Thread: Encoding data in 7 bit words, need some ideas.

I need to repackage data to be sent in 7 bit words with the MSB padded. I have tried different approaches but the last byte comes out wrong. The 16 bit value 0x87E5 would be encoded into three bytes like this:

original.

10000111 11100101

encoded.

01000011 01111001 00100000

I have been trying to do it like this but the third byte comes out wrong. This is what I have tried so far, there might be better / more clever way of achiving the same thing though. Interested to hear your input on this.

// 1.) ........10000111 11100101 00000000
// 2.) ...>> 01000011 11110010 10000000
// 3.) .................>> 01111001 01000000
// 4.) ...............................>> 00100000

Code:

	unsigned char byte[4] = {0};
	int large = 0x87E5;  		          // 10000111 11100101 00000000 00000000
	large >>= 1;				  // 01000011 11110010 10000000 00000000
	printf("%x\n", large);
	large = htons(large);		 
	memcpy(&byte, &large, 3);	  
	binary(byte[0]);			  // 01000011 >OK
	
	byte[1] >>= 1;
	binary(byte[1]);			  // 01111001 >OK
	

	large >>= 1;
	memcpy(&byte[2], &large, 3);
	byte[4] >>= 1;
	binary(byte[4]);			  // 01111111 >wrong! should be 00100000?

BTW, the call to the binary() function prints in binary to stdout, thanks Dino for the hint on how to do this.