Add placeholder expression for zero-based item index: {n} and {+n}

Close #1482

man/man1/fzf.1

@@ -301,7 +301,9 @@ e.g. \fBfzf --multi --preview='head -10 {+}'\fR
 When using a field index expression, leading and trailing whitespace is stripped
 from the replacement string. To preserve the whitespace, use the \fBs\fR flag.
 
-Also, \fB{q}\fR is replaced to the current query string.
+Also, \fB{q}\fR is replaced to the current query string, and \fB{n}\fR is
+replaced to zero-based ordinal index of the line. Use \fB{+n}\fR if you want
+all index numbers when multiple lines are selected.
 
 Note that you can escape a placeholder pattern by prepending a backslash.
 

src/terminal.go

@@ -24,7 +24,7 @@ import (
 var placeholder *regexp.Regexp
 
 func init() {
-	placeholder = regexp.MustCompile("\\\\?(?:{[+s]*[0-9,-.]*}|{q})")
+	placeholder = regexp.MustCompile("\\\\?(?:{[+s]*[0-9,-.]*}|{q}|{\\+?n})")
 }
 
 type jumpMode int
@@ -227,6 +227,7 @@ const (
 type placeholderFlags struct {
 	plus          bool
 	preserveSpace bool
+	number        bool
 	query         bool
 }
 
@@ -1198,6 +1199,9 @@ func parsePlaceholder(match string) (bool, string, placeholderFlags) {
 		case 's':
 			flags.preserveSpace = true
 			skipChars++
+		case 'n':
+			flags.number = true
+			skipChars++
 		case 'q':
 			flags.query = true
 		default:
@@ -1253,7 +1257,16 @@ func replacePlaceholder(template string, stripAnsi bool, delimiter Delimiter, fo
 
 		if match == "{}" {
 			for idx, item := range items {
-				replacements[idx] = quoteEntry(item.AsString(stripAnsi))
+				if flags.number {
+					n := int(item.text.Index)
+					if n < 0 {
+						replacements[idx] = ""
+					} else {
+						replacements[idx] = strconv.Itoa(n)
+					}
+				} else {
+					replacements[idx] = quoteEntry(item.AsString(stripAnsi))
+				}
 			}
 			return strings.Join(replacements, " ")
 		}
@@ -1351,7 +1364,7 @@ func (t *Terminal) buildPlusList(template string, forcePlus bool) (bool, []*Item
 	//   2. or it contains {+} and we have more than one item already selected.
 	// To do so, we pass an empty Item instead of nil to trigger an update.
 	if current == nil {
-		current = &Item{}
+		current = &minItem
 	}
 
 	var sels []*Item

test/test_go.rb

@@ -1400,21 +1400,21 @@ class TestGoFZF < TestBase
 
   def test_preview_flags
     tmux.send_keys %(seq 10 | sed 's/^/:: /; s/$/  /' |
-        #{FZF} --multi --preview 'echo {{2}/{s2}/{+2}/{+s2}/{q}}'), :Enter
+        #{FZF} --multi --preview 'echo {{2}/{s2}/{+2}/{+s2}/{q}/{n}/{+n}}'), :Enter
-    tmux.until { |lines| lines[1].include?('{1/1  /1/1  /}') }
+    tmux.until { |lines| lines[1].include?('{1/1  /1/1  //0/0}') }
     tmux.send_keys '123'
-    tmux.until { |lines| lines[1].include?('{////123}') }
+    tmux.until { |lines| lines[1].include?('{////123//}') }
     tmux.send_keys 'C-u', '1'
     tmux.until { |lines| lines.match_count == 2 }
-    tmux.until { |lines| lines[1].include?('{1/1  /1/1  /1}') }
+    tmux.until { |lines| lines[1].include?('{1/1  /1/1  /1/0/0}') }
     tmux.send_keys :BTab
-    tmux.until { |lines| lines[1].include?('{10/10  /1/1  /1}') }
+    tmux.until { |lines| lines[1].include?('{10/10  /1/1  /1/9/0}') }
     tmux.send_keys :BTab
-    tmux.until { |lines| lines[1].include?('{10/10  /1 10/1   10  /1}') }
+    tmux.until { |lines| lines[1].include?('{10/10  /1 10/1   10  /1/9/0 9}') }
     tmux.send_keys '2'
-    tmux.until { |lines| lines[1].include?('{//1 10/1   10  /12}') }
+    tmux.until { |lines| lines[1].include?('{//1 10/1   10  /12//0 9}') }
     tmux.send_keys '3'
-    tmux.until { |lines| lines[1].include?('{//1 10/1   10  /123}') }
+    tmux.until { |lines| lines[1].include?('{//1 10/1   10  /123//0 9}') }
   end
 
   def test_preview_q_no_match