Simula le variabili int con byte o corti

Question

Come sai, non esiste una variabile int su JavaCard 2.2.x. Ho bisogno di variabili intere per l'applet che sto sviluppando.

Desidero sapere come simulare una variabile int e la sua aritmetica utilizzando le variabili byte e short.

Answer 1

variabili sono facoltative in Java Card 2.2, pertanto può essere una variabile int. Tuttavia, molte implementazioni non danno fastidio, probabilmente perché nessuna delle chiamate API gestisce gli interi. Lo stesso vale per la classe javacardx.framework.math.BigNumber. Se sei fortunato è presente, anche se ti aiuterà solo con addizione e moltiplicazione (ma può essere implementato usando le funzioni native, il che renderebbe molto più veloce).

Si noti che se le funzioni sono necessarie con parsimonia, è preferibile utilizzare alcune variabili locali short e utilizzare quelle per eseguire i calcoli in linea. Questo ha il vantaggio di avere prestazioni molto migliori. Poiché non è possibile restituire più di una variabile (e gli argomenti Java sono passati per valore) è difficile restituire due cortometraggi in base al valore.

È possibile prendere alcuni suggerimenti da, ad es. le implementazioni Java BigInteger che utilizzano gli array long per rappresentare numeri interi di dimensioni arbitrarie. Un problema è che i normali operatori restituiscono una nuova istanza ogni volta che vengono chiamati. Questa non è una buona opzione su Java Card, in quanto nuove istanze verranno create nella memoria persistente. Quindi è più facile creare combinato

Answer 2

risposta speciale di Natale.

Completamente testato utilizzando i test di unità, ma ancora solo su Java SE.

Ha bisogno di un po 'di lavoro per istanziare il backing array.

Alcuni codici possono ancora essere ottimizzati inserendo internamente l'operando di sinistra.

Si noti che questo codice utilizza *= - assegnazione della risposta alla prima variabile - piuttosto che * in quanto non è una buona idea per creare istanze di oggetti durante il runtime Java Card (sono creati nella memoria persistente).

Elimina tutti i commenti di JavaDoc, poiché altrimenti non si adatterebbe alle dimensioni massime del post.

/** 
* Free for use by all, please keep this line and the author line intact. 
* 
* @author Maarten Bodewes 
*/ 
public final class JCInteger { 

    private static final short BYTE_SIZE = 8; 
    private static final short SHORT_SIZE = 16; 
    private static final short INTEGER_SIZE = 32; 

    private static final short HIGH = 0; 
    private static final short LOW = 1; 

    private final short[] values; 

    private JCInteger(final byte memoryType) { 

     // TODO this should be backed by an array in RAM, using JCSystem.makeTransientByteArray() 
     // using either JCSystem.CLEAR_ON_RESET or JCSystem.CLEAR_ON_DESELECT 

     values = new short[(short) 2]; 
    } 

    public static JCInteger createInstance(final byte memoryType) { 
     return new JCInteger(memoryType); 
    } 

    public JCInteger assign(final JCInteger rightHandOperand) { 
     values[HIGH] = rightHandOperand.values[HIGH]; 
     values[LOW] = rightHandOperand.values[LOW]; 
     return this; 
    } 

    public JCInteger assign(final short high, final short low) { 
     values[HIGH] = high; 
     values[LOW] = low; 
     return this; 
    } 

    public JCInteger assignSigned(final short signedValue) { 
     if (signedValue >= 0) { 
      values[HIGH] = (short) 0x0000; 
     } else { 
      values[HIGH] = (short) 0xFFFF; 
     } 
     values[LOW] = signedValue; 
     return this; 
    } 

    public JCInteger assignUnsigned(final short unsignedValue) { 
     values[HIGH] = (short) 0x0000; 
     values[LOW] = unsignedValue; 
     return this; 
    } 

    public short getHigh() { 
     // no pun intended 
     return values[HIGH]; 
    } 

    public short getLow() { 
     return values[LOW]; 
    } 

    public short[] getBackingShortArray() { 
     return values; 
    } 

    public JCInteger negate() { 

     // basically invert, then increase, note that -Integer.MIN_VALUE = Integer.MIN_VALUE (as it is in Java) 
     values[HIGH] = (short)~values[HIGH]; 
     values[LOW] = (short)~values[LOW]; 
     increment(); 
     return this; 
    } 

    public JCInteger increment() { 
     values[LOW]++; 
     if (values[LOW] == 0) { 
      values[HIGH]++; 
     } 
     return this; 
    } 

    public JCInteger decrement() { 
     values[LOW]--; 
     if (values[LOW] == -1) { 
      values[HIGH]--; 
     } 
     return this; 
    } 

    public JCInteger add(final JCInteger y) { 
     addUnsignedLow(y.values[LOW]); 
     values[HIGH] += y.values[HIGH]; 
     return this; 
    } 

    public JCInteger subtract(final JCInteger y) { 
     // subtracts by adding the negated i 
     // negation is identical to invert + increase 
     // however the increase is performed to the result of adding the inverted value 

     // invert 
     final short xlInv = (short) ~y.values[LOW]; 
     final short xhInv = (short) ~y.values[HIGH]; 

     // add 
     addUnsignedLow(xlInv); 
     values[HIGH] += xhInv; 

     // increase 
     increment(); 
     return this; 
    } 

    public JCInteger multiply(JCInteger y) { 
     // uses the fact that: 
     // x * y = 
     // (x1 * 2^16 + x0) * (y1 * 2^16 + y0) = 
     // (x1 * y1 * 2^32) + x1 * y0 * 2^16 + x0 * y1 * 2^16 + x0 * y0 = 
     // x1 * y0 * 2^16 + x0 * y1 * 2^16 + x0 * y0 (because anything * 2^32 overflows all the bits) = 
     // x1 * y0 * 2^16 + x0 * y1 * 2^16 + z1 | z0 (where z1 = high 16 bits of x0 * y* and z0 is the low part) = 
     // r1 | r0 where r1 = x1 * y0 + x0 * y1 + z1 and r0 = z0 
     // r1 is only 16 bits so x1 * y0 and x0 * y0 may overflow, as may the additions, hopefully leaving the sign 
     // bit correctly set 

     boolean xPositive = this.isPositive(); 
     if (!xPositive) { 
      this.negate(); 
     } 

     final short xh = this.values[HIGH]; 
     final short xl = this.values[LOW]; 

     short yh = y.values[HIGH]; 
     short yl = y.values[LOW]; 

     // --- if signed then negate y --- 
     final boolean yPositive; 
     if ((yh & 0x8000) == 0) { 
      yPositive = true; 
     } else { 
      // negation (complement then increase) 
      yh = (short) ~yh; 
      yl = (short) ~yl; 
      yl++; 
      if (yl == 0) { 
       yh++; 
      } 
      yPositive = false; 
     } 

     // calculates z1 and z0 and stores it in the current values 
     multiplyUnsigned(xl, yl, values); 

     // perform the calculation for the high parts 
     values[HIGH] += (short) (xh * yl + xl * yh); 

     // make sure we return a correctly signed value 
     if ((xPositive && !yPositive) || (!xPositive && yPositive)) { 
      this.negate(); 
     } 

     return this; 
    } 

    public JCInteger divide(JCInteger y) { 

     // --- pre-calculations on y --- 

     // put y in yh and yl 
     short yh = y.values[HIGH]; 
     short yl = y.values[LOW]; 

     if (yh == 0 && yl == 0) { 
      // division by zero 
      throw new ArithmeticException(); 
     } 

     final boolean yPositive; 
     if ((yh & 0x8000) == 0) { 
      yPositive = true; 
     } else { 
      // negation (complement then increase) 
      yh = (short) ~yh; 
      yl = (short) ~yl; 
      yl++; 
      if (yl == 0) { 
       yh++; 
      } 
      yPositive = false; 
     } 

     final short divisorSize = (short) (INTEGER_SIZE - numberOfLeadingZeros(yh, yl)); 

     // --- pre-calculations on x --- 

     final boolean xPositive = this.isPositive(); 
     if (!xPositive) { 
      this.negate(); 
     } 
     final short dividentSize = (short) (INTEGER_SIZE - numberOfLeadingZeros()); 

     // --- setup the maximum number of shifts --- 

     final short maxShifts = (short) (dividentSize - divisorSize); 

     // --- slightly superfluous check if divisor is higher than dividend --- 

     if (maxShifts < 0) { 
      // return 0, no division can be performed 
      values[HIGH] = 0; 
      values[LOW] = 0; 
      return this; 
     } 

     // --- shift divisor left until the highest bit is aligned with the highest bit of the dividend --- 

     if (maxShifts <= JCInteger.SHORT_SIZE) { 
      yh = (short) (((yl & 0xFFFF) >>> (SHORT_SIZE - maxShifts)) | (yh << maxShifts)); 
      yl <<= maxShifts; 
     } else { 
      yh = (short) (yl << (maxShifts - SHORT_SIZE)); 
      yl = 0; 
     } 

     short rh = 0, rl = 0; 
     for (short i = maxShifts; i >= 0; i--) { 
      final short xho = values[HIGH]; 
      final short xlo = values[LOW]; 

      // --- subtract (add complement and increment does the job) --- 

      // add complement 
      addUnsignedLow((short) ~yl); 
      values[HIGH] += (short) ~yh; 

      // increase to create subtraction 
      increment(); 

      if (isPositive()) { 
       // --- we have subtracted y * 2^n, so include 2^n to the result --- 
       if (i >= SHORT_SIZE) { 
        rh |= 1 << (i - SHORT_SIZE); 
       } else { 
        rl |= 1 << i; 
       } 
      } else { 
       // --- we could not subtract, so restore --- 
       values[HIGH] = xho; 
       values[LOW] = xlo; 
      } 

      // --- shift right by 1 --- 
      // first do low shift as high shift changes value 
      yl = (short) ((yh << (JCInteger.SHORT_SIZE - 1)) | ((yl & 0xFFFF) >>> 1)); 
      yh = (short) ((yh & 0xFFFF) >>> 1); 
     } 

     values[HIGH] = rh; 
     values[LOW] = rl; 

     // make sure we return a correctly signed value (may mess up sign bit on overflows?) 
     if ((xPositive && !yPositive) || (!xPositive && yPositive)) { 
      this.negate(); 
     } 

     return this; 
    } 

    public JCInteger remainder(JCInteger y) { 

     // --- pre-calculations on y --- 

     // put y in yh and yl 
     short yh = y.values[HIGH]; 
     short yl = y.values[LOW]; 

     if (yh == 0 && yl == 0) { 
      // division by zero 
      throw new ArithmeticException(); 
     } 

     if ((yh & 0x8000) != 0) { 
      // negation (complement then increase) 
      yh = (short) ~yh; 
      yl = (short) ~yl; 
      yl++; 
      if (yl == 0) { 
       yh++; 
      } 
     } 

     final short divisorSize = (short) (INTEGER_SIZE - numberOfLeadingZeros(yh, yl)); 

     // --- pre-calculations on x --- 

     final boolean xPositive = this.isPositive(); 
     if (!xPositive) { 
      this.negate(); 
     } 
     final short dividentSize = (short) (INTEGER_SIZE - numberOfLeadingZeros()); 

     // --- setup the maximum number of shifts --- 

     final short maxShifts = (short) (dividentSize - divisorSize); 

     // --- slightly superfluous check if divisor is higher than dividend --- 

     if (maxShifts < 0) { 
      if (!xPositive) { 
       return this.negate(); 
      } 
      return this; 
     } 

     // --- shift divisor left until the highest bit is aligned with the highest bit of the dividend --- 

     if (maxShifts <= JCInteger.SHORT_SIZE) { 
      yh = (short) (((yl & 0xFFFF) >>> (SHORT_SIZE - maxShifts)) | (yh << maxShifts)); 
      yl <<= maxShifts; 
     } else { 
      yh = (short) (yl << (maxShifts - SHORT_SIZE)); 
      yl = 0; 
     } 

     for (short i = maxShifts; i >= 0; i--) { 
      final short xho = values[HIGH]; 
      final short xlo = values[LOW]; 

      // --- subtract (add complement and increment does the job) --- 

      // add complement 
      addUnsignedLow((short) ~yl); 
      values[HIGH] += (short) ~yh; 

      // increase to create subtraction 
      increment(); 

      if (!isPositive()) { 
       values[HIGH] = xho; 
       values[LOW] = xlo; 
      } 

      // --- shift right by 1 --- 
      // first do low shift as high shift changes value 
      yl = (short) ((yh << (JCInteger.SHORT_SIZE - 1)) | ((yl & 0xFFFF) >>> 1)); 
      yh = (short) ((yh & 0xFFFF) >>> 1); 
     } 

     if (!xPositive) { 
      negate(); 
     } 

     return this; 
    } 

    public JCInteger leftShift(short shiftDistance) { 
     shiftDistance = (short) (shiftDistance & 0x1F); 
     if (shiftDistance == 0) { 
      return this; 
     } 

     final short low = values[LOW]; 
     final short high = values[HIGH]; 

     // TODO test if we can do without if on Java Card (is integer value calculated? cannot really be. 
     if (shiftDistance < SHORT_SIZE) { 
      values[HIGH] = (short) (((low & 0xFFFF) >>> (SHORT_SIZE - shiftDistance)) | (high << shiftDistance)); 
      values[LOW] <<= shiftDistance; 
     } else { 
      values[HIGH] = (short) (low << (shiftDistance - SHORT_SIZE)); 
      values[LOW] = 0; 
     } 

     return this; 
    } 

    public JCInteger signedRightShift(short shiftDistance) { 
     shiftDistance = (short) (shiftDistance & 0x1F); 
     if (shiftDistance == 0) { 
      return this; 
     } 

     final short low = values[LOW]; 
     final short high = values[HIGH]; 

     if (shiftDistance < SHORT_SIZE) { 
      values[HIGH] = (short) (high >>> shiftDistance); 
      values[LOW] = (short) ((high << (SHORT_SIZE - shiftDistance)) | ((low & 0xFFFF) >>> shiftDistance)); 
     } else { 
      if ((high & 0x8000) == 0) { 
       values[HIGH] = 0; 
       values[LOW] = (short) ((high & 0xFFFF) >>> (shiftDistance - SHORT_SIZE)); 
      } else { 
       values[HIGH] = (short) 0xFFFF; 
       values[LOW] = (short) (high >>> (shiftDistance - SHORT_SIZE)); 
      } 
     } 

     return this; 
    } 

    public JCInteger unsignedRightShift(short shiftDistance) { 
     shiftDistance = (short) (shiftDistance & 0x1F); 
     if (shiftDistance == 0) { 
      return this; 
     } 

     final short low = values[LOW]; 
     final short high = values[HIGH]; 

     if (shiftDistance < SHORT_SIZE) { 
      values[HIGH] = (short) ((high & 0xFFFF) >>> shiftDistance); 
      values[LOW] = (short) ((high << (SHORT_SIZE - shiftDistance)) | ((low & 0xFFFF) >>> shiftDistance)); 
     } else { 
      values[HIGH] = 0; 
      values[LOW] = (short) ((high & 0xFFFF) >>> (shiftDistance - SHORT_SIZE)); 
     } 

     return this; 
    } 

    public JCInteger complement() { 
     this.values[HIGH] = (short) ~this.values[HIGH]; 
     this.values[LOW] = (short) ~this.values[LOW]; 
     return this; 
    } 

    public JCInteger xor(final JCInteger y) { 
     this.values[HIGH] ^= y.values[HIGH]; 
     this.values[LOW] ^= y.values[LOW]; 
     return this; 
    } 

    public JCInteger and(final JCInteger y) { 
     this.values[HIGH] &= y.values[HIGH]; 
     this.values[LOW] &= y.values[LOW]; 
     return this; 
    } 

    public JCInteger or(final JCInteger y) { 
     this.values[HIGH] |= y.values[HIGH]; 
     this.values[LOW] |= y.values[LOW]; 
     return this; 
    } 

    public short signum() { 
     if (values[HIGH] == 0 && values[LOW] == 0) { 
      return 0; 
     } 

     // get sign bit (>>> 15) negate, -1 for neg, 0 for pos, then times 2 (<< 2) which leaves -2 for neg 0 for pos 
     // and finally add 1, to get the result -1 or 1 for negative and positive, respectively 
     return (short) ((-((values[HIGH] >>> 15) & 1) * 2) + 1); 
    } 

    public short numberOfLeadingZeros() { 
     short t = values[HIGH]; 

     if (t != 0) { 
      for (short i = 0; i < SHORT_SIZE; i++) { 
       if (t < 0) { 
        return i; 
       } 
       t <<= 1; 
      } 
     } 

     t = values[LOW]; 

     if (t != 0) { 
      for (short i = SHORT_SIZE; i < INTEGER_SIZE; i++) { 
       if (t < 0) { 
        return i; 
       } 
       t <<= 1; 
      } 
     } 

     return INTEGER_SIZE; 
    } 

    public short compareTo(JCInteger anotherInteger) { 
     final short xh = values[HIGH]; 
     final short yh = anotherInteger.values[HIGH]; 

     if (xh < yh) { 
      return -1; 
     } else if (xh > yh) { 
      return 1; 
     } 

     // --- xh == yh --- 

     final short xl = values[LOW]; 
     final short yl = anotherInteger.values[LOW]; 

     // TODO think of better way than four ifs 
     if (xl < 0 && yl >= 0) { 
      return 1; 
     } else if (xl >= 0 && yl < 0) { 
      return -1; 
     } else if (xl > yl) { 
      return 1; 
     } else if (xl < yl) { 
      return -1; 
     } 

     return 0; 
    } 

    public boolean equals(Object obj) { 

     if (!(obj instanceof JCInteger)) { 
      return false; 
     } 

     final JCInteger otherInt = (JCInteger) obj; 
     return values[HIGH] == otherInt.values[HIGH] 
       && values[LOW] == otherInt.values[LOW]; 
    } 

    public short encode(final byte[] bArray, short bOff) { 
     // use javacard.framework.Util.setShort() instead 
     bArray[bOff++] = (byte) (values[HIGH] >>> BYTE_SIZE); 
     bArray[bOff++] = (byte) (values[HIGH] & 0xFF); 
     bArray[bOff++] = (byte) (values[LOW] >>> BYTE_SIZE); 
     bArray[bOff++] = (byte) (values[LOW] & 0xFF); 
     return bOff; 
    } 

    public JCInteger decode(final byte[] bArray, short bOff) { 
     values[HIGH] = (short) ((bArray[bOff++] << BYTE_SIZE) | (bArray[bOff++] & 0xFF)); 
     values[LOW] = (short) ((bArray[bOff++] << BYTE_SIZE) | (bArray[bOff++] & 0xFF)); 
     return this; 
    } 

    private boolean isPositive() { 
     return (values[HIGH] & 0x8000) == 0; 
    } 

    private void addUnsignedLow(final short yl) { 
     final short xl = values[LOW]; 
     values[HIGH] += carryOnUnsignedAddition(xl, yl); 
     values[LOW] = (short) (xl + yl); 
    } 

    private static short carryOnUnsignedAddition(final short x, final short y) { 
     // implementation without any conditionals on the highest bits of x, y and r = x + y 
     final short r = (short) (x + y); 
     // uses only the sign bit on the variables including the result to see if carry will happen 
     return (short) ((((x & y) | (x & ~y & ~r) | (~x & y & ~r)) >>> 15) & 1); 
    } 

    private static short[] multiplyUnsigned(short x, short y, short[] r) { 

     // uses the fact that: 
     // x * y = 
     // (x1 * 2^8 + x0) * (y1 * 2^8 + y0) = 
     // (x1 * y1 * 2^16) + x1 * y0 * 2^8 + x0 * y1 * 2^8 + x0 * y0 

     final short x1 = (short) ((x >>> BYTE_SIZE) & 0xFF); 
     final short x0 = (short) (x & 0xFF); 

     final short y1 = (short) ((y >>> BYTE_SIZE) & 0xFF); 
     final short y0 = (short) (y & 0xFF); 

     // TODO check uppiest bit of rh and rl 

     // calculate z2 * 2^(2 * 8) = x1 * y1 * 2^(2 * 8) = x1 * y1 << 16, 
     // store it as partial result in rh 
     short rh = (short) (x1 * y1); 

     // calculate z0 = x0 * y0 
     short rl = (short) (x0 * y0); 

     short toAdd, result; 

     // calculate x1 * y0* 2^8 
     short x1y0 = (short) (x1 * y0); 
     rh += (x1y0 >>> 8) & 0xFF; 
     toAdd = (short) ((x1y0 << 8) & 0xFF00); 
     result = (short) (rl + toAdd); 
     rh += carryOnUnsignedAddition(rl, toAdd); 
     rl = result; 

     // calculate x0 * y1* 2^8 
     short x0y1 = (short) (x0 * y1); 
     rh += (x0y1 >>> 8) & 0xFF; 
     toAdd = (short) ((x0y1 << 8) & 0xFF00); 
     result = (short) (rl + toAdd); 
     rh += carryOnUnsignedAddition(rl, toAdd); 
     rl = result; 

     r[HIGH] = rh; 
     r[LOW] = rl; 
     return r; 
    } 

    private static short numberOfLeadingZeros(short ih, short il) { 

     if (ih != 0) { 
      for (short i = 0; i < SHORT_SIZE; i++) { 
       if (ih < 0) { 
        return i; 
       } 
       ih <<= 1; 
      } 
     } 

     if (il != 0) { 
      for (short i = SHORT_SIZE; i < INTEGER_SIZE; i++) { 
       if (il < 0) { 
        return i; 
       } 
       il <<= 1; 
      } 
     } 

     return INTEGER_SIZE; 
    } 
}