Take an array of values and interpolate missing numbers

linear-interpolation-of-array.php

<?php
function interpolate_row( $row = array() ) {
	if ( ! is_array( $row ) || empty( $row ) ) {
		return $row;
	}

	$items = array();
	$item  = array(
		'start'         => null,
		'end'           => null,
		'empty_indexes' => array(),
	);
	foreach ( $row as $index => $current_val ) {
		$prev_val = $row[ $index - 1 ];
		$next_val = $row[ $index + 1 ];
		if ( empty( $current_val ) && $current_val != '0' ) {

			// Look behind
			if ( ( ! empty( $prev_val ) || $prev_val == '0' ) && is_numeric( $prev_val ) ) {
				$item['start'] = $prev_val + 0; // + 0 so PHP can do smart numeric type casting
			}

			// Look ahead
			if ( ( ! empty( $next_val ) || $next_val == '0' ) && is_numeric( $next_val ) ) {
				$item['end'] = $next_val + 0; // + 0 so PHP can do smart numeric type casting
			}

			$item['empty_indexes'][] = $index;

			// If we have a start and end value we can reset for a new item
			if (
				( ! empty( $item['start'] ) || $item['start'] === 0 ) &&
				( ! empty( $item['end'] ) || $item['end'] === 0 )
			) {
				$items[] = $item;
				$item    = array(
					'start'         => null,
					'end'           => null,
					'empty_indexes' => array(),
				);
			}

			// Bad data to interpolate, reset for a new item
			if (
				( empty( $item['start'] ) && $item['start'] !== 0 ) &&
				( ! empty( $item['end'] ) || $item['end'] === 0 )
			) {
				$item = array(
					'start'         => null,
					'end'           => null,
					'empty_indexes' => array(),
				);
			}
		}
	}
	foreach ( $items as $item ) {
		if ( $item['start'] === $item['end'] ) {
			foreach ( $item['empty_indexes'] as $row_index ) {
				$row[ $row_index ] = strval( $item['start'] );
			}
		} else {
			$numerator = $item['end'] - $item['start'];
			$divisor   = count( $item['empty_indexes'] ) + 1;
			$step      = $numerator / $divisor;
			foreach ( $item['empty_indexes'] as $index => $row_index ) {
				$incremental_value = $step * ( $index + 1 );
				$row[ $row_index ] = $item['start'] + $incremental_value;
				$row[ $row_index ] = strval( $row[ $row_index ] );
			}
		}
	}
	return $row;
}

test.php

<?php
// Pass this to interpolate_row()
$input = array (
  0 => 'foo',
  1 => 'bar',
  2 => '',
  3 => '10',
  4 => '',
  5 => '',
  6 => '40',
  7 => '',
  8 => '',
  9 => '10',
  10 => '5',
  11 => '',
  12 => '',
  13 => '',
  14 => '5',
  15 => '0',
  16 => '',
  17 => '',
  18 => '',
  19 => '100',
  20 => 'baz',
);

// This is the expected result
$result = array (
  0 => 'foo',
  1 => 'bar',
  2 => '',
  3 => '10',
  4 => '20',
  5 => '30',
  6 => '40',
  7 => '30',
  8 => '20',
  9 => '10',
  10 => '5',
  11 => '5',
  12 => '5',
  13 => '5',
  14 => '5',
  15 => '0',
  16 => '25',
  17 => '50',
  18 => '75',
  19 => '100',
  20 => 'baz',
)