Lesson 15 · Mode indicator and character count

Mode indicator and character count

Every QR data stream opens with a header saying how it is encoded and how many characters follow. Today you prepend the 4-bit mode indicator and the character count, completing the payload for HELLO WORLD.

The goal

Prefix the alphanumeric mode indicator and a 9-bit character count to the packed data.

Start here - the target

TO DO

Scenario: Header plus packed data forms the payload

Giventhe packed alphanumeric bits for "HELLO WORLD", and the alphanumeric mode indicator 0010 with a 9-bit character count for Version 1

Whenthe mode indicator (0010) and the count (11, as 000001011) are written before the packed data

Thenthe full payload is 74 bits long (4 + 9 + 61)

Andit begins 00100000010110110000101101111000... , the mode nibble 0010 followed by the count 000001011 followed by the first pair

Background

A decoder reading a bitstream needs to know two things before anything else: which mode the data uses and how many characters it contains. So every segment opens with a 4-bit mode indicator - 0010 for alphanumeric, 0001 for numeric, 0100 for byte - followed by a character count whose bit width depends on the mode and the symbol version. For Version 1 alphanumeric, the count is 9 bits.

"HELLO WORLD" has 11 characters, so the count field is 11 in 9 bits: 000001011. Prepend the mode 0010 and this count to the 61 packed bits and you have a complete 74-bit payload beginning 0010 000001011 01100001011 .... That is the entire message content; what remains is not about the text at all but about filling the symbol to its fixed capacity, which the next two lessons handle with a terminator and padding.

Make it work

// Alphanumeric mode = 0010. Version 1 uses a 9-bit count.
w.writeBits(0b0010, 4)          // mode indicator
w.writeBits(len(text), 9)       // character count
encodeAlnum(w, text)            // then the packed pairs

CheckpointDONE

The full data payload for a string is assembled. Commit and stop here.