Add word-wrap support, with wrap length provided by the user

We could try to automatically detect the terminal width and wrap at that
point, but this would increase the binary footprint for all users even
if not using this feature.

Instead, we can allow users to specify their preferred line length limit
(if any), and those who want to bear the cost of checking the terminal
size can do so if they wish. This also makes the feature more testable.

Original patch by Sascha Grunert <sgrunert@suse.com>

help.go

@@ -64,6 +64,11 @@ var HelpPrinter helpPrinter = printHelp
 // HelpPrinterCustom is a function that writes the help output. It is used as
 // the default implementation of HelpPrinter, and may be called directly if
 // the ExtraInfo field is set on an App.
+//
+// In the default implementation, if the customFuncs argument contains a
+// "wrapAt" key, which is a function which takes no arguments and returns
+// an int, this int value will be used to produce a "wrap" function used
+// by the default template to wrap long lines.
 var HelpPrinterCustom helpPrinterCustom = printHelpCustom
 
 // VersionPrinter prints the version for the App
@@ -286,12 +291,29 @@ func ShowCommandCompletions(ctx *Context, command string) {
 // The customFuncs map will be combined with a default template.FuncMap to
 // allow using arbitrary functions in template rendering.
 func printHelpCustom(out io.Writer, templ string, data interface{}, customFuncs map[string]interface{}) {
+
+	const maxLineLength = 10000
+
 	funcMap := template.FuncMap{
 		"join":    strings.Join,
 		"indent":  indent,
 		"nindent": nindent,
 		"trim":    strings.TrimSpace,
+		"wrap":    func(input string, offset int) string { return wrap(input, offset, maxLineLength) },
+		"offset":  offset,
 	}
+
+	if customFuncs["wrapAt"] != nil {
+		if wa, ok := customFuncs["wrapAt"]; ok {
+			if waf, ok := wa.(func() int); ok {
+				wrapAt := waf()
+				customFuncs["wrap"] = func(input string, offset int) string {
+					return wrap(input, offset, wrapAt)
+				}
+			}
+		}
+	}
+
 	for key, value := range customFuncs {
 		funcMap[key] = value
 	}
@@ -402,3 +424,55 @@ func indent(spaces int, v string) string {
 func nindent(spaces int, v string) string {
 	return "\n" + indent(spaces, v)
 }
+
+func wrap(input string, offset int, wrapAt int) string {
+	var sb strings.Builder
+
+	lines := strings.Split(input, "\n")
+
+	padding := strings.Repeat(" ", offset)
+
+	for i, line := range lines {
+		if i != 0 {
+			sb.WriteString(padding)
+		}
+
+		sb.WriteString(wrapLine(line, offset, wrapAt, padding))
+
+		if i != len(lines)-1 {
+			sb.WriteString("\n")
+		}
+	}
+
+	return sb.String()
+}
+
+func wrapLine(input string, offset int, wrapAt int, padding string) string {
+	if wrapAt <= offset || len(input) <= wrapAt-offset {
+		return input
+	}
+
+	lineWidth := wrapAt - offset
+	words := strings.Fields(input)
+	if len(words) == 0 {
+		return input
+	}
+
+	wrapped := words[0]
+	spaceLeft := lineWidth - len(wrapped)
+	for _, word := range words[1:] {
+		if len(word)+1 > spaceLeft {
+			wrapped += "\n" + padding + word
+			spaceLeft = lineWidth - len(word)
+		} else {
+			wrapped += " " + word
+			spaceLeft -= 1 + len(word)
+		}
+	}
+
+	return wrapped
+}
+
+func offset(input string, fixed int) int {
+	return len(input) + fixed
+}