start working on flag helpers

toolbox3d/params.go

@@ -0,0 +1,67 @@
+package toolbox3d
+
+import (
+	"flag"
+	"reflect"
+	"strconv"
+	"unicode"
+)
+
+// AddFlags adds the numeric fields of a struct to a
+// FlagSet.
+//
+// If f is nil, the command-line FlagSet is used.
+//
+// Fields should be annotated with `default:...` to
+// indicate the default integer or floating point values.
+//
+// This may panic() if the default for a field is
+// incorrectly formatted.
+func AddFlags(obj any, f *flag.FlagSet) {
+	if f == nil {
+		f = flag.CommandLine
+	}
+	val := reflect.ValueOf(obj)
+	fields := reflect.VisibleFields(val.Type())
+	for _, field := range fields {
+		flagName := flagNameForField(field.Name)
+		defaultStr := field.Tag.Get("default")
+		usageStr := field.Tag.Get("usage")
+		var err error
+		switch field.Type.Kind() {
+		case reflect.Int:
+			var defaultVal int
+			if defaultStr != "" {
+				defaultVal, err = strconv.Atoi(defaultStr)
+				if err != nil {
+					panic(err)
+				}
+			}
+			f.IntVar(val.FieldByIndex(field.Index).Addr().Interface().(*int),
+				flagName, defaultVal, usageStr)
+		case reflect.Float64:
+			var defaultVal float64
+			if defaultStr != "" {
+				defaultVal, err = strconv.ParseFloat(defaultStr, 64)
+				if err != nil {
+					panic(err)
+				}
+			}
+			f.Float64Var(val.FieldByIndex(field.Index).Addr().Interface().(*float64),
+				flagName, defaultVal, usageStr)
+		}
+	}
+}
+
+func flagNameForField(field string) string {
+	var result []rune
+	for _, x := range field {
+		if unicode.IsUpper(x) {
+			if len(result) > 0 {
+				result = append(result, '-')
+			}
+			result = append(result, unicode.ToLower(x))
+		}
+	}
+	return string(result)
+}