[czj]

Carriers ask for a DataMatrix on parcel labels, and Gamba drew QR codes and linear barcodes but not that. This is the last part of #74, after QR codes in #103 and linear barcodes in #104. Closes #74.

data_matrix encodes the data as a square ECC 200 symbol and draws it as vector rectangles, with one path and one fill and no dependency.

• All square sizes: the smallest of the 24, from 10×10 to 144×144, that holds the data; larger sizes get several data regions and interleaved Reed-Solomon blocks. Data past 1558 codewords raises.
• Compact digits: ASCII encodation puts digits two to a codeword; bytes past 127 go behind an upper shift.
• Same options as QR codes: size (with its one-module quiet zone), module_size, at, position, vposition, quiet_zone and color. There is no level: error correction is fixed by the size.
• One drawing for both: QR codes and DataMatrix share draw_matrix_code and, in Gamba's own API, one box.
• Read back in CI: CI installs dmtx-utils, whose dmtxread reads every test symbol back from the rendered page.

Proof

test/data_matrix_test.rb reads 11 symbols back with dmtxread from pages rendered at 150 dpi: a letter, digits, a URL, UTF-8 bytes, and random ASCII of 10 to 1500 characters on a seed, from 10×10 to 144×144. Sizes are pinned (6 digits fill a 10×10, 3116 fill a 144×144, 3117 raise), along with the one fill and the solid bottom edge's position in the content stream, and a box in Gamba's own API. In a scratch run, all 15 symbols I drew decoded, from one character to the largest size. The QR and barcode tests still pass. rake test: 1369 runs, 0 failures; RuboCop is clean.

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Generated by Claude Code