Correctly handle common prefixes in substring completion

Substring completion would previously not complete the longest common
substring if that substring was a prefix of all the completion
alternatives.  Now it does.  An explanation of this bug

Substring completion is implemented by passing the `prefix' symbol as
part of the pattern passed to completion-pcm--merge-completions.  This
symbol is supposed to cause completion-pcm--merge-completions to
"grow" a completion of a common substring only from the "right" of the
symbol (a common suffix), not from the "left" of the symbol (a common
prefix).  Yes, this is the opposite of what the name `prefix' would
imply.

When processing a symbolic element of the pattern,
completion-pcm--merge-completions first finds the common prefix of all
the completions in that part of the pattern (using try-completion).
Then for `prefix' and other elements which want to complete a common
suffix, the common prefix is removed from each element and then the
common suffix is calculated with completion--common-suffix.

If the common prefix covers the entirety of all the alternatives
(i.e. when "unique" is true in the code), it's also a common suffix.
In that case, the common suffix calculation (if it runs) is basically
a no-op which will produce an empty string, since we removed the
common prefix before running it.

Before this change, `prefix' elements would unconditionally discard
the common prefix, which produced the wrong result in the case that
common prefix == common suffix.  For example:

  (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
  -> ("b")

Now we detect this situation and include the common prefix in this
case for `prefix' elements.  Then we get:

  (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
  -> ("b" "a")

which is correct.

* lisp/minibuffer.el (completion-pcm--merge-completions): Don't ignore
a common suffix in a `prefix' pattern element when it's also a common
prefix.
* test/lisp/minibuffer-tests.el (completion-substring-test-5): Add a
test.

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 5d7cbcedd41f99cc473152cca22496cfcd9b49a7..52a286018a1384804a814980e7d0eeb5d54137be 100644 (file)
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -4025,7 +4025,9 @@ the same set of elements."
                      (unique (or (and (eq prefix t) (setq prefix fixed))
                                  (and (stringp prefix)
                                       (eq t (try-completion prefix comps))))))
-                (unless (or (eq elem 'prefix)
+                ;; if the common prefix is unique, it also is a common
+                ;; suffix, so we should add it for `prefix' elements
+                (unless (or (and (eq elem 'prefix) (not unique))
                             (equal prefix ""))
                   (push prefix res))
                 ;; If there's only one completion, `elem' is not useful

diff --git a/test/lisp/minibuffer-tests.el b/test/lisp/minibuffer-tests.el
index ff58d35eb3e77b2285db303818fc487ae36ba3f5..4f92d7f841c507a5629bc45bc9c4c0d085e8e4a6 100644 (file)
--- a/test/lisp/minibuffer-tests.el
+++ b/test/lisp/minibuffer-tests.el
@@ -298,6 +298,19 @@
                   "jab" '("dabjabstabby" "many") nil 3)))
            6)))
 
+(ert-deftest completion-substring-test-5 ()
+  ;; merge-completions needs to work correctly when
+  (should (equal
+           (completion-pcm--merge-completions '("ab" "sab") '(prefix "b"))
+           '("b" "a" prefix)))
+  (should (equal
+           (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
+           '("b" "a")))
+  ;; substring completion should successfully complete the entire string
+  (should (equal
+           (completion-substring-try-completion "b" '("ab" "ab") nil 0)
+           '("ab" . 2))))
+
 (ert-deftest completion-flex-test-1 ()
   ;; Fuzzy match
   (should (equal