Calculates who gets how much money back when paying expenses for a group of people

groupbalances.php

<?php
/**
 * When you travel in a group and, for practical reasons, some people pay
 * expenses for the whole group (hotel rooms, train tickets, ...), this class
 * will help you to sort out who gets how much money back in the end.
 */

namespace foo;

use Exception;

class Calc
{
    /** @var array */
    protected $names = array();

    /** @var array */
    protected $transactions = array();

    /**
     *
     * @param array $lines Lines of formatted input
     * @throws Exception
     */
    public function import($lines)
    {
        foreach ($lines as $line) {
            $line = trim($line);

            // Skip empty lines
            if ($line === '') {
                continue;
            }

            // Skip comments
            if (substr($line, 0, 1) === ';') {
                continue;
            }

            $elements = preg_split('/\s{2,}/', $line);

            if (count($elements) !== 4) {
                throw new Exception(sprintf('Line not well-formed: %s'), $line);
            }

            $this->process($elements);
        }

        if (isset($this->names['*'])) {
            unset($this->names['*']);
        }

        $this->names = array_keys($this->names);

        $this->expandMacros();
    }

    public function debug()
    {
        echo "Events:\n";
        echo "=======\n\n";

        foreach ($this->transactions as $trans) {
            printf("% 10s lends % 10s   % 5s   (%s)\n",
                    $trans['from'], $trans['to'], $trans['amount'], $trans['description']);
        }

        echo "\n";
    }

    /**
     *
     * @param array $elements
     */
    protected function process($elements)
    {
        list($from, $to, $amount, $desc) = $elements;

        if (strpos($to, ',') !== false) {
            $tos = explode(',', $to);
            $to = array();

            foreach ($tos as $t) {
                $t = trim($t);

                if ($t === '') {
                    continue;
                }

                $to[] = $t;
            }
        } else {
            $to = array($to);
        }

        $amount = (float) $amount;

        $this->names[$from] = true;

        foreach ($to as $t) {
            $this->names[$t] = true;
        }

        $this->transactions[] = array(
            'from'        => $from,
            'to'          => $to,
            'amount'      => $amount,
            'description' => $desc
        );
    }

    protected function expandMacros()
    {
        for ($i = 0; $i < count($this->transactions); $i++) {
            $trans = &$this->transactions[$i];

            if ($trans['to'] === array('*')) {
                $trans['to'] = $this->names;
            }

            if ($trans['description'] === '"') {
                $trans['description'] = $this->transactions[$i - 1]['description'];
            }
        }
        unset($trans);

        $newTrans = array();

        foreach ($this->transactions as $trans) {
            if (count($trans['to']) > 1) {
                foreach ($trans['to'] as $to) {
                    $newTrans[] = array(
                        'from'        => $trans['from'],
                        'to'          => $to,
                        'amount'      => $trans['amount'] / count($trans['to']),
                        'description' => $trans['description']
                    );
                }
            } else {
                $newTrans[] = array(
                    'from'        => $trans['from'],
                    'to'          => $trans['to'][0],
                    'amount'      => $trans['amount'],
                    'description' => $trans['description']
                );
            }
        }

        $this->transactions = $newTrans;
    }

    public function getNames()
    {
        return $this->names;
    }

    public function getExpensesByName($name, $withoutOwn = false)
    {
        $expenses = 0.0;

        foreach ($this->transactions as $trans) {
            if ($trans['from'] === $name) {
                if ($withoutOwn && $trans['to'] === $name) {
                    continue;
                }
                $expenses += $trans['amount'];
            }
        }

        return $expenses;
    }

    public function getDebtsByName($name, $withoutOwn = false)
    {
        $expenses = 0.0;

        foreach ($this->transactions as $trans) {
            if ($trans['to'] === $name) {
                if ($withoutOwn && $trans['from'] === $name) {
                    continue;
                }
                $expenses += $trans['amount'];
            }
        }

        return $expenses;
    }

    public function getDebtorsByName($name, $withoutOwn = false)
    {
        $debtors = array();

        foreach ($this->transactions as $trans) {
            if ($trans['from'] === $name) {
                if ($withoutOwn && $trans['to'] === $name) {
                    continue;
                }
                if (!isset($debtors[$trans['to']])) {
                    $debtors[$trans['to']] = 0.0;
                }

                $debtors[$trans['to']] += $trans['amount'];
            }
        }

        return $debtors;
    }
}

$demo = <<<EOT
; Who loaned money to whom?
;
;   * := "everybody", expands to (Tom, Bob, Mia) in this case
;
;   " := "duplicate previous description"
;
; Mia paid for the groups' train tickets.
; Bob paid for Tom's cappuccino.
; Tom paid for Mia's and Bob's lunch.
; Bob paid for the groups' hotel rooms.
;
;From        To        Amount      Description
;-------------------------------------------------
 Mia         *          12.00      train tickets

 Bob         Tom         3.10      cappuccino

 Tom         Mia        10.30      lunch
 Tom         Bob        11.70       "

 Bob         *          97.20      hotel rooms
EOT;

$lines = explode("\n", $demo);

$c = new Calc();
$c->import($lines);
$c->debug();

echo "Money loaned:\n";
echo "=============\n\n";

$sum = 0.0;

foreach ($c->getNames() as $name) {
    printf("% 10s : %s\n", $name, $c->getExpensesByName($name));
    /*foreach ($c->getDebtorsByName($name) as $debtor => $amount) {
        printf("    % 10s : %s\n", $debtor, $amount);
    }*/
    $sum += $c->getExpensesByName($name);
}
printf("SUM: %s\n", $sum);

echo "\n";


echo "Money borrowed:\n";
echo "===============\n\n";

$sum = 0.0;

foreach ($c->getNames() as $name) {
    printf("% 10s : %s\n", $name, $c->getDebtsByName($name));
    $sum += $c->getDebtsByName($name);
}
printf("SUM: %s\n", $sum);

echo "\n";



echo "Balances:\n";
echo "=========\n\n";

$sum = 0.0;

foreach ($c->getNames() as $name) {
    printf("% 10s : %s\n", $name, $c->getExpensesByName($name) - $c->getDebtsByName($name));
    $sum += $c->getExpensesByName($name) - $c->getDebtsByName($name);
}
printf("SUM: %s\n", round($sum, 2));

echo "\n";


exit;

// This doesn't work so well, yet. The idea is to print a list of the least
// number of transactions necessary to equalize all balances

echo "Transactions:\n";
echo "==============\n\n";

$balances = array();

foreach ($c->getNames() as $name) {
    $balances[$name] = $c->getExpensesByName($name) - $c->getDebtsByName($name);
}

arsort($balances);

foreach ($balances as $key => $value) {
    if ($value > 0.0) {
        $poss[$key] = $value;
    } else {
        $negs[$key] = $value;
    }
}

foreach ($poss as $key => $value) {
    while ($value > 0.0) {
    }
}