misc-code/iterators.lisp

;;;; iterators.lisp

(defpackage #:iterators-simple
  (:use :cl)
  (:export #:with-iterator
           #:call-iterator
           #:define-iterator
           #:create-iterator))

(in-package #:iterators-simple)

;;; "iterators" goes here. Hacks and glory await!

(defun make-numeric-iterator (function second-argument &optional (initial 1))
  "Create a closure acting as a numeric iterator.
This takes a FUNCTION, a SECOND-ARGUMENT to a function,
and an INITIAL value for N defaulting at 1.

This applies the function to N, and the SECOND-ARGUMENT,
setting N to the value."
  (let ((n initial))
    #'(lambda ()
        (setf n (funcall function n second-argument)))))

(defun make-stream-iterator (stream-or-file-name type)
  (let ((stream (if (stringp stream-or-file-name)
                    (open stream-or-file-name)
                    stream-or-file-name)))
    (ecase type
      (:line
       #'(lambda ()
           (read-line stream nil nil)))
      (:char
       #'(lambda ()
           (read-char stream nil nil)))
      (:byte
       #'(lambda ()
           (read-byte stream nil nil))))))

(defun make-list-iterator (&rest list)
  (let ((list list))
    #'(lambda ()
        (if (= 0 (length list))
            nil
            (pop list)))))

(defun call-iterator (iterator)
  (funcall iterator))

(defun create-iterator (type &rest args)
  "Create an integer of type TYPE, using args ARGS."
  (ecase type
    (:numeric
     (make-numeric-iterator (elt args 0) (elt args 1) (if (elt args 2) (elt args 2) 1)))
    (:stream
     (make-stream-iterator (elt args 0) (elt args 1)))
    (:list
     (apply #'make-list-iterator args))))

(defmacro with-iterator ((name type &rest args) &body body)
  (let ((iterator-expression (ecase type
                               (:numeric
                                `(make-numeric-iterator ,(elt args 0) ,(elt args 1) ,(if (elt args 2) (elt args 2) 1)))
                               (:stream
                                `(make-stream-iterator ,(elt args 0) ,(elt args 1)))
                               (:list
                                `(make-list-iterator ,@args)))))
    `(let ((,name ,iterator-expression))
       (flet ((,name ()
                (call-iterator ,name)))
         ,@body))))

(defmacro define-iterator (name type &rest args)
  (let ((iter-expression (ecase type
                           (:numeric
                            `(make-numeric-iterator ,(elt args 0) ,(elt args 1) ,(if (elt args 2) (elt args 2) 1)))
                           (:stream
                            `(make-stream-iterator ,(elt args 0) ,(elt args 1)))
                           (:list
                            `(make-list-iterator ,@args)))))
    `(defvar ,name ,iter-expression)))