feat: add base-string and improve string

By building strings up with streams, we can yield the preferrable
`(simple-array character (*))` type instead of a `vector`.

Author: fosskers

CHANGELOG.md

@@ -8,10 +8,13 @@
   SEXP strings.
 - Reducer: `quantities` for counting unique occurrences of streamed items.
 - Reducer: `partition` for splitting the stream results.
+- Reducer: `base-string` for reducing into a `simple-base-string`.
 
 #### Changed
 
-- The `vector`, `string`, and `average` reducers are now faster and use less memory.
+- Reducer: `vector` and `average` are now faster and use less memory.
+- Reducer: `string` is now more efficient and yields the specialized
+  `(simple-array character (*))` type.
 - `comp` is now a macro and uses less memory for long composition chains.
 
 #### Fixed

transducers/package.lisp

@@ -1,7 +1,8 @@
 (defpackage transducers
   (:use :cl)
   (:shadow #:map #:concatenate #:log #:step #:split
-           #:cons #:count #:first #:last #:max #:min #:find #:string #:vector #:hash-table
+           #:string #:base-string #:vector #:hash-table
+           #:cons #:count #:first #:last #:max #:min #:find
            #:random)
   ;; --- Entry Points --- ;;
   (:export #:transduce)

transducers/reducers.lisp

@@ -16,15 +16,27 @@ reversal."
         ((and a? (not i?)) acc)
         (t '())))
 
-(declaim (ftype (function (&optional (or cl:string null) character) cl:string) string))
 (defun string (&optional (acc nil a?) (input #\z i?))
-  "Reducer: Collect a stream of characters into to a single string."
-  (cond ((and a? i?) (vector-push-extend input acc) acc)
-        ((and a? (not i?)) acc)
-        (t (make-array 16 :element-type 'character :adjustable t :fill-pointer 0))))
+  "Reducer: Collect a stream of characters into to a `simple-string'."
+  (cond ((and a? i?)
+         (write-char input acc)
+         acc)
+        ((and a? (not i?)) (get-output-stream-string acc))
+        (t (make-string-output-stream :element-type 'character))))
+
+#+nil
+(transduce (map #'char-upcase) #'string "hello")
+
+(defun base-string (&optional (acc nil a?) (input #\z i?))
+  "Reducer: Collect a stream of characters into to a single string of `base-char'."
+  (cond ((and a? i?)
+         (write-char input acc)
+         acc)
+        ((and a? (not i?)) (get-output-stream-string acc))
+        (t (make-string-output-stream :element-type 'base-char))))
 
 #+nil
-(string-transduce (map #'char-upcase) #'string "hello")
+(transduce (map #'char-upcase) #'base-string "hello")
 
 (declaim (ftype (function (&optional (or cl:vector null) t) cl:vector) vector))
 (defun vector (&optional (acc nil a?) (input nil i?))