Livt.Net

src/ipv4/Ipv4HeaderBuilder.lvt

namespace Livt.Net

/**
 * Pure fixed-header IPv4 encoding from explicit endpoint metadata.
 * Version/IHL=0x45, DSCP=0, DF=1, fragment offset=0 and TTL=64.
 */
class Ipv4HeaderBuilder
{
	/**
	 * Emits bytes 0..19: version/DSCP, total length, identification, flags/offset,
	 * TTL/protocol, checksum, source address and destination address.
	 * Arrays use network order, totalLength is 20..65535 and checksum is 0..65535.
	 * The caller validates bounds; out-of-header indices return zero.
	 */
	public static inline fn GetByte[](
		index: int, totalLength: int, identification: byte[2],
		sourceIp: byte[4], destinationIp: byte[4], protocol: byte, checksum: int) byte
	{
		if (index < 0) {
			return 0x00
		}

		if (index == 0) {
			return 0x45
		}

		if (index == 1) {
			return 0x00
		}

		if (index == 2) {
			return NetworkOrder.HighByte(totalLength)
		}

		if (index == 3) {
			return NetworkOrder.LowByte(totalLength)
		}

		if (index < 6) {
			return identification[index - 4]
		}

		if (index == 6 || index == 8) {
			return 0x40
		}

		if (index == 7) {
			return 0x00
		}

		if (index == 9) {
			return protocol
		}

		if (index == 10) {
			return NetworkOrder.HighByte(checksum)
		}

		if (index == 11) {
			return NetworkOrder.LowByte(checksum)
		}

		if (index < 16) {
			return sourceIp[index - 12]
		}

		if (index < 20) {
			return destinationIp[index - 16]
		}

		return 0x00
	}
}