QR Code Generator — Swift source

// qr-code-generator — Swift 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.

/// Error-correction level indicator bits (ISO/IEC 18004 table C.1).
let ecFormatBits: [Character: UInt16] = ["L": 0b01, "M": 0b00, "Q": 0b11, "H": 0b10]

/// 15-bit format info string (MSB first) for (ecLevel, mask); nil on unknown level.
func formatInfo(ecLevel: Character, mask: UInt16) -> String? {
    ecFormatBits[ecLevel].map { formatInfoBch(data5: ($0 << 3) | mask) }
}

/// 15-bit format info string (MSB first) for a 5-bit data value.
func formatInfoBch(data5: UInt16) -> String {
    var v = data5 << 10
    let g: UInt16 = 0x537 // BCH(15,5) generator polynomial
    for i in stride(from: 14, through: 10, by: -1) {
        if v & (UInt16(1) << i) != 0 {
            v ^= g << (i - 10) // polynomial long division
        }
    }
    let full = ((data5 << 10) | (v & 0x3FF)) ^ 0x5412 // apply the mask pattern
    let bits = String(full, radix: 2)
    return String(repeating: "0", count: 15 - bits.count) + bits
}

// Example:
//   formatInfo(ecLevel: "L", mask: 0) -> "111011111000100" (0x77C4, matching the published tables)