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113 lines
2.5 KiB
113 lines
2.5 KiB
package ber
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import (
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"errors"
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"fmt"
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"io"
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)
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func readIdentifier(reader io.Reader) (Identifier, int, error) {
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identifier := Identifier{}
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read := 0
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// identifier byte
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b, err := readByte(reader)
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if err != nil {
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if Debug {
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fmt.Printf("error reading identifier byte: %v\n", err)
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}
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return Identifier{}, read, err
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}
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read++
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identifier.ClassType = Class(b) & ClassBitmask
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identifier.TagType = Type(b) & TypeBitmask
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if tag := Tag(b) & TagBitmask; tag != HighTag {
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// short-form tag
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identifier.Tag = tag
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return identifier, read, nil
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}
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// high-tag-number tag
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tagBytes := 0
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for {
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b, err := readByte(reader)
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if err != nil {
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if Debug {
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fmt.Printf("error reading high-tag-number tag byte %d: %v\n", tagBytes, err)
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}
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return Identifier{}, read, err
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}
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tagBytes++
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read++
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// Lowest 7 bits get appended to the tag value (x.690, 8.1.2.4.2.b)
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identifier.Tag <<= 7
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identifier.Tag |= Tag(b) & HighTagValueBitmask
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// First byte may not be all zeros (x.690, 8.1.2.4.2.c)
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if tagBytes == 1 && identifier.Tag == 0 {
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return Identifier{}, read, errors.New("invalid first high-tag-number tag byte")
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}
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// Overflow of int64
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// TODO: support big int tags?
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if tagBytes > 9 {
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return Identifier{}, read, errors.New("high-tag-number tag overflow")
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}
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// Top bit of 0 means this is the last byte in the high-tag-number tag (x.690, 8.1.2.4.2.a)
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if Tag(b)&HighTagContinueBitmask == 0 {
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break
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}
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}
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return identifier, read, nil
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}
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func encodeIdentifier(identifier Identifier) []byte {
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b := []byte{0x0}
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b[0] |= byte(identifier.ClassType)
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b[0] |= byte(identifier.TagType)
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if identifier.Tag < HighTag {
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// Short-form
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b[0] |= byte(identifier.Tag)
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} else {
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// high-tag-number
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b[0] |= byte(HighTag)
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tag := identifier.Tag
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b = append(b, encodeHighTag(tag)...)
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}
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return b
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}
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func encodeHighTag(tag Tag) []byte {
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// set cap=4 to hopefully avoid additional allocations
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b := make([]byte, 0, 4)
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for tag != 0 {
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// t := last 7 bits of tag (HighTagValueBitmask = 0x7F)
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t := tag & HighTagValueBitmask
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// right shift tag 7 to remove what was just pulled off
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tag >>= 7
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// if b already has entries this entry needs a continuation bit (0x80)
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if len(b) != 0 {
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t |= HighTagContinueBitmask
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}
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b = append(b, byte(t))
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}
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// reverse
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// since bits were pulled off 'tag' small to high the byte slice is in reverse order.
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// example: tag = 0xFF results in {0x7F, 0x01 + 0x80 (continuation bit)}
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// this needs to be reversed into 0x81 0x7F
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for i, j := 0, len(b)-1; i < len(b)/2; i++ {
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b[i], b[j-i] = b[j-i], b[i]
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}
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return b
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}
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