;;;; -*- Mode: Lisp; Syntax: ANSI-Common-Lisp; Base: 10 -*-
;;;; *************************************************************************
;;;; FILE IDENTIFICATION
;;;;
;;;; Name:          pipes.lisp
;;;; Purpose:       Pipes based on ideas from Norvig's PAIP book
;;;; Programmers:   Kevin M. Rosenberg and Peter Norvig
;;;; Date Started:  Apr 2000
;;;;
;;;; $Id: pipes.lisp,v 1.1 2002/11/02 17:49:10 kevin Exp $
;;;;
;;;; This file is Copyright (c) 2000-2002 by Kevin M. Rosenberg and
;;;; Copyright (c) 1998-2002 by Peter Norvig. 
;;;;
;;;; KMRCL users are granted the rights to distribute and use this software
;;;; as governed by the terms of the Lisp Lesser GNU Public License
;;;; (http://opensource.franz.com/preamble.html), also known as the LLGPL.
;;;; *************************************************************************

(in-package :pipes)


(defconstant +empty-pipe+ nil)

(defmacro make-pipe (head tail)
  "create a pipe by eval'ing head and delaying tail."
  `(cons ,head #'(lambda () ,tail)))

(defun pipe-tail (pipe)
  "return tail of pipe or list, and destructively update 
   the tail if it is a function."
  ;; pipes should never contain functions as values
  (if (functionp (rest pipe))
    (setf (rest pipe) (funcall (rest pipe)))
    (rest pipe)))

(defun pipe-head (pipe) (first pipe))

(defun pipe-elt (pipe n)
  "nth element of pipe, 0 based."
  (if (= n 0) (pipe-head pipe)
     (pipe-elt (pipe-tail pipe) (- n 1))))


(defun enumerate (pipe &key count key (result pipe))
  "go through all or count elements of pipe,
   possibly applying the key function. "
  (if (or (eq pipe +empty-pipe+) (eql count 0))
    result
    (progn
      (unless (null key) (funcall key (pipe-head pipe)))
      (enumerate (pipe-tail pipe)
                 :count (if count (1- count))
                 :key key
                 :result result))))

(defun pipe-display (pipe &optional count)
  (enumerate pipe :count count))

(defun pipe-force (pipe)
  (enumerate pipe))

(defun pipe-filter (predicate pipe)
  "keep only items in (non-null) pipe satisfying predicate"
     (if (eq pipe +empty-pipe+)
      +empty-pipe+
      (let ((head (pipe-head pipe))
            (tail (pipe-tail pipe)))
      (if (funcall predicate head)
        (make-pipe head (pipe-filter predicate tail))
        (pipe-filter predicate tail)))))
               

(defun pipe-map (fn pipe)
  "Map fn over pipe, delaying all but the first fn call,
   collecting res<ults"
  (if (eq pipe +empty-pipe+)
    +empty-pipe+
    (make-pipe (funcall fn (pipe-head pipe))
               (pipe-map fn (pipe-tail pipe)))))


(defun pipe-map-filtering (fn pipe &optional filter-test)
  "Map fn over pipe, delaying all but the first fn call,
   collecting results"
  (if (eq pipe +empty-pipe+)
    +empty-pipe+
    (let* ((head (pipe-head pipe))
           (tail (pipe-tail pipe))
           (result (funcall fn head)))
      (if (or (and filter-test (funcall filter-test result))
              result)
        (make-pipe result (pipe-map-filtering fn tail filter-test))
        (pipe-map-filtering fn tail filter-test)))))
      
      
(defun pipe-append (pipex pipey)
  "return a pipe that appends two pipes"
  (if (eq pipex +empty-pipe+)
    pipey
    (make-pipe (pipe-head pipex)
               (pipe-append (pipe-tail pipex) pipey))))

(defun pipe-mappend (fn pipe)
  "lazily map fn over pipe, appending results"
  (if (eq pipe +empty-pipe+)
    +empty-pipe+
    (let ((x (funcall fn (pipe-head pipe))))
      (make-pipe (pipe-head x)
                 (pipe-append (pipe-tail x)
                               (pipe-mappend fn (pipe-tail pipe)))))))

(defun pipe-mappend-filtering (fn pipe &optional filter-test)
  "Map fn over pipe, delaying all but the first fn call,
   appending results, filtering along the way"
  (if (eq pipe +empty-pipe+)
    +empty-pipe+
    (let* ((head (pipe-head pipe))
           (tail (pipe-tail pipe))
           (result (funcall fn head)))
      (if (or (and filter-test (funcall filter-test result))
              result)
        (make-pipe (pipe-head result)
                   (pipe-append (pipe-tail result)
                                 (pipe-mappend-filtering fn tail filter-test)))
        (pipe-mappend-filtering fn tail filter-test)))))