QR Code Generator — C# source

// qr-code-generator — C# 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.
using System.Collections.Generic;

static class QrFormatInfo
{
    // Error-correction level indicator bits (ISO/IEC 18004 table C.1).
    static readonly Dictionary<char, int> EcFormatBits = new()
    {
        ['L'] = 0b01, ['M'] = 0b00, ['Q'] = 0b11, ['H'] = 0b10,
    };

    /// 15-bit format info string (MSB first) for (ecLevel, mask); null on unknown level.
    public static string? FormatInfo(char ecLevel, int mask)
        => EcFormatBits.TryGetValue(ecLevel, out var bits)
            ? FormatInfoBch((bits << 3) | mask)
            : null;

    /// 15-bit format info string (MSB first) for a 5-bit data value.
    public static string FormatInfoBch(int data5)
    {
        int v = data5 << 10;
        const 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 Convert.ToString(full, 2).PadLeft(15, '0');
    }
}

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