commit 408290f7c2a968a0de255813e125a9ebb0a9dda6
Author: Alex Flint <alex.flint@gmail.com>
Date:   Sat Oct 31 16:15:24 2015 -0700

    basic first version working

diff --git a/parse.go b/parse.go
new file mode 100644
index 0000000..b58b51e
--- /dev/null
+++ b/parse.go
@@ -0,0 +1,268 @@
+package arguments
+
+import (
+	"fmt"
+	"os"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+// MustParse processes command line arguments and exits upon failure.
+func MustParse(dest interface{}) {
+	err := Parse(dest)
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+}
+
+// Parse processes command line arguments and stores the result in args.
+func Parse(dest interface{}) error {
+	return ParseFrom(dest, os.Args)
+}
+
+// ParseFrom processes command line arguments and stores the result in args.
+func ParseFrom(dest interface{}, args []string) error {
+	v := reflect.ValueOf(dest)
+	if v.Kind() != reflect.Ptr {
+		panic(fmt.Sprintf("%s is not a pointer type", v.Type().Name()))
+	}
+	v = v.Elem()
+
+	// Parse the spec
+	spec, err := extractSpec(v.Type())
+	if err != nil {
+		return err
+	}
+
+	// Process args
+	err = processArgs(v, spec, args)
+	if err != nil {
+		return err
+	}
+
+	// Validate
+	return validate(spec)
+}
+
+// spec represents information about an argument extracted from struct tags
+type spec struct {
+	field      reflect.StructField
+	index      int
+	long       string
+	short      string
+	multiple   bool
+	required   bool
+	positional bool
+	help       string
+	wasPresent bool
+}
+
+// extractSpec gets specifications for each argument from the tags in a struct
+func extractSpec(t reflect.Type) ([]*spec, error) {
+	if t.Kind() != reflect.Struct {
+		panic(fmt.Sprintf("%s is not a struct pointer", t.Name()))
+	}
+
+	var specs []*spec
+	for i := 0; i < t.NumField(); i++ {
+		// Check for the ignore switch in the tag
+		field := t.Field(i)
+		tag := field.Tag.Get("arg")
+		if tag == "-" {
+			continue
+		}
+
+		spec := spec{
+			long:  strings.ToLower(field.Name),
+			field: field,
+			index: i,
+		}
+
+		// Get the scalar type for this field
+		scalarType := field.Type
+		if scalarType.Kind() == reflect.Slice {
+			spec.multiple = true
+			scalarType = scalarType.Elem()
+			if scalarType.Kind() == reflect.Ptr {
+				scalarType = scalarType.Elem()
+			}
+		}
+
+		// Check for unsupported types
+		switch scalarType.Kind() {
+		case reflect.Array, reflect.Chan, reflect.Func, reflect.Interface,
+			reflect.Map, reflect.Ptr, reflect.Struct,
+			reflect.Complex64, reflect.Complex128:
+			return nil, fmt.Errorf("%s.%s: %s fields are not supported", t.Name(), field.Name, scalarType.Kind())
+		}
+
+		// Look at the tag
+		if tag != "" {
+			for _, key := range strings.Split(tag, ",") {
+				var value string
+				if pos := strings.Index(key, ":"); pos != -1 {
+					value = key[pos+1:]
+					key = key[:pos]
+				}
+
+				switch {
+				case strings.HasPrefix(key, "--"):
+					spec.long = key[2:]
+				case strings.HasPrefix(key, "-"):
+					if len(key) != 2 {
+						return nil, fmt.Errorf("%s.%s: short arguments must be one character only", t.Name(), field.Name)
+					}
+					spec.short = key[1:]
+				case key == "required":
+					spec.required = true
+				case key == "positional":
+					spec.positional = true
+				case key == "help":
+					spec.help = value
+				default:
+					return nil, fmt.Errorf("unrecognized tag '%s' on field %s", key, tag)
+				}
+			}
+		}
+		specs = append(specs, &spec)
+	}
+	return specs, nil
+}
+
+// processArgs processes arguments using a pre-constructed spec
+func processArgs(dest reflect.Value, specs []*spec, args []string) error {
+	// construct a map from arg name to spec
+	specByName := make(map[string]*spec)
+	for _, spec := range specs {
+		if spec.long != "" {
+			specByName[spec.long] = spec
+		}
+		if spec.short != "" {
+			specByName[spec.short] = spec
+		}
+	}
+
+	// process each string from the command line
+	var allpositional bool
+	var positionals []string
+
+	// must use explicit for loop, not range, because we manipulate i inside the loop
+	for i := 0; i < len(args); i++ {
+		arg := args[i]
+		if arg == "--" {
+			allpositional = true
+			continue
+		}
+
+		if !strings.HasPrefix(arg, "-") || allpositional {
+			positionals = append(positionals, arg)
+			continue
+		}
+
+		// check for an equals sign, as in "--foo=bar"
+		var value string
+		opt := strings.TrimLeft(arg, "-")
+		if pos := strings.Index(opt, "="); pos != -1 {
+			value = opt[pos+1:]
+			opt = opt[:pos]
+		}
+
+		// lookup the spec for this option
+		spec, ok := specByName[opt]
+		if !ok {
+			return fmt.Errorf("unknown argument %s", arg)
+		}
+		spec.wasPresent = true
+
+		// deal with the case of multiple values
+		if spec.multiple {
+			var values []string
+			if value == "" {
+				for i++; i < len(args) && !strings.HasPrefix(args[i], "-"); i++ {
+					values = append(values, args[i])
+				}
+			} else {
+				values = append(values, value)
+			}
+			setSlice(dest, spec, values)
+			continue
+		}
+
+		// if it's a flag and it has no value then set the value to true
+		if spec.field.Type.Kind() == reflect.Bool && value == "" {
+			value = "true"
+		}
+
+		// if we have something like "--foo" then the value is the next argument
+		if value == "" {
+			if i+1 == len(args) || strings.HasPrefix(args[i+1], "-") {
+				return fmt.Errorf("missing value for %s", arg)
+			}
+			value = args[i+1]
+			i++
+		}
+
+		err := setScalar(dest.Field(spec.index), value)
+		if err != nil {
+			return fmt.Errorf("error processing %s: %v", arg, err)
+		}
+	}
+	return nil
+}
+
+// validate an argument spec after arguments have been parse
+func validate(spec []*spec) error {
+	for _, arg := range spec {
+		if arg.required && !arg.wasPresent {
+			return fmt.Errorf("--%s is required", strings.ToLower(arg.field.Name))
+		}
+	}
+	return nil
+}
+
+// parse a value as the apropriate type and store it in the struct
+func setSlice(dest reflect.Value, spec *spec, values []string) error {
+	// TODO
+	return nil
+}
+
+// set a value from a string
+func setScalar(v reflect.Value, s string) error {
+	if !v.CanSet() {
+		return fmt.Errorf("field is not writable")
+	}
+
+	switch v.Kind() {
+	case reflect.String:
+		v.Set(reflect.ValueOf(s))
+	case reflect.Bool:
+		x, err := strconv.ParseBool(s)
+		if err != nil {
+			return err
+		}
+		v.Set(reflect.ValueOf(x))
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		x, err := strconv.ParseInt(s, 10, v.Type().Bits())
+		if err != nil {
+			return err
+		}
+		v.Set(reflect.ValueOf(x).Convert(v.Type()))
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		x, err := strconv.ParseUint(s, 10, v.Type().Bits())
+		if err != nil {
+			return err
+		}
+		v.Set(reflect.ValueOf(x).Convert(v.Type()))
+	case reflect.Float32, reflect.Float64:
+		x, err := strconv.ParseFloat(s, v.Type().Bits())
+		if err != nil {
+			return err
+		}
+		v.Set(reflect.ValueOf(x).Convert(v.Type()))
+	default:
+		return fmt.Errorf("not a scalar type: %s", v.Kind())
+	}
+	return nil
+}
diff --git a/parse_test.go b/parse_test.go
new file mode 100644
index 0000000..4864ebc
--- /dev/null
+++ b/parse_test.go
@@ -0,0 +1,88 @@
+package arguments
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func split(s string) []string {
+	return strings.Split(s, " ")
+}
+
+func TestStringSingle(t *testing.T) {
+	var args struct {
+		Foo string
+	}
+	err := ParseFrom(&args, split("--foo bar"))
+	require.NoError(t, err)
+	assert.Equal(t, "bar", args.Foo)
+}
+
+func TestMixed(t *testing.T) {
+	var args struct {
+		Foo  string `arg:"-f"`
+		Bar  int
+		Ham  bool
+		Spam float32
+	}
+	args.Bar = 3
+	err := ParseFrom(&args, split("-spam=1.2 -ham -f xyz"))
+	require.NoError(t, err)
+	assert.Equal(t, "xyz", args.Foo)
+	assert.Equal(t, 3, args.Bar)
+	assert.Equal(t, true, args.Ham)
+	assert.Equal(t, 1.2, args.Spam)
+}
+
+func TestRequired(t *testing.T) {
+	var args struct {
+		Foo string `arg:"required"`
+	}
+	err := ParseFrom(&args, nil)
+	require.Error(t, err, "--foo is required")
+}
+
+func TestShortFlag(t *testing.T) {
+	var args struct {
+		Foo string `arg:"-f"`
+	}
+
+	err := ParseFrom(&args, split("-f xyz"))
+	require.NoError(t, err)
+	assert.Equal(t, "xyz", args.Foo)
+
+	err = ParseFrom(&args, split("-foo xyz"))
+	require.NoError(t, err)
+	assert.Equal(t, "xyz", args.Foo)
+
+	err = ParseFrom(&args, split("--foo xyz"))
+	require.NoError(t, err)
+	assert.Equal(t, "xyz", args.Foo)
+}
+
+func TestCaseSensitive(t *testing.T) {
+	var args struct {
+		Lower bool `arg:"-v"`
+		Upper bool `arg:"-V"`
+	}
+
+	err := ParseFrom(&args, split("-v"))
+	require.NoError(t, err)
+	assert.True(t, args.Lower)
+	assert.False(t, args.Upper)
+}
+
+func TestCaseSensitive2(t *testing.T) {
+	var args struct {
+		Lower bool `arg:"-v"`
+		Upper bool `arg:"-V"`
+	}
+
+	err := ParseFrom(&args, split("-V"))
+	require.NoError(t, err)
+	assert.False(t, args.Lower)
+	assert.True(t, args.Upper)
+}