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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package asn1

import (
        "reflect"
        "strconv"
        "strings"
)

// ASN.1 objects have metadata preceding them:
//   the tag: the type of the object
//   a flag denoting if this object is compound or not
//   the class type: the namespace of the tag
//   the length of the object, in bytes

// Here are some standard tags and classes

const (
        tagBoolean         = 1
        tagInteger         = 2
        tagBitString       = 3
        tagOctetString     = 4
        tagOID             = 6
        tagEnum            = 10
        tagUTF8String      = 12
        tagSequence        = 16
        tagSet             = 17
        tagPrintableString = 19
        tagT61String       = 20
        tagIA5String       = 22
        tagUTCTime         = 23
        tagGeneralizedTime = 24
        tagGeneralString   = 27
)

const (
        classUniversal       = 0
        classApplication     = 1
        classContextSpecific = 2
        classPrivate         = 3
)

type tagAndLength struct {
        class, tag, length int
        isCompound         bool
}

// ASN.1 has IMPLICIT and EXPLICIT tags, which can be translated as "instead
// of" and "in addition to". When not specified, every primitive type has a
// default tag in the UNIVERSAL class.
//
// For example: a BIT STRING is tagged [UNIVERSAL 3] by default (although ASN.1
// doesn't actually have a UNIVERSAL keyword). However, by saying [IMPLICIT
// CONTEXT-SPECIFIC 42], that means that the tag is replaced by another.
//
// On the other hand, if it said [EXPLICIT CONTEXT-SPECIFIC 10], then an
// /additional/ tag would wrap the default tag. This explicit tag will have the
// compound flag set.
//
// (This is used in order to remove ambiguity with optional elements.)
//
// You can layer EXPLICIT and IMPLICIT tags to an arbitrary depth, however we
// don't support that here. We support a single layer of EXPLICIT or IMPLICIT
// tagging with tag strings on the fields of a structure.

// fieldParameters is the parsed representation of tag string from a structure field.
type fieldParameters struct {
        optional     bool   // true iff the field is OPTIONAL
        explicit     bool   // true iff an EXPLICIT tag is in use.
        application  bool   // true iff an APPLICATION tag is in use.
        defaultValue *int64 // a default value for INTEGER typed fields (maybe nil).
        tag          *int   // the EXPLICIT or IMPLICIT tag (maybe nil).
        stringType   int    // the string tag to use when marshaling.
        set          bool   // true iff this should be encoded as a SET

        // Invariants:
        //   if explicit is set, tag is non-nil.
}

// Given a tag string with the format specified in the package comment,
// parseFieldParameters will parse it into a fieldParameters structure,
// ignoring unknown parts of the string.
func parseFieldParameters(str string) (ret fieldParameters) {
        for _, part := range strings.Split(str, ",") {
                switch {
                case part == "optional":
                        ret.optional = true
                case part == "explicit":
                        ret.explicit = true
                        if ret.tag == nil {
                                ret.tag = new(int)
                        }
                case part == "ia5":
                        ret.stringType = tagIA5String
                case part == "printable":
                        ret.stringType = tagPrintableString
                case strings.HasPrefix(part, "default:"):
                        i, err := strconv.ParseInt(part[8:], 10, 64)
                        if err == nil {
                                ret.defaultValue = new(int64)
                                *ret.defaultValue = i
                        }
                case strings.HasPrefix(part, "tag:"):
                        i, err := strconv.Atoi(part[4:])
                        if err == nil {
                                ret.tag = new(int)
                                *ret.tag = i
                        }
                case part == "set":
                        ret.set = true
                case part == "application":
                        ret.application = true
                        if ret.tag == nil {
                                ret.tag = new(int)
                        }
                }
        }
        return
}

// Given a reflected Go type, getUniversalType returns the default tag number
// and expected compound flag.
func getUniversalType(t reflect.Type) (tagNumber int, isCompound, ok bool) {
        switch t {
        case objectIdentifierType:
                return tagOID, false, true
        case bitStringType:
                return tagBitString, false, true
        case timeType:
                return tagUTCTime, false, true
        case enumeratedType:
                return tagEnum, false, true
        case bigIntType:
                return tagInteger, false, true
        }
        switch t.Kind() {
        case reflect.Bool:
                return tagBoolean, false, true
        case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
                return tagInteger, false, true
        case reflect.Struct:
                return tagSequence, true, true
        case reflect.Slice:
                if t.Elem().Kind() == reflect.Uint8 {
                        return tagOctetString, false, true
                }
                if strings.HasSuffix(t.Name(), "SET") {
                        return tagSet, true, true
                }
                return tagSequence, true, true
        case reflect.String:
                return tagPrintableString, false, true
        }
        return 0, false, false
}