
                                   (apply #'string item))
                                 (parse-ansible-pattern line '()))))
          (delete-region bol eol)
          (dolist (str expansion)
            (insert str)))))))

(defun first-token (str)
  (let ((tokens (split-string str)))
    (if (consp tokens)
        (car tokens)
      nil)))

(defun getent-hosts (host)
  (first-token
   (shell-command-to-string
    (format "getent hosts '%s'" host))))

(defun ansible-insert-host-address ()
  "Append the IP address of the host in the current line to the
line.  Example: 'localhost' -> 'localhost ansible_host=127.0.0.1'"
  (interactive)
  (beginning-of-line)
  (let ((bol (point)))
    (end-of-line)
    (let ((eol (point)))
      (let ((line (buffer-substring-no-properties bol eol)))
        (let ((name (first-token line)))
          (if name
              (let ((address (getent-hosts name)))
                (if address
                    (progn (delete-region bol eol)
                           (insert name)
                           (insert " ansible_host=")
                           (insert address)
                           (forward-line))))))))))