rdelim: Speed up 'read-string' (aka. 'get-string-all'.)

This yields a 20% improvement on the "read-string" benchmark.

* module/ice-9/rdelim.scm (read-string): Rewrite as a 'case-lambda',
  with a tight loop around 'read-char', and without using
  'read-string!'.
* test-suite/tests/rdelim.test ("read-string")["longer than 100 chars,
  with limit"]: New test.
* benchmark-suite/benchmarks/ports.bm ("rdelim")["read-string"]: New
  benchmark.

benchmark-suite/benchmarks/ports.bm

@@ -1,6 +1,6 @@
 ;;; ports.bm --- Port I/O.         -*- mode: scheme; coding: utf-8; -*-
 ;;;
-;;; Copyright (C) 2010, 2011, 2012 Free Software Foundation, Inc.
+;;; Copyright (C) 2010, 2011, 2012, 2014 Free Software Foundation, Inc.
 ;;;
 ;;; This program is free software; you can redistribute it and/or
 ;;; modify it under the terms of the GNU Lesser General Public License
@@ -89,4 +89,10 @@
     (benchmark "read-line" 1000
                (let ((port (with-fluids ((%default-port-encoding "UTF-8"))
                              (open-input-string str))))
-                 (sequence (read-line port) 1000)))))
+                 (sequence (read-line port) 1000))))
+
+  (let ((str (large-string "Hello, world.\n")))
+    (benchmark "read-string" 200
+               (let ((port (with-fluids ((%default-port-encoding "UTF-8"))
+                             (open-input-string str))))
+                 (read-string port)))))

module/ice-9/rdelim.scm

@@ -1,6 +1,7 @@
 ;;; installed-scm-file
 
-;;;; Copyright (C) 1997, 1999, 2000, 2001, 2006, 2010, 2013 Free Software Foundation, Inc.
+;;;; Copyright (C) 1997, 1999, 2000, 2001, 2006, 2010, 2013,
+;;;;   2014 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
@@ -148,26 +149,29 @@ left in the port."
                 (lp (1+ n)))))
         (- n start))))
 
-(define* (read-string #:optional (port (current-input-port)) (count #f))
-  "Read all of the characters out of PORT and return them as a string.
+(define* read-string
+  (case-lambda*
+   "Read all of the characters out of PORT and return them as a string.
 If the COUNT argument is present, treat it as a limit to the number of
 characters to read.  By default, there is no limit."
-  (check-arg (or (not count) (index? count)) "bad count" count)
-  (let loop ((substrings '())
-             (total-chars 0)
-             (buf-size 100))		; doubled each time through.
-    (let* ((buf (make-string (if count
-                                 (min buf-size (- count total-chars))
-                                 buf-size)))
-           (nchars (read-string! buf port))
-           (new-total (+ total-chars nchars)))
-      (cond
-       ((= nchars buf-size)
-        ;; buffer filled.
-        (loop (cons buf substrings) new-total (* buf-size 2)))
-       (else
-        (string-concatenate-reverse
-         (cons (substring buf 0 nchars) substrings)))))))
+   ((#:optional (port (current-input-port)))
+    ;; Fast path.
+    ;; This creates more garbage than using 'string-set!' as in
+    ;; 'read-string!', but currently that is faster nonetheless.
+    (let loop ((chars '()))
+      (let ((char (read-char port)))
+        (if (eof-object? char)
+            (list->string (reverse! chars))
+            (loop (cons char chars))))))
+   ((port count)
+    ;; Slower path.
+    (let loop ((chars '())
+               (total 0))
+      (let ((char (read-char port)))
+        (if (or (eof-object? char) (>= total count))
+            (list->string (reverse chars))
+            (loop (cons char chars) (+ 1 total))))))))
+
 
 ;;; read-line [PORT [HANDLE-DELIM]] reads a newline-terminated string
 ;;; from PORT.  The return value depends on the value of HANDLE-DELIM,

test-suite/tests/rdelim.test

@@ -1,7 +1,7 @@
 ;;;; rdelim.test --- Delimited I/O.      -*- mode: scheme; coding: utf-8; -*-
 ;;;; Ludovic Courtès <ludo@gnu.org>
 ;;;;
-;;;; 	Copyright (C) 2011, 2013 Free Software Foundation, Inc.
+;;;; 	Copyright (C) 2011, 2013, 2014 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
@@ -209,7 +209,13 @@
       (let* ((s (string-concatenate (make-list 20 "hello, world!")))
              (p (open-input-string s)))
         (and (string=? (read-string p) s)
-             (string=? (read-string p) "")))))
+             (string=? (read-string p) ""))))
+
+    (pass-if-equal "longer than 100 chars, with limit"
+        "hello, world!"
+      (let* ((s (string-concatenate (make-list 20 "hello, world!")))
+             (p (open-input-string s)))
+        (read-string p 13))))
 
   (with-test-prefix "read-string!"