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>

app.go

@@ -74,6 +74,8 @@ type App struct {
 	Copyright string
 	// Writer writer to write output to
 	Writer io.Writer
+	// If greater than zero, wrap help text lines at this length (experimental).
+	WrapAt int
 	// ErrWriter writes error output
 	ErrWriter io.Writer
 	// ExitErrHandler processes any error encountered while running an App before

app_test.go

@@ -1197,7 +1197,7 @@ func TestAppHelpPrinter(t *testing.T) {
 	}()
 
 	var wasCalled = false
-	HelpPrinter = func(w io.Writer, template string, data interface{}) {
+	HelpPrinter = func(w io.Writer, template string, data interface{}, wrapAt int) {
 		wasCalled = true
 	}
 

help.go

@@ -42,10 +42,10 @@ var helpSubcommand = &Command{
 }
 
 // Prints help for the App or Command
-type helpPrinter func(w io.Writer, templ string, data interface{})
+type helpPrinter func(w io.Writer, templ string, data interface{}, wrapAt int)
 
 // Prints help for the App or Command with custom template function.
-type helpPrinterCustom func(w io.Writer, templ string, data interface{}, customFunc map[string]interface{})
+type helpPrinterCustom func(w io.Writer, templ string, data interface{}, customFunc map[string]interface{}, wrapAt int)
 
 // HelpPrinter is a function that writes the help output. If not set explicitly,
 // this calls HelpPrinterCustom using only the default template functions.
@@ -78,7 +78,7 @@ func ShowAppHelp(c *Context) error {
 	}
 
 	if c.App.ExtraInfo == nil {
-		HelpPrinter(c.App.Writer, template, c.App)
+		HelpPrinter(c.App.Writer, template, c.App, c.App.WrapAt)
 		return nil
 	}
 
@@ -87,7 +87,7 @@ func ShowAppHelp(c *Context) error {
 			"ExtraInfo": c.App.ExtraInfo,
 		}
 	}
-	HelpPrinterCustom(c.App.Writer, template, c.App, customAppData())
+	HelpPrinterCustom(c.App.Writer, template, c.App, customAppData(), c.App.WrapAt)
 
 	return nil
 }
@@ -189,7 +189,7 @@ func ShowCommandHelpAndExit(c *Context, command string, code int) {
 func ShowCommandHelp(ctx *Context, command string) error {
 	// show the subcommand help for a command with subcommands
 	if command == "" {
-		HelpPrinter(ctx.App.Writer, SubcommandHelpTemplate, ctx.App)
+		HelpPrinter(ctx.App.Writer, SubcommandHelpTemplate, ctx.App, ctx.App.WrapAt)
 		return nil
 	}
 
@@ -200,7 +200,7 @@ func ShowCommandHelp(ctx *Context, command string) error {
 				templ = CommandHelpTemplate
 			}
 
-			HelpPrinter(ctx.App.Writer, templ, c)
+			HelpPrinter(ctx.App.Writer, templ, c, ctx.App.WrapAt)
 
 			return nil
 		}
@@ -261,9 +261,22 @@ 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{}) {
+func printHelpCustom(out io.Writer, templ string, data interface{}, customFuncs map[string]interface{}, wrapAt int) {
+
+	wrapFunc := func(input string, offset int) string {
+		return input
+	}
+
+	if wrapAt > 0 {
+		wrapFunc = func(input string, offset int) string {
+			return wrap(input, offset, wrapAt)
+		}
+	}
+
 	funcMap := template.FuncMap{
-		"join": strings.Join,
+		"join":   strings.Join,
+		"wrap":   wrapFunc,
+		"offset": offset,
 	}
 	for key, value := range customFuncs {
 		funcMap[key] = value
@@ -284,8 +297,8 @@ func printHelpCustom(out io.Writer, templ string, data interface{}, customFuncs
 	_ = w.Flush()
 }
 
-func printHelp(out io.Writer, templ string, data interface{}) {
-	HelpPrinterCustom(out, templ, data, nil)
+func printHelp(out io.Writer, templ string, data interface{}, wrapAt int) {
+	HelpPrinterCustom(out, templ, data, nil, wrapAt)
 }
 
 func checkVersion(c *Context) bool {
@@ -366,3 +379,53 @@ func checkCommandCompletions(c *Context, name string) bool {
 	ShowCommandCompletions(c, name)
 	return true
 }
+
+func wrap(input string, offset int, wrapAt int) string {
+	var sb strings.Builder
+
+	lines := strings.Split(input, "\n")
+
+	for i, line := range lines {
+		if i != 0 {
+			sb.WriteString(strings.Repeat(" ", offset))
+		}
+
+		sb.WriteString(wrapLine(line, offset, wrapAt))
+
+		if i != len(lines)-1 {
+			sb.WriteString("\n")
+		}
+	}
+
+	return sb.String()
+}
+
+func wrapLine(input string, offset int, wrapAt int) string {
+	if wrapAt <= offset || len(input) <= wrapAt-offset {
+		return input
+	}
+
+	lineWidth := wrapAt - offset
+	words := strings.Fields(strings.TrimSpace(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" + strings.Repeat(" ", offset) + 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
+}