QR Code Generator — Java source

// qr-code-generator — Java port: QR format information (BCH).
//
// Display snippet: assembling the module matrix is too long to show here;
// this ports formatInfoString(ecLevel, mask) from the TypeScript lib
// (src/lib/qr.ts), split into the EC-bits lookup and the BCH remainder.
// The full encoder (modes, Reed-Solomon EC, masking, matrix) lives in the
// TypeScript/Go sources.
//
// The 15-bit format string protects 5 data bits — 2 error-correction level
// bits (L=01 M=00 Q=11 H=10) followed by the 3-bit mask pattern — with a
// BCH(15,5) code using generator 0x537, then XORs the result with the
// fixed 0x5412 pattern so an all-zero string is never valid.

/** QR format information (BCH), ported from the canonical TS lib. */
public final class QrFormatInfo {

    /** Error-correction level indicator bits (ISO/IEC 18004 table C.1); null = unknown. */
    private static Integer ecFormatBits(char ecLevel) {
        switch (ecLevel) {
            case 'L': return 0b01;
            case 'M': return 0b00;
            case 'Q': return 0b11;
            case 'H': return 0b10;
            default:  return null;
        }
    }

    /** 15-bit format info string (MSB first) for (ecLevel, mask); null on unknown level. */
    public static String formatInfo(char ecLevel, int mask) {
        Integer bits = ecFormatBits(ecLevel);
        return bits == null ? null : formatInfoBch((bits << 3) | mask);
    }

    /** 15-bit format info string (MSB first) for a 5-bit data value. */
    public static String formatInfoBch(int data5) {
        int v = data5 << 10;
        final int G = 0x537; // BCH(15,5) generator polynomial
        for (int i = 14; i >= 10; i--) {
            if ((v & (1 << i)) != 0) {
                v ^= G << (i - 10); // polynomial long division
            }
        }
        int full = ((data5 << 10) | (v & 0x3FF)) ^ 0x5412; // apply the mask pattern
        return String.format("%15s", Integer.toBinaryString(full)).replace(' ', '0');
    }
}

// Example:
//   QrFormatInfo.formatInfo('L', 0) -> "111011111000100" (0x77C4, matching the published tables)