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// SDLProtocolMessageDisassembler.m
//
#import "SDLProtocolMessageDisassembler.h"
#import "SDLProtocolHeader.h"
#import "SDLProtocolMessage.h"
NS_ASSUME_NONNULL_BEGIN
@implementation SDLProtocolMessageDisassembler
// Use to break up a large message into a sequence of smaller messages,
// each of which is less than 'mtu' number of bytes total size.
+ (NSArray<SDLProtocolMessage *> *)disassemble:(SDLProtocolMessage *)incomingMessage withLimit:(NSUInteger)mtu {
// Questions:
// What message IDs does the current system use? Same messageIDs? Same CorrelationIDs?
// What gets simply copied from incoming header to created headers; and what needs adjustment?
// How big is the message header?
NSUInteger headerSize = incomingMessage.header.size;
// The size of the message is too big to send in one chunk.
// So lets break it up.
// Just how big IS this message?
NSUInteger incomingPayloadSize = (incomingMessage.data.length - headerSize);
// How many messages do we need to create to hold that many bytes?
// Note: this does NOT count the special first message which acts as a descriptor.
NSUInteger numberOfMessagesRequired = ceil((float)incomingPayloadSize / (float)(mtu - headerSize));
// And how many data bytes go in each message?
NSUInteger numberOfDataBytesPerMessage = mtu - headerSize;
// Create the outgoing array to hold the messages we will create.
NSMutableArray<SDLProtocolMessage *> *outgoingMessages = [NSMutableArray arrayWithCapacity:numberOfMessagesRequired + 1];
// Create the first message
SDLProtocolHeader *firstFrameHeader = [incomingMessage.header copy];
firstFrameHeader.frameType = SDLFrameTypeFirst;
UInt32 payloadData[2];
payloadData[0] = CFSwapInt32HostToBig((UInt32)incomingMessage.payload.length);
payloadData[1] = CFSwapInt32HostToBig((UInt32)numberOfMessagesRequired);
NSMutableData *firstFramePayload = [NSMutableData dataWithBytes:payloadData length:sizeof(payloadData)];
firstFrameHeader.bytesInPayload = (UInt32)firstFramePayload.length;
SDLProtocolMessage *firstMessage = [SDLProtocolMessage messageWithHeader:firstFrameHeader andPayload:firstFramePayload];
outgoingMessages[0] = firstMessage;
// Create the middle messages (the ones carrying the actual data).
for (int n = 0; n < numberOfMessagesRequired - 1; n++) {
// Frame # after 255 must cycle back to 1, not 0.
// A 0 signals last frame.
UInt8 frameNumber = (n % 255) + 1;
SDLProtocolHeader *nextFrameHeader = [incomingMessage.header copy];
nextFrameHeader.frameType = SDLFrameTypeConsecutive;
nextFrameHeader.frameData = frameNumber;
NSUInteger offsetOfDataForThisFrame = headerSize + (n * numberOfDataBytesPerMessage);
NSData *nextFramePayload = [incomingMessage.data subdataWithRange:NSMakeRange(offsetOfDataForThisFrame, numberOfDataBytesPerMessage)];
nextFrameHeader.bytesInPayload = (UInt32)nextFramePayload.length;
SDLProtocolMessage *nextMessage = [SDLProtocolMessage messageWithHeader:nextFrameHeader andPayload:nextFramePayload];
outgoingMessages[n + 1] = nextMessage;
}
// Create the last message
SDLProtocolHeader *lastFrameHeader = [incomingMessage.header copy];
lastFrameHeader.frameType = SDLFrameTypeConsecutive;
lastFrameHeader.frameData = SDLFrameInfoConsecutiveLastFrame;
NSUInteger numberOfMessagesCreatedSoFar = numberOfMessagesRequired - 1;
NSUInteger numberOfDataBytesSentSoFar = numberOfMessagesCreatedSoFar * numberOfDataBytesPerMessage;
NSUInteger numberOfDataBytesInLastMessage = incomingPayloadSize - numberOfDataBytesSentSoFar;
NSUInteger offsetOfDataForLastFrame = headerSize + numberOfDataBytesSentSoFar;
NSData *lastFramePayload = [incomingMessage.data subdataWithRange:NSMakeRange(offsetOfDataForLastFrame, numberOfDataBytesInLastMessage)];
lastFrameHeader.bytesInPayload = (UInt32)lastFramePayload.length;
SDLProtocolMessage *lastMessage = [SDLProtocolMessage messageWithHeader:lastFrameHeader andPayload:lastFramePayload];
outgoingMessages[numberOfMessagesRequired] = lastMessage;
return outgoingMessages;
}
@end
NS_ASSUME_NONNULL_END
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