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JavaScript - positive and negative around index wrapping / overlapping to address items in array

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In this article, we're going to have a look at how properly in JavaScript calculate index number when we try to address index by negative value or we exceed index to overlap range to normal index.

1. Double modulo example

This section present solution that is based on fact if we will use two times modulo we are able to escape negative calues too.

// ONLINE-RUNNER:browser;

function escapeIndex(index, count) {
	return (index % count + count) % count;
}

// Usage example:

var arraySize = 10;

console.log(  escapeIndex(  0, arraySize ) ); // 0
console.log(  escapeIndex(  1, arraySize ) ); // 1
console.log(  escapeIndex(  2, arraySize ) ); // 2
console.log(  escapeIndex(  9, arraySize ) ); // 9

console.log( escapeIndex(  10, arraySize ) ); // 0
console.log( escapeIndex(  11, arraySize ) ); // 1
console.log( escapeIndex(  19, arraySize ) ); // 9
console.log( escapeIndex(  20, arraySize ) ); // 0
console.log( escapeIndex(  21, arraySize ) ); // 1

console.log( escapeIndex(  -1, arraySize ) ); // 9
console.log( escapeIndex(  -9, arraySize ) ); // 1
console.log( escapeIndex( -10, arraySize ) ); // 0
console.log( escapeIndex( -11, arraySize ) ); // 9
console.log( escapeIndex( -19, arraySize ) ); // 1
console.log( escapeIndex( -20, arraySize ) ); // 0
console.log( escapeIndex( -21, arraySize ) ); // 9

/*
  ----------------[ Calculations map ]--------------------

  normal index       0   1   2   3   4   5   6   7   8   9

  index exceeded    10  11  12  13  14  15  16  17  18  19
                    20  21  22  23  24  25  26  27  28  29
                    ...

  index negative   -10  -9  -8  -7  -6  -5  -4  -3  -2  -1
                   -20 -19 -18 -17 -16 -15 -14 -13 -12 -11
                   -30 -29 -28 -27 -26 -25 -24 -23 -22 -21
                    ...

                     |   |   |   |   |   |   |   |   |   |  
                     |   |   |   |   |   |   |   |   |   |  
                     
  result:            0   1   2   3   4   5   6   7   8   9

*/

2. Modulo with 2 cases example

This section shows solution which uses if statement to split calculations for two cases: negative indesxes and positive.

Used cases are based on:

  1. modulo for positive indexes:
    itemIndex % arraySize,
  2. adding decreased with 1 maximal integer number in JavaScript and later use modulo for negative indexes:
    (itemIndex + 0x1FFFFFFFFFFFFE) % arraySize.

Maximum integer number in JavaScript:

ExponentalHexadecimalDecimal
2^53 - 10x1FFFFFFFFFFFFF9007199254740991

Runnabe examples are presented is below: 

// ONLINE-RUNNER:browser;

function escapeIndex(itemIndex, arraySize) {
    if (itemIndex < 0) {
        return (itemIndex + 0x1FFFFFFFFFFFFE) % arraySize;
    }
    return itemIndex % arraySize;
}

// Usage example:

var arraySize = 10;

console.log(  escapeIndex(  0, arraySize ) ); // 0
console.log(  escapeIndex(  1, arraySize ) ); // 1
console.log(  escapeIndex(  2, arraySize ) ); // 2
console.log(  escapeIndex(  9, arraySize ) ); // 9

console.log( escapeIndex(  10, arraySize ) ); // 0
console.log( escapeIndex(  11, arraySize ) ); // 1
console.log( escapeIndex(  19, arraySize ) ); // 9
console.log( escapeIndex(  20, arraySize ) ); // 0
console.log( escapeIndex(  21, arraySize ) ); // 1

console.log( escapeIndex(  -1, arraySize ) ); // 9
console.log( escapeIndex(  -9, arraySize ) ); // 1
console.log( escapeIndex( -10, arraySize ) ); // 0
console.log( escapeIndex( -11, arraySize ) ); // 9
console.log( escapeIndex( -19, arraySize ) ); // 1
console.log( escapeIndex( -20, arraySize ) ); // 0
console.log( escapeIndex( -21, arraySize ) ); // 9

3. Wrapped array class example

In this section simple class for array wraping is presented.

// ONLINE-RUNNER:browser;

function escapeIndex(index, count) {
  	if(index < 0) {
      	return (index + 0x1FFFFFFFFFFFFE) % count;
  	}
  	return index % count;
}

function WrappedArray(array) {
  	this.getValue = function(index) {
        var safeIndex = escapeIndex(index, array.length);
            return array[safeIndex];
        };
  	this.setValue = function(index, value) {
    	var safeIndex = escapeIndex(index, array.length);
      		array[safeIndex] = value;;
    	};
  	this.iterate = function(start, stop, callback) {
    	var safeStart = escapeIndex(start, array.length);
      	var safeStop = escapeIndex(stop, array.length);
      	if (safeStart < safeStop) {
        	for (var i = safeStart; i < safeStop; ++i) {
            	var result = callback(i, array[i]);
              if (result !== undefined) {
                array[i] = result;
              }
            }
        } else {
          	for (var i = safeStart; i < array.length; ++i) {
            	var result = callback(i, array[i]);
            	if (result !== undefined) {
              		array[i] = result;
            	}
          	}
          	for (var i = 0; i < safeStop; ++i) {
            	var result = callback(i, array[i]);
            	if (result !== undefined) {
              		array[i] = result;
            	}
          	}
        }
    };
}

// Usage example:

var array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
var wrapper = new WrappedArray(array);

// Example 1
console.log( wrapper.getValue(  0 ) ); //  1
console.log( wrapper.getValue(  9 ) ); // 10
console.log( wrapper.getValue( -1 ) ); // 10
console.log( wrapper.getValue( 10 ) ); //  1

// Example 2
console.log( 'Iterating: <0, 9>' );
wrapper.iterate(0, 10, function(index, value) {
  // 1 2 3 4 5 6 7 8 9 10
console.log( index + ': ' + value );
});

// Example 3
console.log( 'Iterating: <-1, 1>' );
wrapper.iterate(-1, 2, function(index, value) {
    // 10 1 2
	console.log( index + ': ' + value );
});

// Example 4
console.log( 'Iterating: <7, 6>' );
wrapper.iterate(7, 7, function(index, value) {
  	// 8 9 10 1 2 3 4 5 6 7
	console.log( index + ': ' + value );
});

// Example 5
console.log( 'Iterating with zero setting: <-2, 1> = 8, 9, 0, 1' );
wrapper.iterate(-2, 2, function(index, value) {
  	// sets 10 9 1 2 to 0
  	return 0;
});
wrapper.iterate(0, 10, function(index, value) {
  // 0 0 3 4 5 6 7 8 0 0
  console.log( index + ': ' + value );
});

 

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