ecmascript:

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

var traceEnabled = false;

function tracePrint (outputString)
{
	if (traceEnabled) Browser.println ('[PositionInterpolator2d]' + outputString);
}
function alwaysPrint (outputString)
{
	Browser.println ('[PositionInterpolator2d]' + 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 != 1)
	{
	  alwaysPrint ('*** warning:  check sizes of key and keyValue, should be identical!');
	}
	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 < key.length; index++)
	{
		if ((key[index] < 0) || (key[index] > 1))
		{
		   alwaysPrint ('*** warning:  key[' + index + '] =' + key[index] + ', out of range [0..1]');
		}
	}
}

function set_fraction (inputFloat, timestamp) {
	fraction = inputFloat;
	outputResult = new SFVec2f ();
	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 = key.length - 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)');
		// update outputResult with final keyValue
		outputResult = keyValue[keyValue.length];

		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])');
		// update outputResult with corresponding keyValue
		outputResult = keyValue[previousFractionIndex];
	}
	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);
		// no arithmetic operators provided for SFVec2f, treat element by element
		nextKeyValue  = keyValue[previousFractionIndex + 1];
		priorKeyValue = keyValue[previousFractionIndex];
		tracePrint (' nextKeyValue =' + nextKeyValue);
		tracePrint ('priorKeyValue =' + priorKeyValue);
		deltaKeyValue = new SFVec2f (	nextKeyValue[0] - priorKeyValue[0],
						nextKeyValue[1] - priorKeyValue[1]);
		//	tracePrint ('deltaKeyValue =' + deltaKeyValue);
		// update outputResult
		outputResult[0] = keyValue[previousFractionIndex][0]
			        + percentFraction * deltaKeyValue[0];
		outputResult[1] = keyValue[previousFractionIndex][1]
			        + percentFraction * deltaKeyValue[1];
	}
	value_changed = outputResult;
	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
}

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