Lesson 38 · Capstone: HELLO WORLD - Build a QR Code Encoder

Capstone: HELLO WORLD

The finale runs the whole pipeline - encode, error-correct, lay out, mask, and format - and asserts the finished 21x21 grid matches the known-good reference module for module, a symbol a real scanner reads as HELLO WORLD.

The goal

Encode HELLO WORLD end to end and assert the final grid equals the reference symbol.

Start here - the target

TO DO

Scenario: The finished symbol matches the reference

Givena fresh encoder and the input "HELLO WORLD" at error-correction level Q

Whenencode runs the full pipeline: data codewords, Reed-Solomon, layout, mask selection (mask 6), and format information

Thenthe resulting 21x21 grid equals the reference symbol in the background below, module for module (all 441 modules), with (0,0) dark, the top-left finder's first row a solid run of 7 dark modules, and the dark module (13,8) dark

Anda QR scanner decoding the rendered symbol reads back exactly the string "HELLO WORLD"

Background

This is the promise the whole project was built to keep. Encode("HELLO WORLD", "Q") runs every stage you built: the string becomes data codewords (modes, packing, padding), Reed-Solomon over GF(256) adds the recovery codewords, the codewords are laid into the grid along the zigzag around the function patterns, all eight masks are scored and mask 6 wins, and the level-Q, mask-6 format information is written beside the finders. The output is the exact reference symbol below - and a real scanner pointed at it reads back HELLO WORLD.

#######....#..#######
#.....#.##..#.#.....#
#.###.#..#.##.#.###.#
#.###.#.#####.#.###.#
#.###.#.##.#..#.###.#
#.....#..#..#.#.....#
#######.#.#.#.#######
........##.##........
.#.####.##..###.##.#.
#.####.#....####.###.
..#.#.##...#..##.....
#.##.#...#.##...##...
##.########.###.#####
........#...#..#.#...
#######..##..##..####
#.....#.#.#..#..#.###
#.###.#.##.#..#...###
#.###.#.#.###...#.#..
#.###.#..#....#....##
#.....#.###..###..##.
#######..#.#.......#.

Every # and . above is a decision your code now makes for itself: the finders and timing from the layout chapter, the format bits along row and column 8, and the masked data everywhere else. From finite-field addition being XOR all the way to a scannable symbol, you have built the honest core of every QR generator - the field arithmetic, Reed-Solomon error correction, and layout that the larger versions extend with more blocks, bigger grids, and alignment patterns. That is a real QR encoder, and it is yours.

Make it work

grid := Encode("HELLO WORLD", "Q")
// grid == reference (see background), and rendering it to an
// image and scanning it returns "HELLO WORLD".
fmt.Print(grid.Render(4)) // ASCII, 4-module quiet zone

CheckpointDONE

You have built a working QR code encoder. The project is complete - commit and stop here.