bistromathic: properly compute the lcp, as expected by readline

Currently completion on "at" proposes only "atan", but does not
actually complete "at" into "atan".

* examples/c/bistromathic/parse.y (completion): Install the lcp in
matches[0].
* examples/c/bistromathic/bistromathic.test: Check that case.

2 files changed, 23 insertions, 2 deletions

diff --git a/examples/c/bistromathic/bistromathic.test b/examples/c/bistromathic/bistromathic.test
index 0bec1cb3..a877bea3 100755
--- a/examples/c/bistromathic/bistromathic.test
+++ b/examples/c/bistromathic/bistromathic.test
@@ -85,6 +85,7 @@ run 0 '> 1 / 0
 ## Completion.  ##
 ## ------------ ##
 
+# Check completion after an operator.
 sed -e 's/\\t/	/g' >input <<EOF
 (1+\t\t
 EOF
@@ -94,3 +95,10 @@ run 0 '> (1+
 atan                     exp                      sqrt
 > (1+
 > err: 1.4: syntax error: expected - or ( or double precision number or function or variable before end of file'
+
+# Check the completion of a word.
+sed -e 's/\\t/	/g' >input <<EOF
+(at\t\t
+EOF
+run 0 '> (atan ( ''
+> err: 1.9: syntax error: expected - or ( or double precision number or function or variable before end of file'
diff --git a/examples/c/bistromathic/parse.y b/examples/c/bistromathic/parse.y
index d424cdfe..e1315ee5 100644
--- a/examples/c/bistromathic/parse.y
+++ b/examples/c/bistromathic/parse.y
@@ -384,8 +384,7 @@ completion (const char *text, int start, int end)
   const int len = strlen (text);
   // Need initial prefix and final NULL.
   char **matches = calloc (ntokens + symbol_count () + 2, sizeof *matches);
-  int match = 0;
-  matches[match++] = strdup (text);
+  int match = 1;
   for (int i = 0; i < ntokens; ++i)
     if (tokens[i] == YYTRANSLATE (TOK_VAR))
       {
@@ -406,6 +405,20 @@ completion (const char *text, int start, int end)
           matches[match++] = strdup (token);
       }
 
+  // Find the longest common prefix, and install it in matches[0], as
+  // required by readline.
+  if (match == 1)
+    matches[0] = strdup (text);
+  else
+    {
+      int lcplen = strlen (matches[1]);
+      for (int i = 2; i < match && lcplen; ++i)
+        for (int j = 0; j < lcplen; ++j)
+          if (matches[1][j] != matches[i][j])
+            lcplen = j;
+      matches[0] = strndup (matches[1], lcplen);
+    }
+
   if (yydebug)
     {
       fprintf (stderr, "completion(\"%.*s[%.*s]%s\") = ",