PhabricatorComposeChartFunction.php
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	<?php

	final class PhabricatorComposeChartFunction
	extends PhabricatorHigherOrderChartFunction {

	const FUNCTIONKEY = 'compose';

	protected function newArguments() {
	return array(
	$this->newArgument()
	->setName('f')
	->setType('function')
	->setRepeatable(true),
	);
	}

	public function evaluateFunction(array $xv) {
	$original_positions = array_keys($xv);
	$remaining_positions = $original_positions;
	foreach ($this->getFunctionArguments() as $function) {
	$xv = $function->evaluateFunction($xv);

	// If a function evaluates to "null" at some positions, we want to return
	// "null" at those positions and stop evaluating the function.

	// We also want to pass "evaluateFunction()" a natural list containing
	// only values it should evaluate: keys should not be important and we
	// should not pass "null". This simplifies implementation of functions.

	// To do this, first create a map from original input positions to
	// function return values.
	$xv = array_combine($remaining_positions, $xv);

	// If a function evaluated to "null" at any position where we evaluated
	// it, the result will be "null". We remove the position from the
	// vector so we stop evaluating it.
	foreach ($xv as $x => $y) {
	if ($y !== null) {
	continue;
	}

	unset($xv[$x]);
	}

	// Store the remaining original input positions for the next round, then
	// throw away the array keys so we're passing the next function a natural
	// list with only non-"null" values.
	$remaining_positions = array_keys($xv);
	$xv = array_values($xv);

	// If we have no more inputs to evaluate, we can bail out early rather
	// than passing empty vectors to functions for evaluation.
	if (!$xv) {
	break;
	}
	}


	$yv = array();
	$xv = array_combine($remaining_positions, $xv);
	foreach ($original_positions as $position) {
	if (isset($xv[$position])) {
	$y = $xv[$position];
	} else {
	$y = null;
	}
	$yv[$position] = $y;
	}

	return $yv;
	}

	public function getDataRefs(array $xv) {
	// TODO: This is not entirely correct. The correct implementation would
	// map "x -> y" at each stage of composition and pull and aggregate all
	// the datapoint refs. In practice, we currently never compose functions
	// with a data function somewhere in the middle, so just grabbing the first
	// result is close enough.

	// In the future, we may: notably, "x -> shift(-1 month) -> ..." to
	// generate a month-over-month overlay is a sensible operation which will
	// source data from the middle of a function composition.

	foreach ($this->getFunctionArguments() as $function) {
	return $function->getDataRefs($xv);
	}

	return array();
	}

	}

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