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;; Copyright (C) 2020 Free Software Foundation, Inc.
;;
;; This library is free software; you can redistribute it and/or
;; modify it under the terms of the GNU Lesser General Public
;; License as published by the Free Software Foundation; either
;; version 3 of the License, or (at your option) any later version.
;;
;; This library is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;; Lesser General Public License for more details.
;;
;; You should have received a copy of the GNU Lesser General Public
;; License along with this library; if not, write to the Free Software
;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
(define-module (srfi srfi-171 meta)
#:use-module (srfi srfi-9)
#:use-module ((rnrs bytevectors) #:select (bytevector-length bytevector-u8-ref))
#:export (reduced reduced?
unreduce
ensure-reduced
preserving-reduced
list-reduce
vector-reduce
string-reduce
bytevector-u8-reduce
port-reduce
generator-reduce))
;; A reduced value is stops the transduction.
(define-record-type <reduced>
(reduced val)
reduced?
(val unreduce))
(define (ensure-reduced x)
"Ensure that @var{x} is reduced"
(if (reduced? x)
x
(reduced x)))
;; helper function that wraps a reduced value twice since reducing functions (like list-reduce)
;; unwraps them. tconcatenate is a good example: it re-uses it's reducer on it's input using list-reduce.
;; If that reduction finishes early and returns a reduced value, list-reduce would "unreduce"
;; that value and try to continue the transducing process.
(define (preserving-reduced reducer)
(lambda (a b)
(let ((return (reducer a b)))
(if (reduced? return)
(reduced return)
return))))
;; This is where the magic tofu is cooked
(define (list-reduce f identity lst)
(if (null? lst)
identity
(let ((v (f identity (car lst))))
(if (reduced? v)
(unreduce v)
(list-reduce f v (cdr lst))))))
(define (vector-reduce f identity vec)
(let ((len (vector-length vec)))
(let loop ((i 0) (acc identity))
(if (= i len)
acc
(let ((acc (f acc (vector-ref vec i))))
(if (reduced? acc)
(unreduce acc)
(loop (+ i 1) acc)))))))
(define (string-reduce f identity str)
(let ((len (string-length str)))
(let loop ((i 0) (acc identity))
(if (= i len)
acc
(let ((acc (f acc (string-ref str i))))
(if (reduced? acc)
(unreduce acc)
(loop (+ i 1) acc)))))))
(define (bytevector-u8-reduce f identity vec)
(let ((len (bytevector-length vec)))
(let loop ((i 0) (acc identity))
(if (= i len)
acc
(let ((acc (f acc (bytevector-u8-ref vec i))))
(if (reduced? acc)
(unreduce acc)
(loop (+ i 1) acc)))))))
(define (port-reduce f identity reader port)
(let loop ((val (reader port)) (acc identity))
(if (eof-object? val)
acc
(let ((acc (f acc val)))
(if (reduced? acc)
(unreduce acc)
(loop (reader port) acc))))))
(define (generator-reduce f identity gen)
(let loop ((val (gen)) (acc identity))
(if (eof-object? val)
acc
(let ((acc (f acc val)))
(if (reduced? acc)
(unreduce acc)
(loop (gen) acc))))))
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