ed: implement join command (#176)

* This feature was in Todo-list comment for a long time
* "j" by itself is an error, to match GNU ed
* "1j" and "1,1j" address a single line not a range; no change occurs
* "1,3j" is interpreted as "join lines 2 and 3 to the end of line 1 (line 4 becomes the new line 2)
* I tested this by diffing output against GNU ed output, given the same join command and the same input

bin/ed

@@ -56,7 +56,6 @@ License: gpl
 # Todo:
 #        - Implement the following commands from the v7 docs:
 #                g - global command
-#                j - join
 #                k - mark
 #                l - "list" lines, show special chars
 #                m - move
@@ -250,6 +249,8 @@ while (1) {
             if ($EXTENDED_MESSAGES && defined $Error) {
                 print "$Error\n";
             }
+        } elsif ($command eq 'j') {
+            &edJoin;
         } elsif ($command eq 'n') {
             edPrint(1);
         }
@@ -289,6 +290,31 @@ sub edPrint {
     $CurrentLineNum = $adrs[1];
 }
 
+# merge lines back into $lines[$adrs[0]]
+sub edJoin {
+    if (!defined($adrs[0])) { # no currentLine inference
+        edWarn('invalid address');
+        return;
+    }
+    if (!defined($adrs[1])) { # expect range
+        return;
+    }
+    if ($adrs[0] == $adrs[1]) { # nop
+        return;
+    }
+
+    my $buf = $lines[$adrs[0]];
+    my $start = $adrs[0] + 1;
+    for my $i ($start .. $adrs[1]) {
+        chomp $buf;
+        $buf .= $lines[$i];
+    }
+    $lines[$adrs[0]] = $buf;
+    splice @lines, $start, $adrs[1] - $adrs[0];
+    $NeedToSave = 1;
+    $UserHasBeenWarned = 0;
+}
+
 #
 # Perform text substitution
 #
@@ -776,7 +802,7 @@ sub edParse {
                   (([+-])?(\d+))?         # [+]num | -num
                   (([\+]+)|([-^]+))?        # + | -
                 )?
-                 ([acdeEfhHinpPqQrswW=])?        # command char
+                 ([acdeEfhHijnpPqQrswW=])?        # command char
                  \s*(\S+)?                # argument (filename, etc.)
                  )$/x);
 
@@ -1080,6 +1106,10 @@ Display last error
 
 Insert text
 
+=item j
+
+Join a range of lines into a single line
+
 =item n
 
 Print from buffer with line number prefix