path-utils: Uniform maximum length on all platforms.

When necessary, replace some bytes from the middle rather than
hashing the path. (This means it is still human-readable, mostly.)

gui-lib/framework/private/path-utils.rkt

@@ -2,7 +2,6 @@
 
 (require "sig.rkt"
          racket/list
-         openssl/md5
          "../preferences.rkt")
 
 (import)
@@ -97,57 +96,85 @@
      (build-path base name-element)]))
 
 
+(define candidate-separators
+  `(#"!" #"%" #"_" #"|" #":" #">" #"^" #"$" #"@" #"*" #"?"))
+
+(define separator-regexps
+  (map (compose1 byte-regexp regexp-quote) candidate-separators))
 
 ; encode-as-path-element : dir-path path-element -> path-element
 ; N.B. generate-backup-name may supply a relative directory, but
 ; we should always use a complete one.
-; Using simplify-path does that and ensures no 'up or 'same
-; Using ! is not completely robust, but works well enough for Emacs.
+; That is handled by simplify+explode-path->bytes.
 ; Windows has limitations on path lengths. Racket handles MAX_PATH
 ; by using "\\?\" paths when necessary, but individual elements must
-; be shorter than lpMaximumComponentLength. If necessary, we avoid
-; this by hashing the path.
+; be shorter than lpMaximumComponentLength.
+; We respect this limit (on all platforms, for consistency)
+; by replacing some bytes from the middle if necessary.
 (define (encode-as-path-element base-maybe-relative name)
-  (define windows?
-    (eq? 'windows (system-path-convention-type)))
   (define illegal-rx
-    (if windows?
-        #rx#"\\\\"
-        #rx#"/"))
-  (define pth
-    (simplify-path (build-path base-maybe-relative name)))
+    (case (system-path-convention-type)
+      [(windows) #rx#"\\\\"]
+      [else #rx#"/"]))
+  (define l-bytes
+    (simplify+explode-path->bytes (build-path base-maybe-relative name)))
+  (define separator-byte
+    (or (let ([all-components (apply bytes-append l-bytes)])
+          (for/first ([sep (in-list candidate-separators)]
+                      [rx (in-list separator-regexps)]
+                      #:unless (regexp-match? rx all-components))
+            sep))
+        #"!"))
   (define legible-name-bytes
-    (let ([elements (explode-path pth)])
-      (apply
-       bytes-append
-       (add-between
-        (cons (regexp-replace* illegal-rx
-                               (path->bytes (car elements))
-                               #"!")
-              (for/list ([elem (in-list (cdr elements))])
-                (regexp-replace* illegal-rx
-                                 (path-element->bytes elem)
-                                 #"!")))
-        #"!"))))
-  (cond
-    [(or (not windows?)
-         (< (bytes-length legible-name-bytes)
-            (lpMaximumComponentLength)))
-     (bytes->path-element legible-name-bytes)]
-    [else
-     (string->path-element
-      (regexp-replace*
-       #rx"\\\\" ; NOT illegal-rx : this is a string regexp
-       (md5 (open-input-bytes (path->bytes pth)))
-       "!"))]))
-
-
+    (apply
+     bytes-append
+     separator-byte
+     (add-between
+      (for/list ([elem (in-list l-bytes)])
+        (regexp-replace* illegal-rx
+                         (path-element->bytes elem)
+                         separator-byte))
+      separator-byte)))
+  (define num-legible-bytes
+    (bytes-length legible-name-bytes))
+  (bytes->path-element
+   (cond
+     [(< num-legible-bytes
+         (lpMaximumComponentLength))
+      legible-name-bytes]
+     [else
+      (define replacement
+        (bytes-append separator-byte #"..." separator-byte))
+      (define num-excess-bytes
+        (+ (- num-legible-bytes
+              (lpMaximumComponentLength))
+           5 ; extra margin of safety
+           (bytes-length replacement)))
+      (define num-bytes-to-keep-per-side
+        (floor (/ (- num-legible-bytes num-excess-bytes)
+                  2)))
+      (bytes-append
+       (subbytes legible-name-bytes 0 num-bytes-to-keep-per-side)
+       replacement
+       (subbytes legible-name-bytes (- num-legible-bytes
+                                       num-bytes-to-keep-per-side)))])))
+
+
+;; simplify+explode-path->bytes : path? -> (listof bytes?)
+;; Useful because path-element->bytes doesn't work on root paths.
+;; Using simplify-path ensures no 'up or 'same.
+(define (simplify+explode-path->bytes pth)
+  (define elems
+    (explode-path (simplify-path pth)))
+  (cons (path->bytes (car elems))
+        (map path-element->bytes (cdr elems))))
 
 ;; lpMaximumComponentLength : -> real?
 ;; Returns the maximum length of an element of a "\\?\" path on Windows.
 ;; For now, assuming 255, but really this should be
 ;; "the value returned in the lpMaximumComponentLength parameter
 ;; of the GetVolumeInformation function".
+;; See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#maxpath
 (define (lpMaximumComponentLength)
   255)