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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 | 1x 2x 2x 2x 18x 18x 12x 18x 18x 18x 12x 12x 2x 2x 2x 2x 2x 2x 2x | import {CursorDataView} from '../../../../CursorDataView';
export function readObjectIdentifier(cdv: CursorDataView, length: number) {
// encoding of an ordered list of subidentifiers, concatenated together (8.19.2)
// Each subidentifier is represented as a series of (one or more) octets.
// Bit 8 of each octet indicates whether it is the last in the series:
// - bit 8 of the last octet is zero; bit 8 of each preceding octet is one.
// - Bits 7 to 1 of the octets in the series collectively encode the subidentifier.
const array = [] as number[];
let subIdentifierValue = 0;
for (let i = 0; i < length; i++) {
const v = cdv.read();
if (subIdentifierValue === 0) {
// leading octet.
Iif (v === 0x80) {
throw new Error(
'The subidentifier shall be encoded in the fewest possible octets. (8.19.2)'
);
}
}
const value = v & 0b0111_1111;
// Conceptually, these groups of bits are concatenated to form an
// unsigned binary number whose most significant bit is bit 7 of
// the first octet and whose least significant bit is bit 1
// of the last octet.
subIdentifierValue = subIdentifierValue * 128 + value;
if ((v & 0b1000_0000) === 0) {
// last octet
array.push(subIdentifierValue);
subIdentifierValue = 0;
}
}
// (8.19.3) The number of subidentifiers (N) shall be one less than
// the number of object identifier components in the
// object identifier value being encoded.
// (8.19.4) The numerical value of the first subidentifier is derived
// from the values of the first two object identifiercomponents
// in the object identifier value being encoded, using the formula:
// (X*40) + Y
// where X is the value of the first object identifier component and Y is the value of the second object identifier
// component.
Iif (array.length === 0) {
throw new Error('Object identifier must have at least one subidentifier');
}
const firstSubIdentifier = array.shift() as number;
const y = firstSubIdentifier % 40;
const x = (firstSubIdentifier - y) / 40;
array.unshift(y);
array.unshift(x);
return array.join('.');
}
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