add usage.go

1 files changed, 112 insertions, 0 deletions

diff --git a/usage.go b/usage.go
new file mode 100644
index 0000000..06a3ebd
--- /dev/null
+++ b/usage.go
@@ -0,0 +1,112 @@
+package arg
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+)
+
+// Usage prints usage information to stdout information and exits with status zero
+func Usage(dest ...interface{}) {
+	if err := WriteUsage(os.Stdout, dest...); err != nil {
+		fmt.Println(err)
+	}
+	os.Exit(0)
+}
+
+// Fail prints usage information to stdout and exits with non-zero status
+func Fail(msg string, dest ...interface{}) {
+	fmt.Println(msg)
+	if err := WriteUsage(os.Stdout, dest...); err != nil {
+		fmt.Println(err)
+	}
+	os.Exit(1)
+}
+
+// WriteUsage writes usage information to the given writer
+func WriteUsage(w io.Writer, dest ...interface{}) error {
+	spec, err := extractSpec(dest...)
+	if err != nil {
+		return err
+	}
+	writeUsage(w, spec)
+	return nil
+}
+
+func synopsis(spec *spec, form string) string {
+	if spec.dest.Kind() == reflect.Bool {
+		return form
+	} else {
+		return form + " " + strings.ToUpper(spec.long)
+	}
+}
+
+// writeUsage writes usage information to the given writer
+func writeUsage(w io.Writer, specs []*spec) {
+	var positionals, options []*spec
+	for _, spec := range specs {
+		if spec.positional {
+			positionals = append(positionals, spec)
+		} else {
+			options = append(options, spec)
+		}
+	}
+
+	fmt.Fprint(w, "usage: ")
+
+	// write the option component of the one-line usage message
+	for _, spec := range options {
+		if !spec.required {
+			fmt.Fprint(w, "[")
+		}
+		fmt.Fprint(w, synopsis(spec, "--"+spec.long))
+		if !spec.required {
+			fmt.Fprint(w, "]")
+		}
+		fmt.Fprint(w, " ")
+	}
+
+	// write the positional component of the one-line usage message
+	for _, spec := range positionals {
+		up := strings.ToUpper(spec.long)
+		if spec.multiple {
+			fmt.Fprintf(w, "[%s [%s ...]]", up)
+		} else {
+			fmt.Fprint(w, up)
+		}
+		fmt.Fprint(w, " ")
+	}
+	fmt.Fprint(w, "\n")
+
+	// write the list of positionals
+	if len(positionals) > 0 {
+		fmt.Fprint(w, "\npositional arguments:\n")
+		for _, spec := range positionals {
+			fmt.Fprintf(w, "  %s\n", spec.long)
+		}
+	}
+
+	// write the list of options
+	if len(options) > 0 {
+		fmt.Fprint(w, "\noptions:\n")
+		const colWidth = 25
+		for _, spec := range options {
+			left := fmt.Sprint(synopsis(spec, "--"+spec.long))
+			if spec.short != "" {
+				left += ", " + fmt.Sprint(synopsis(spec, "-"+spec.short))
+			}
+			fmt.Print(left)
+			if spec.help != "" {
+				if len(left)+2 < colWidth {
+					fmt.Fprint(w, strings.Repeat(" ", colWidth-len(left)))
+				} else {
+					fmt.Fprint(w, "\n"+strings.Repeat(" ", colWidth))
+				}
+				fmt.Fprint(w, spec.help)
+			}
+			fmt.Fprint(w, "\n")
+		}
+	}
+}