ecmascript:

// internal global persistent variables
var previousFraction;
var previousFractionIndex;
var blockSize;
var outputArray;

function tracePrint (outputString)
{
	var traceEnabled = false;
	if (traceEnabled) Browser.println ('[CoordinateInterpolator2D]' + outputString);
}
function alwaysPrint (outputString)
{
	Browser.println ('[CoordinateInterpolator2D]' + outputString);
}
function initialize ()
{
	key      = keyHolderNode.key;
	keyValue = keyValueHolderNode.point;
	previousFractionIndex = -1;
	previousFraction = 0;
	// check key array ranges [0..1] and is monotonically increasing
	// check that size of keyValue array is integer multiple of size of key array
	tracePrint ('key            =' + key);
	tracePrint ('key.length= ' + key.length);
	tracePrint ('keyValue=   ' + keyValue);
	tracePrint ('keyValue.length=' + keyValue.length);
	blockSize =  keyValue.length/key.length;
	tracePrint ('blockSize=' + blockSize);
	if (blockSize != Math.round(blockSize))
	{
	  alwaysPrint ('*** warning:  blockSize not an integer multiple. check sizes of key and keyValue');
	}
	if (key[0] != 0)
	{
	  alwaysPrint ('*** warning:  key[0] != 0');
	}
	if (key[key.length-1] != 1)
	{
	  alwaysPrint ('*** warning:  key[' + (key.length - 1) + '] != 1, reset from' + key[key.length-1] + ' to 1');
	  key[key.length-1] = 1;
	}
	for (index = 0; index < blockSize; index++)
	{
		if ((key[index] < 0) || (key[index] > 1))
		{
		   alwaysPrint ('*** warning:  key[' + index + '] =' + key[index] + ', out of range [0..1]');
		}
	}
	// instantiate default array, later computations just update it
	outputArray = new MFVec2f ();
	for (index = 0; index < blockSize; index++)
	{
		// dynamically grow outputArray to match initial block
		outputArray[index] = keyValue[index];
	}
	tracePrint ('initial outputArray=' + outputArray);
}

function set_fraction (inputFloat, timestamp) {
	fraction = inputFloat;
	tracePrint ('previousFractionIndex=' + previousFractionIndex
		 + ', fraction=' + fraction + ', previousFraction=' + previousFraction);

	if (fraction < 0)
	{
		tracePrint ('*** illegal fraction' + fraction + ' set to 0');
		fraction = 0;
		previousFractionIndex = 0; // first
	}
	else if (fraction > 1)
	{
		alwaysPrint ('*** illegal fraction' + fraction + ' set to 1');
		fraction = 1;
		previousFractionIndex = blockSize - 1; // last
	}
	else if (previousFractionIndex == -1)
	{
		previousFractionIndex = 0; // first
		tracePrint ('previousFractionIndex initialized for first event');
	}
	else if ((fraction >= previousFraction) && (fraction >= key[previousFractionIndex+1]))
	{
		previousFractionIndex++;
	}
	else if (fraction < previousFraction) // regress, or loop repeat without reaching one
	{
		previousFractionIndex = 0;
		while ((fraction >= key[previousFractionIndex+1]) && (previousFractionIndex < blockSize))
		{
			previousFractionIndex++;
		}
		tracePrint ('reset/reincrement previousFractionIndex to' + previousFractionIndex);
	}

	if (fraction == 1) // use final block
	{
		tracePrint ('(fraction == 1)');
		for (index = 0; index < blockSize; index++)
		{
			// update outputArray with final four keyValues
			outputArray[4 - index] = keyValue[keyValue.length - index];
		}
		previousFractionIndex = -1; // setup for restart
		tracePrint ('finished final fraction==1 block');
	}
	// when fraction matches index, calculate value_changed from corresponding keyValue array
	else if (fraction == key[previousFractionIndex])
	{
		tracePrint ('(fraction == key[previousFractionIndex])');
		for (index = 0; index < blockSize; index++)
		{
			// update outputArray - need to interpolate next
			outputArray[index] = keyValue[blockSize * (previousFractionIndex) + index];
		}
	}
	else // calculate value_changed by interpolating between adjacent keyValue arrays
	{
		partialFraction = fraction                     - key[previousFractionIndex];
		deltaFraction   = key[previousFractionIndex+1] - key[previousFractionIndex];
		percentFraction = partialFraction / deltaFraction;
	//	tracePrint ('deltaFraction   =' + deltaFraction);
	//	tracePrint ('partialFraction =' + partialFraction);
		tracePrint ('percentFraction =' + percentFraction);
		for (index = 0; index < blockSize; index++)
		{
			// no arithmetic operators provided for SFVec2f, treat element by element
			nextKeyValue  = keyValue[blockSize * (previousFractionIndex + 1) + index];
			priorKeyValue = keyValue[blockSize * (previousFractionIndex)     + index];
			deltaKeyValue = new SFVec2f (
						nextKeyValue[0] - priorKeyValue[0],
						nextKeyValue[1] - priorKeyValue[1]);
		//	tracePrint ('deltaKeyValue =' + deltaKeyValue);
			// update outputArray
			outputArray[index][0] = keyValue[blockSize * (previousFractionIndex) + index][0]
			   + percentFraction * deltaKeyValue[0];
			outputArray[index][1] = keyValue[blockSize * (previousFractionIndex) + index][1]
			   + percentFraction * deltaKeyValue[1];
		}
	}
	value_changed = outputArray;
	previousFraction = fraction;
	tracePrint ('value_changed=' + value_changed);
}

function set_key (inputArray, timestamp) {
	key = inputArray;       // update key Vector2FloatArray
	keyHolderNode.key = key; // update holder
	initialize (timestamp); // reverify key, keyValue sizes
	key_changed = key;	// eventOut
}

function set_keyValue (inputArray, timestamp) {
	keyValue = inputArray;  	// update keyValue Vector2FloatArray
	keyValueHolderNode.point = keyValue; // update holder
	initialize (timestamp); 	// reverify key, keyValue sizes
	keyValue_changed = keyValue;	// eventOut
}