Use "kong" as tag keys fixes #9 (#10)

tag.go

@@ -0,0 +1,136 @@
+package kong
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"unicode/utf8"
+)
+
+type Tag struct {
+	Cmd         bool
+	Arg         bool
+	Required    bool
+	Optional    bool
+	Help        string
+	Type        string
+	Default     string
+	Format      string
+	Placeholder string
+	Env         string
+	Short       rune
+}
+
+func parseCSV(s string) ([]string, error) {
+	num := 0
+	parts := []string{}
+	current := []rune{}
+
+	add := func() {
+		parts = append(parts, string(current))
+		current = []rune{}
+		num++
+	}
+
+	quotes := false
+
+	runes := []rune(s)
+	for idx := 0; idx < len(runes); idx++ {
+		r := runes[idx]
+		next := rune(0)
+		eof := false
+		if idx < len(runes)-1 {
+			next = runes[idx+1]
+		} else {
+			eof = true
+		}
+		if !quotes && r == ',' {
+			add()
+			continue
+		}
+		if r == '\\' {
+			if next == '\'' {
+				idx++
+				r = '\''
+			}
+		} else if r == '\'' {
+			if quotes {
+				quotes = false
+				if next == ',' || eof {
+					continue
+				}
+				return parts, fmt.Errorf("%v has an unexpected char at pos %v", s, idx)
+			} else {
+				quotes = true
+				continue
+			}
+		}
+		current = append(current, r)
+	}
+	if quotes {
+		return parts, fmt.Errorf("%v is not quoted properly", s)
+	}
+
+	add()
+
+	return parts, nil
+}
+
+func parseTag(fv reflect.Value, s string) (*Tag, error) {
+	t := &Tag{}
+	if s == "" {
+		return t, nil
+	}
+
+	parts, err := parseCSV(s)
+	if err != nil {
+		return t, err
+	}
+
+	for _, part := range parts {
+		is := func(m string) bool { return part == m }
+		value := func(m string) (string, bool) {
+			split := strings.SplitN(part, "=", 2)
+			if split[0] != m {
+				return "", false
+			}
+			if len(split) == 1 {
+				return "", true
+			}
+			return split[1], true
+		}
+
+		if is("cmd") {
+			t.Cmd = true
+		} else if is("arg") {
+			t.Arg = true
+		} else if is("required") {
+			t.Required = true
+		} else if is("optional") {
+			t.Optional = true
+		} else if v, ok := value("default"); ok {
+			t.Default = v
+		} else if v, ok := value("help"); ok {
+			t.Help = v
+		} else if v, ok := value("type"); ok {
+			t.Type = v
+		} else if v, ok := value("placeholder"); ok {
+			t.Placeholder = v
+		} else if v, ok := value("env"); ok {
+			t.Env = v
+		} else if v, ok := value("rune"); ok {
+			t.Short, _ = utf8.DecodeRuneInString(v)
+			if t.Short == utf8.RuneError {
+				t.Short = 0
+			}
+		} else {
+			return t, fmt.Errorf("%v is an unknown kong key", part)
+		}
+	}
+
+	if t.Placeholder == "" {
+		t.Placeholder = strings.ToUpper(dashedString(fv.Type().Name()))
+	}
+
+	return t, nil
+}