QR Code Generator — Kotlin source

// qr-code-generator — Kotlin 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); null = unknown. */
fun ecFormatBits(ecLevel: Char): Int? = when (ecLevel) {
    'L' -> 0b01
    'M' -> 0b00
    'Q' -> 0b11
    'H' -> 0b10
    else -> null
}

/** 15-bit format info string (MSB first) for (ecLevel, mask); null on unknown level. */
fun formatInfo(ecLevel: Char, mask: Int): String? =
    ecFormatBits(ecLevel)?.let { formatInfoBch((it shl 3) or mask) }

/** 15-bit format info string (MSB first) for a 5-bit data value. */
fun formatInfoBch(data5: Int): String {
    var v = data5 shl 10
    val g = 0x537 // BCH(15,5) generator polynomial
    for (i in 14 downTo 10) {
        if (v and (1 shl i) != 0) v = v xor (g shl (i - 10)) // polynomial long division
    }
    val full = ((data5 shl 10) or (v and 0x3FF)) xor 0x5412 // apply the mask pattern
    return full.toString(2).padStart(15, '0')
}

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