DusanKasan / Knapsack

src/collection_functions.php

<?php

namespace DusanKasan\Knapsack;

use ArrayIterator;
use DusanKasan\Knapsack\Exceptions\ItemNotFound;
use DusanKasan\Knapsack\Exceptions\NoMoreItems;
use Traversable;

/**
 * Converts $collection to array. If $collection is not array or Traversable, an array [$collection] will be returned.
 * If there are multiple items with the same key, only the last will be preserved.
 *
 * @param array|Traversable $collection
 * @return array
 */
function toArray($collection)
{
    $arr = [];
    foreach ($collection as $key => $value) {
        $arr[$key] = $value;
    }

    return $arr;
}

/**
 * Returns a lazy collection of distinct items in $collection.
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function distinct($collection)
{
    $generatorFactory = function () use ($collection) {
        $distinctValues = [];

        foreach ($collection as $key => $value) {
            if (!in_array($value, $distinctValues)) {
                $distinctValues[] = $value;
                yield $key => $value;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns number of items in $collection.
 *
 * @param array|Traversable $collection
 * @return int
 */
function size($collection)
{
    $result = 0;
    foreach ($collection as $value) {
        $result++;
    }

    return $result;
}

/**
 * Returns a non-lazy collection with items from $collection in reversed order.
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function reverse($collection)
{
    $generatorFactory = function () use ($collection) {
        $array = [];
        foreach ($collection as $key => $value) {
            $array[] = [$key, $value];
        }

        return map(
            indexBy(
                array_reverse($array),
                function($item) {
                    return $item[0];
                }
            ),
            function($item) {
                return $item[1];
            }
        );
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of values from $collection (i.e. the keys are reset).
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function values($collection)
{
    $generatorFactory = function () use ($collection) {
        foreach ($collection as $value) {
            yield $value;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of keys from $collection.
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function keys($collection)
{
    $generatorFactory = function () use ($collection) {
        foreach ($collection as $key => $value) {
            yield $key;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of items from $collection repeated infinitely.
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function cycle($collection)
{
    $generatorFactory = function () use ($collection) {
        while (true) {
            foreach ($collection as $key => $value) {
                yield $key => $value;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a non-lazy collection of shuffled items from $collection.
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function shuffle($collection)
{
    $buffer = [];
    foreach ($collection as $key => $value) {
        $buffer[] = [$key, $value];
    }

    \shuffle($buffer);

    return dereferenceKeyValue($buffer);
}

/**
 * Returns true if $collection does not contain any items.
 *
 * @param array|Traversable $collection
 * @return bool
 */
function isEmpty($collection)
{
    foreach ($collection as $value) {
        return false;
    }

    return true;
}

/**
 * Returns true if $collection does contain any items.
 *
 * @param array|Traversable $collection
 * @return bool
 */
function isNotEmpty($collection)
{
    return !isEmpty($collection);
}

/**
 * Returns a collection where keys are distinct values from $collection and values are number of occurrences of each
 * value.
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function frequencies($collection)
{
    return countBy($collection, '\DusanKasan\Knapsack\identity');
}

/**
 * Returns the first item of $collection or throws ItemNotFound if #collection is empty.
 *
 * @param array|Traversable $collection
 * @return mixed
 */
function first($collection)
{
    return getNth($collection, 0);
}

/**
 * Returns the last item of $collection or throws ItemNotFound if #collection is empty.
 *
 * @param array|Traversable $collection
 * @return mixed
 */
function last($collection)
{
    return first(reverse($collection));
}

/**
 * Returns a lazy collection of items of $collection where value of each item is set to the return value of calling
 * $function on its value and key.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function map($collection, callable $function)
{
    $generatorFactory = function () use ($collection, $function) {
        foreach ($collection as $key => $value) {
            yield $key => $function($value, $key);
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of items from $collection for which $function returns true.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function filter($collection, callable $function)
{
    $generatorFactory = function () use ($collection, $function) {
        foreach ($collection as $key => $value) {
            if ($function($value, $key)) {
                yield $key => $value;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection with items from all $collections passed as argument appended together
 *
 * @param array|Traversable ...$collections
 * @return Collection
 */
function concat(...$collections)
{
    $generatorFactory = function () use ($collections) {
        foreach ($collections as $collection) {
            foreach ($collection as $key => $value) {
                yield $key => $value;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Reduces the collection to single value by iterating over the collection and calling $reduction while
 * passing $startValue and current key/item as parameters. The output of $function is used as $startValue in
 * next iteration. The output of $function on last element is the return value of this function.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @param mixed $startValue
 * @return mixed
 */
function reduce($collection, callable $function, $startValue)
{
    $tmp = duplicate($startValue);

    foreach ($collection as $key => $value) {
        $tmp = $function($tmp, $value, $key);
    }

    return $tmp;
}

/**
 * Flattens multiple levels of nesting in collection. If $levelsToFlatten is not specified, flattens all levels of
 * nesting.
 *
 * @param array|Traversable $collection
 * @param int $levelsToFlatten -1 to flatten everything
 * @return Collection
 */
function flatten($collection, $levelsToFlatten = -1)
{
    $generatorFactory = function () use ($collection, $levelsToFlatten) {
        $flattenNextLevel = $levelsToFlatten < 0 || $levelsToFlatten > 0;
        $childLevelsToFlatten = $levelsToFlatten > 0 ? $levelsToFlatten - 1 : $levelsToFlatten;

        foreach ($collection as $key => $value) {
            if ($flattenNextLevel && (is_array($value) || $value instanceof Traversable)) {
                foreach (flatten($value, $childLevelsToFlatten) as $childKey => $childValue) {
                    yield $childKey => $childValue;
                }
            } else {
                yield $key => $value;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a non-lazy collection sorted using $collection($item1, $item2, $key1, $key2 ). $collection should
 * return true if first item is larger than the second and false otherwise.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value1, $value2, $key1, $key2)
 * @return Collection
 */
function sort($collection, callable $function)
{
    $array = iterator_to_array(
        values(
            map(
                $collection,
                function ($value, $key) {
                    return [$key, $value];
                }
            )
        )
    );

    uasort(
        $array,
        function ($a, $b) use ($function) {
            return $function($a[1], $b[1], $a[0], $b[0]);
        }
    );

    return dereferenceKeyValue($array);
}

/**
 * Returns a lazy collection that is a part of $collection starting from $from position and ending in $to position.
 * If $to is not provided, the returned collection is contains all items from $from until end of $collection. All items
 * before $from are iterated over, but not included in result.
 *
 * @param array|Traversable $collection
 * @param int $from
 * @param int $to -1 to slice until end
 * @return Collection
 */
function slice($collection, $from, $to = -1)
{
    $generatorFactory = function () use ($collection, $from, $to) {
        $index = 0;
        foreach ($collection as $key => $value) {
            if ($index >= $from && ($index < $to || $to == -1)) {
                yield $key => $value;
            } elseif ($index >= $to && $to >= 0) {
                break;
            }

            $index++;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a non-lazy collection of items grouped by the result of $function.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function groupBy($collection, callable $function)
{
    $result = [];

    foreach ($collection as $key => $value) {
        $newKey = $function($value, $key);

        $group = isset($result[$newKey]) ? $result[$newKey] : new Collection([]);
        $result[$newKey] = $group->append($value);
    }

    return Collection::from($result);
}

/**
 * Executes $function for each item in $collection
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function each($collection, callable $function)
{
    $generatorFactory = function () use ($collection, $function) {
        foreach ($collection as $key => $value) {
            $function($value, $key);

            yield $key => $value;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns an item with $key key from $collection. If that key is not present, throws ItemNotFound.
 *
 * @param array|Traversable $collection
 * @param mixed $key
 * @return mixed
 */
function get($collection, $key)
{
    foreach ($collection as $valueKey => $value) {
        if ($key === $valueKey) {
            return $value;
        }
    }

    throw new ItemNotFound;
}

/**
 * Returns an item with $key key from $collection. If that key is not present, returns $default.
 *
 * @param array|Traversable $collection
 * @param mixed $key
 * @param mixed $default value returned if key is not found
 * @return mixed
 */
function getOrDefault($collection, $key, $default)
{
    try {
        return get($collection, $key);
    } catch (ItemNotFound $e) {
        return $default;
    }
}

/**
 * Returns the first item from $collection for which $function returns true. If item like that is not present, returns
 * $default.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @param mixed $default
 * @return mixed
 */
function find($collection, callable $function, $default = null)
{
    foreach ($collection as $key => $value) {
        if ($function($value, $key)) {
            return $value;
        }
    }

    return $default;
}

/**
 * Returns a lazy collection by changing keys of $collection for each item to the result of $function for
 * that item.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function indexBy($collection, callable $function)
{
    $generatorFactory = function () use ($collection, $function) {
        foreach ($collection as $key => $value) {
            yield $function($value, $key) => $value;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a non-lazy collection of items whose keys are the return values of $function and values are the number of
 * items in this collection for which the $function returned this value.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function countBy($collection, callable $function)
{
    return map(
        groupBy($collection, $function),
        '\DusanKasan\Knapsack\size'
    );
}

/**
 * Returns true if $function returns true for every item in $collection
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return bool
 */
function every($collection, callable $function)
{
    foreach ($collection as $key => $value) {
        if (!$function($value, $key)) {
            return false;
        }
    }

    return true;
}

/**
 * Returns true if $function returns true for at least one item in $collection.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return bool
 */
function some($collection, callable $function)
{
    foreach ($collection as $key => $value) {
        if ($function($value, $key)) {
            return true;
        }
    }

    return false;
}

/**
 * Returns true if $needle is found in $collection values.
 *
 * @param $collection
 * @param mixed $needle
 * @return bool
 */
function contains($collection, $needle)
{
    foreach ($collection as $key => $value) {
        if ($value === $needle) {
            return true;
        }
    }

    return false;
}

/**
 * Reduce that walks from right to the left.
 *
 * @param array|Traversable $collection
 * @param callable $function
 * @param mixed $startValue
 * @return mixed
 */
function reduceRight($collection, callable $function, $startValue)
{
    return reduce(reverse($collection), $function, $startValue);
}

/**
 * Returns a lazy collection of first $numberOfItems items of $collection.
 *
 * @param array|Traversable $collection
 * @param int $numberOfItems
 * @return Collection
 */
function take($collection, $numberOfItems)
{
    return slice($collection, 0, $numberOfItems);
}

/**
 * Returns a lazy collection of all but first $numberOfItems items of $collection.
 *
 * @param array|Traversable $collection
 * @param int $numberOfItems
 * @return Collection
 */
function drop($collection, $numberOfItems)
{
    return slice($collection, $numberOfItems);
}

/**
 * Returns a lazy collection of values, where first value is $value and all subsequent values are computed by applying
 * $function to the last value in the collection. By default this produces an infinite collection. However you can
 * end the collection by throwing a NoMoreItems exception.
 *
 * @param mixed $value
 * @param callable $function ($value, $key)
 * @return Collection
 */
function iterate($value, callable $function)
{
    $duplicated = duplicate($value);
    $generatorFactory = function () use ($duplicated, $function) {
        $value = $duplicated;

        yield $value;

        while (true) {
            try {
                $value = $function($value);
                yield $value;
            } catch (NoMoreItems $e) {
                break;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of items from $collection for which $function returned true.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function reject($collection, callable $function)
{
    return filter(
        $collection,
        function($value, $key) use ($function) {
            return !$function($value, $key);
        }
    );
}

/**
 * Returns a lazy collection of items in $collection without the last $numberOfItems items.
 *
 * @param array|Traversable $collection
 * @param int $numberOfItems
 * @return Collection
 */
function dropLast($collection, $numberOfItems = 1)
{
    $generatorFactory = function () use ($collection, $numberOfItems) {
        $buffer = [];

        foreach ($collection as $key => $value) {
            $buffer[] = [$key, $value];

            if (count($buffer) > $numberOfItems) {
                $val = array_shift($buffer);
                yield $val[0] => $val[1];
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of items from $collection separated by $separator.
 *
 * @param array|Traversable $collection
 * @param mixed $separator
 * @return Collection
 */
function interpose($collection, $separator)
{
    $generatorFactory = function () use ($collection, $separator) {
        foreach (take($collection, 1) as $key => $value) {
            yield $key => $value;
        }

        foreach (drop($collection, 1) as $key => $value) {
            yield $separator;
            yield $key => $value;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of first item from first collection, first item from second, second from first and
 * so on. Accepts any number of collections.
 *
 * @param array|Traversable ...$collections
 * @return Collection
 */
function interleave(...$collections)
{
    $generatorFactory = function () use ($collections) {
        $normalizedCollection = array_map(
            function ($collection) {
                $c = (is_array($collection)) ? new ArrayIterator($collection) : $collection;
                $c->rewind();
                return $c;
            },
            $collections
        );

        do {
            $valid = false;
            foreach ($normalizedCollection as $collection) {
                if ($collection->valid()) {
                    yield $collection->key() => $collection->current();
                    $collection->next();
                    $valid = true;
                }
            }
        } while ($valid);
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of items in $collection with $value added as first element. If $key is not provided
 * it will be next integer index.
 *
 * @param array|Traversable $collection
 * @param mixed $value
 * @param mixed|null $key
 * @return Collection
 */
function prepend($collection, $value, $key = null)
{
    $generatorFactory = function () use ($collection, $value, $key) {
        if ($key === null) {
            yield $value;
        } else {
            yield $key => $value;
        }

        foreach ($collection as $key => $value) {
            yield $key => $value;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of items in $collection with $value added as last element. If $key is not provided
 * it will be next integer index.
 *
 * @param array|Traversable $collection
 * @param mixed $value
 * @param mixed|null $key
 * @return Collection
 */
function append($collection, $value, $key = null)
{
    $generatorFactory = function () use ($collection, $value, $key) {
        foreach ($collection as $k => $v) {
            yield $k => $v;
        }

        if ($key === null) {
            yield $value;
        } else {
            yield $key => $value;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection by removing items from $collection until first item for which $function returns false.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function dropWhile($collection, callable $function)
{
    $generatorFactory = function () use ($collection, $function) {
        $shouldDrop = true;
        foreach ($collection as $key => $value) {
            if ($shouldDrop) {
                $shouldDrop = $function($value, $key);
            }

            if (!$shouldDrop) {
                yield $key => $value;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of items from $collection until first item for which $function returns false.
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function takeWhile($collection, callable $function)
{
    $generatorFactory = function () use ($collection, $function) {
        $shouldTake = true;
        foreach ($collection as $key => $value) {
            if ($shouldTake) {
                $shouldTake = $function($value, $key);
            }

            if ($shouldTake) {
                yield $key => $value;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection. A result of calling map and flatten(1)
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function mapcat($collection, callable $function)
{
    return flatten(map($collection, $function), 1);
}

/**
 * Returns a lazy collection [take($collection, $position), drop($collection, $position)]
 *
 * @param array|Traversable $collection
 * @param int $position
 * @return Collection
 */
function splitAt($collection, $position)
{
    $generatorFactory = function () use ($collection, $position) {
        yield take($collection, $position);
        yield drop($collection, $position);
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection [takeWhile($collection, $function), dropWhile($collection, $function)]
 *
 * @param array|Traversable $collection
 * @param callable $function ($value, $key)
 * @return Collection
 */
function splitWith($collection, callable $function)
{
    $generatorFactory = function () use ($collection, $function) {
        yield takeWhile($collection, $function);
        yield dropWhile($collection, $function);
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection with items from $collection but values that are found in keys of $replacementMap
 * are replaced by their values.
 *
 * @param array|Traversable $collection
 * @param array|Traversable $replacementMap
 * @return Collection
 */
function replace($collection, $replacementMap)
{
    $generatorFactory = function () use ($collection, $replacementMap) {
        foreach ($collection as $key => $value) {
            $newValue = getOrDefault($replacementMap, $value, $value);
            yield $key => $newValue;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of reduction steps.
 *
 * @param array|Traversable $collection
 * @param callable $function
 * @param mixed $startValue
 * @return Collection
 */
function reductions($collection, callable $function, $startValue)
{
    $generatorFactory = function () use ($collection, $function, $startValue) {
        $tmp = duplicate($startValue);

        yield $tmp;
        foreach ($collection as $key => $value) {
            $tmp = $function($tmp, $value, $key);
            yield $tmp;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of every nth ($step) item in  $collection.
 *
 * @param array|Traversable $collection
 * @param int $step
 * @return Collection
 */
function takeNth($collection, $step)
{
    $generatorFactory = function () use ($collection, $step) {
        $index = 0;
        foreach ($collection as $key => $value) {
            if ($index % $step == 0) {
                yield $key => $value;
            }

            $index++;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of collections of $numberOfItems items each, at $step step
 * apart. If $step is not supplied, defaults to $numberOfItems, i.e. the partitions
 * do not overlap. If a $padding collection is supplied, use its elements as
 * necessary to complete last partition up to $numberOfItems items. In case there are
 * not enough padding elements, return a partition with less than $numberOfItems items.
 *
 * @param array|Traversable $collection
 * @param $numberOfItems
 * @param int $step
 * @param array|Traversable $padding
 * @return Collection
 */
function partition($collection, $numberOfItems, $step = -1, $padding = [])
{
    $generatorFactory = function () use ($collection, $numberOfItems, $step, $padding) {
        $buffer = [];
        $itemsToSkip = 0;
        $tmpStep = $step ?: $numberOfItems;

        foreach ($collection as $key => $value) {
            if (count($buffer) == $numberOfItems) {
                yield dereferenceKeyValue($buffer);

                $buffer = array_slice($buffer, $tmpStep);
                $itemsToSkip =  $tmpStep - $numberOfItems;
            }

            if ($itemsToSkip <= 0) {
                $buffer[] = [$key, $value];
            } else {
                $itemsToSkip--;
            }
        }

        yield take(
            concat(dereferenceKeyValue($buffer), $padding),
            $numberOfItems
        );
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection created by partitioning $collection each time $function returned a different value.
 *
 * @param array|Traversable $collection
 * @param callable $function
 * @return Collection
 */
function partitionBy($collection, callable $function)
{
    $generatorFactory = function () use ($collection, $function) {
        $result = null;
        $buffer = [];

        foreach ($collection as $key => $value) {
            $newResult = $function($value, $key);

            if (!empty($buffer) && $result != $newResult) {
                yield dereferenceKeyValue($buffer);
                $buffer = [];
            }

            $result = $newResult;
            $buffer[] = [$key, $value];
        }

        if (!empty($buffer)) {
            yield dereferenceKeyValue($buffer);
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns nth ($position) item from $collection. If $position is greater than $collection size, throws ItemNotFound.
 *
 * @param array|Traversable $collection
 * @param int $position
 * @return mixed
 */
function getNth($collection, $position)
{
    return get(values($collection), $position);
}

/**
 * Returns a lazy collection by picking a $key key from each sub-collection of $collection if possible. Ignores
 * non-collection items.
 *
 * @param array|Traversable $collection
 * @param mixed $key
 * @return Collection
 */
function pluck($collection, $key)
{
    $generatorFactory = function () use ($collection, $key) {

        return map(
            filter(
                $collection,
                function ($item) use ($key) {
                    return isCollection($item) && has($item, $key);
                }
            ),
            function($value) use ($key) {
                return get($value, $key);
            }
        );
    };

    return new Collection($generatorFactory);

}

/**
 * Returns a lazy collection of $value repeated $times times. If $times is not provided the collection is infinite.
 *
 * @param mixed $value
 * @param int $times
 * @return Collection
 */
function repeat($value, $times = -1)
{
    $generatorFactory = function () use ($value, $times) {
        $tmpTimes = $times;

        while ($tmpTimes != 0) {
            yield $value;

            $tmpTimes = $tmpTimes < 0 ? -1 : $tmpTimes - 1;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection of numbers starting at $start, incremented by $step until $end is reached.
 *
 * @param int $start
 * @param int|null $end
 * @param int $step
 * @return Collection
 */
function range($start = 0, $end = null, $step = 1)
{
    $generatorFactory = function () use ($start, $end, $step) {
        return iterate(
            $start,
            function($value) use ($step, $end) {
                $result = $value + $step;

                if ($end !== null && $result > $end) {
                    throw new NoMoreItems;
                }

                return $result;
            }
        );
    };

    return new Collection($generatorFactory);
}

//helpers
/**
 * Returns true if $input is array or Traversable object.
 *
 * @param mixed $input
 * @return bool
 */
function isCollection($input)
{
    return is_array($input) || $input instanceof Traversable;
}

/**
 * Returns duplicated/cloned $input that has no relation to the original one. Used for making sure there are no side
 * effect in functions.
 *
 * @param $input
 * @return mixed
 */
function duplicate($input)
{
    if (is_array($input)) {
        return toArray(
            map(
                $input,
                function($i) {
                    return duplicate($i);
                }
            )
        );
    } elseif (is_object($input)) {
        return clone $input;
    } else {
        return $input;
    }
}

/**
 * Transforms [[$key, $value], [$key2, $value2]] into [$key => $value, $key2 => $value2]. Used as a helper
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function dereferenceKeyValue($collection)
{
    $generatorFactory = function () use ($collection) {
        foreach ($collection as $value) {
            yield $value[0] => $value[1];
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Realizes collection - turns lazy collection into non-lazy one by iterating over it and storing the key/values.
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function realize($collection)
{
    return new Collection(toArray($collection));
}

/**
 * Returns the second item of $collection or throws ItemNotFound if $collection is empty or has 1 item.
 *
 * @param array|Traversable $collection
 * @return mixed
 */
function second($collection)
{
    return first(drop($collection, 1));
}


/**
 * Combines $keys and $values into a lazy collection. The resulting collection has length equal to the size of smaller
 * argument If $strict is true, the size of both collections must be equal, otherwise ItemNotFound is thrown. When
 * strict, the collection is realized immediately.
 *
 * @param array|Traversable $keys
 * @param array|Traversable $values
 * @param bool $strict
 * @return Collection
 */
function combine($keys, $values, $strict = false)
{
    $generatorFactory = function () use ($keys, $values, $strict) {
        $keys = is_array($keys) ? new ArrayIterator($keys) : $keys;

Consider using a different name than the imported variable $keys, or did you forget to import by reference?

        $values = is_array($values) ? new ArrayIterator($values) : $values;

Consider using a different name than the imported variable $values, or did you forget to import by reference?

        $values->rewind();

It seems like you code against a concrete implementation and not the interface Traversable as the method rewind() does only exist in the following implementations of said interface: AppendIterator, ArrayIterator, CachingIterator, CallbackFilterIterator, DirectoryIterator, DoctrineTest\InstantiatorTestAsset\PharAsset, DusanKasan\Knapsack\Collection, EmptyIterator, File_Iterator, FilesystemIterator, FilterIterator, Generator, GlobIterator, HttpMessage, HttpRequestPool, Imagick, ImagickPixelIterator, InfiniteIterator, Issue523, IteratorIterator, LimitIterator, MongoCommandCursor, MongoCursor, MongoGridFSCursor, MultipleIterator, NoRewindIterator, PDepend\Input\Iterator, PDepend\Metrics\AnalyzerIterator, PDepend\Source\AST\ASTArtifactList, PDepend\Source\AST\ASTClassOrInterfaceReferenceIterator, PHPUnit_Runner_Filter_GroupFilterIterator, PHPUnit_Runner_Filter_Group_Exclude, PHPUnit_Runner_Filter_Group_Include, PHPUnit_Runner_Filter_Test, PHPUnit_Util_TestSuiteIterator, PHP_CodeCoverage_Report_Node_Iterator, PHP_Token_Stream, ParentIterator, Phar, PharData, RecursiveArrayIterator, RecursiveCachingIterator, RecursiveCallbackFilterIterator, RecursiveDirectoryIterator, RecursiveFilterIterator, RecursiveIteratorIterator, RecursiveRegexIterator, RecursiveTreeIterator, RegexIterator, SQLiteResult, SimpleXMLIterator, SplDoublyLinkedList, SplFileObject, SplFixedArray, SplHeap, SplMaxHeap, SplMinHeap, SplObjectStorage, SplPriorityQueue, SplQueue, SplStack, SplTempFileObject, Symfony\Component\Finder\Iterator\CustomFilterIterator, Symfony\Component\Finder\Iterator\DateRangeFilterIterator, Symfony\Component\Finder\Iterator\DepthRangeFilterIterator, Symfony\Component\Finder\Iterator\ExcludeDirectoryFilterIterator, Symfony\Component\Finder\Iterator\FileTypeFilterIterator, Symfony\Component\Finder\Iterator\FilecontentFilterIterator, Symfony\Component\Finder\Iterator\FilenameFilterIterator, Symfony\Component\Finder\Iterator\FilterIterator, Symfony\Component\Finder\Iterator\MultiplePcreFilterIterator, Symfony\Component\Finder\Iterator\PathFilterIterator, Symfony\Component\Finder\Iterator\RecursiveDirectoryIterator, Symfony\Component\Finder\Iterator\SizeRangeFilterIterator, Symfony\Component\Finder\Tests\Iterator\InnerNameIterator, Symfony\Component\Finder\Tests\Iterator\InnerSizeIterator, Symfony\Component\Finder\Tests\Iterator\InnerTypeIterator, Symfony\Component\Finder\Tests\Iterator\Iterator, Symfony\Component\Finder\Tests\Iterator\MockFileListIterator, Symfony\Component\Finder\Tests\Iterator\TestMultiplePcreFilterIterator, TestIterator, TestIterator2.

        foreach ($keys as $key) {
            if (!$values->valid()) {

It seems like you code against a concrete implementation and not the interface Traversable as the method valid() does only exist in the following implementations of said interface: AppendIterator, ArrayIterator, CachingIterator, CallbackFilterIterator, DirectoryIterator, DoctrineTest\InstantiatorTestAsset\PharAsset, DusanKasan\Knapsack\Collection, EmptyIterator, File_Iterator, FilesystemIterator, FilterIterator, Generator, GlobIterator, HttpMessage, HttpRequestPool, Imagick, ImagickPixelIterator, InfiniteIterator, Issue523, IteratorIterator, LimitIterator, MongoCommandCursor, MongoCursor, MongoGridFSCursor, MultipleIterator, NoRewindIterator, PDepend\Input\Iterator, PDepend\Metrics\AnalyzerIterator, PDepend\Source\AST\ASTArtifactList, PDepend\Source\AST\ASTClassOrInterfaceReferenceIterator, PHPUnit_Runner_Filter_GroupFilterIterator, PHPUnit_Runner_Filter_Group_Exclude, PHPUnit_Runner_Filter_Group_Include, PHPUnit_Runner_Filter_Test, PHPUnit_Util_TestSuiteIterator, PHP_CodeCoverage_Report_Node_Iterator, PHP_Token_Stream, ParentIterator, Phar, PharData, RecursiveArrayIterator, RecursiveCachingIterator, RecursiveCallbackFilterIterator, RecursiveDirectoryIterator, RecursiveFilterIterator, RecursiveIteratorIterator, RecursiveRegexIterator, RecursiveTreeIterator, RegexIterator, SQLiteResult, SimpleXMLIterator, SplDoublyLinkedList, SplFileObject, SplFixedArray, SplHeap, SplMaxHeap, SplMinHeap, SplObjectStorage, SplPriorityQueue, SplQueue, SplStack, SplTempFileObject, Symfony\Component\Finder\Iterator\CustomFilterIterator, Symfony\Component\Finder\Iterator\DateRangeFilterIterator, Symfony\Component\Finder\Iterator\DepthRangeFilterIterator, Symfony\Component\Finder\Iterator\ExcludeDirectoryFilterIterator, Symfony\Component\Finder\Iterator\FileTypeFilterIterator, Symfony\Component\Finder\Iterator\FilecontentFilterIterator, Symfony\Component\Finder\Iterator\FilenameFilterIterator, Symfony\Component\Finder\Iterator\FilterIterator, Symfony\Component\Finder\Iterator\MultiplePcreFilterIterator, Symfony\Component\Finder\Iterator\PathFilterIterator, Symfony\Component\Finder\Iterator\RecursiveDirectoryIterator, Symfony\Component\Finder\Iterator\SizeRangeFilterIterator, Symfony\Component\Finder\Tests\Iterator\InnerNameIterator, Symfony\Component\Finder\Tests\Iterator\InnerSizeIterator, Symfony\Component\Finder\Tests\Iterator\InnerTypeIterator, Symfony\Component\Finder\Tests\Iterator\Iterator, Symfony\Component\Finder\Tests\Iterator\MockFileListIterator, Symfony\Component\Finder\Tests\Iterator\TestMultiplePcreFilterIterator, TestIterator, TestIterator2.

                break;
            }

            yield $key => $values->current();

It seems like you code against a concrete implementation and not the interface Traversable as the method current() does only exist in the following implementations of said interface: AppendIterator, ArrayIterator, CachingIterator, CallbackFilterIterator, DirectoryIterator, DoctrineTest\InstantiatorTestAsset\PharAsset, DusanKasan\Knapsack\Collection, EmptyIterator, File_Iterator, FilesystemIterator, FilterIterator, Generator, GlobIterator, HttpMessage, HttpRequestPool, Imagick, ImagickPixelIterator, InfiniteIterator, Issue523, IteratorIterator, LimitIterator, MongoCommandCursor, MongoCursor, MongoGridFSCursor, MultipleIterator, NoRewindIterator, PDepend\Input\Iterator, PDepend\Metrics\AnalyzerIterator, PDepend\Source\AST\ASTArtifactList, PDepend\Source\AST\ASTClassOrInterfaceReferenceIterator, PHPUnit_Runner_Filter_GroupFilterIterator, PHPUnit_Runner_Filter_Group_Exclude, PHPUnit_Runner_Filter_Group_Include, PHPUnit_Runner_Filter_Test, PHPUnit_Util_TestSuiteIterator, PHP_CodeCoverage_Report_Node_Iterator, PHP_Token_Stream, ParentIterator, Phar, PharData, RecursiveArrayIterator, RecursiveCachingIterator, RecursiveCallbackFilterIterator, RecursiveDirectoryIterator, RecursiveFilterIterator, RecursiveIteratorIterator, RecursiveRegexIterator, RecursiveTreeIterator, RegexIterator, SQLiteResult, SimpleXMLIterator, SplDoublyLinkedList, SplFileObject, SplFixedArray, SplHeap, SplMaxHeap, SplMinHeap, SplObjectStorage, SplPriorityQueue, SplQueue, SplStack, SplTempFileObject, Symfony\Component\Finder\Iterator\CustomFilterIterator, Symfony\Component\Finder\Iterator\DateRangeFilterIterator, Symfony\Component\Finder\Iterator\DepthRangeFilterIterator, Symfony\Component\Finder\Iterator\ExcludeDirectoryFilterIterator, Symfony\Component\Finder\Iterator\FileTypeFilterIterator, Symfony\Component\Finder\Iterator\FilecontentFilterIterator, Symfony\Component\Finder\Iterator\FilenameFilterIterator, Symfony\Component\Finder\Iterator\FilterIterator, Symfony\Component\Finder\Iterator\MultiplePcreFilterIterator, Symfony\Component\Finder\Iterator\PathFilterIterator, Symfony\Component\Finder\Iterator\RecursiveDirectoryIterator, Symfony\Component\Finder\Iterator\SizeRangeFilterIterator, Symfony\Component\Finder\Tests\Iterator\InnerNameIterator, Symfony\Component\Finder\Tests\Iterator\InnerSizeIterator, Symfony\Component\Finder\Tests\Iterator\InnerTypeIterator, Symfony\Component\Finder\Tests\Iterator\Iterator, Symfony\Component\Finder\Tests\Iterator\MockFileListIterator, Symfony\Component\Finder\Tests\Iterator\TestMultiplePcreFilterIterator, TestIterator, TestIterator2.

            $values->next();

It seems like you code against a concrete implementation and not the interface Traversable as the method next() does only exist in the following implementations of said interface: AppendIterator, ArrayIterator, CachingIterator, CallbackFilterIterator, DirectoryIterator, DoctrineTest\InstantiatorTestAsset\PharAsset, DusanKasan\Knapsack\Collection, EmptyIterator, File_Iterator, FilesystemIterator, FilterIterator, Generator, GlobIterator, HttpMessage, HttpRequestPool, Imagick, ImagickPixelIterator, InfiniteIterator, Issue523, IteratorIterator, LimitIterator, MongoCommandCursor, MongoCursor, MongoGridFSCursor, MultipleIterator, NoRewindIterator, PDepend\Input\Iterator, PDepend\Metrics\AnalyzerIterator, PDepend\Source\AST\ASTArtifactList, PDepend\Source\AST\ASTClassOrInterfaceReferenceIterator, PHPUnit_Runner_Filter_GroupFilterIterator, PHPUnit_Runner_Filter_Group_Exclude, PHPUnit_Runner_Filter_Group_Include, PHPUnit_Runner_Filter_Test, PHPUnit_Util_TestSuiteIterator, PHP_CodeCoverage_Report_Node_Iterator, PHP_Token_Stream, ParentIterator, Phar, PharData, RecursiveArrayIterator, RecursiveCachingIterator, RecursiveCallbackFilterIterator, RecursiveDirectoryIterator, RecursiveFilterIterator, RecursiveIteratorIterator, RecursiveRegexIterator, RecursiveTreeIterator, RegexIterator, SQLiteResult, SimpleXMLIterator, SplDoublyLinkedList, SplFileObject, SplFixedArray, SplHeap, SplMaxHeap, SplMinHeap, SplObjectStorage, SplPriorityQueue, SplQueue, SplStack, SplTempFileObject, Symfony\Component\Finder\Iterator\CustomFilterIterator, Symfony\Component\Finder\Iterator\DateRangeFilterIterator, Symfony\Component\Finder\Iterator\DepthRangeFilterIterator, Symfony\Component\Finder\Iterator\ExcludeDirectoryFilterIterator, Symfony\Component\Finder\Iterator\FileTypeFilterIterator, Symfony\Component\Finder\Iterator\FilecontentFilterIterator, Symfony\Component\Finder\Iterator\FilenameFilterIterator, Symfony\Component\Finder\Iterator\FilterIterator, Symfony\Component\Finder\Iterator\MultiplePcreFilterIterator, Symfony\Component\Finder\Iterator\PathFilterIterator, Symfony\Component\Finder\Iterator\RecursiveDirectoryIterator, Symfony\Component\Finder\Iterator\SizeRangeFilterIterator, Symfony\Component\Finder\Tests\Iterator\InnerNameIterator, Symfony\Component\Finder\Tests\Iterator\InnerSizeIterator, Symfony\Component\Finder\Tests\Iterator\InnerTypeIterator, Symfony\Component\Finder\Tests\Iterator\Iterator, Symfony\Component\Finder\Tests\Iterator\MockFileListIterator, Symfony\Component\Finder\Tests\Iterator\TestMultiplePcreFilterIterator, TestIterator, TestIterator2.

        }

        if ($strict && ($keys->valid() || $values->valid())) {

It seems like you code against a concrete implementation and not the interface Traversable as the method valid() does only exist in the following implementations of said interface: AppendIterator, ArrayIterator, CachingIterator, CallbackFilterIterator, DirectoryIterator, DoctrineTest\InstantiatorTestAsset\PharAsset, DusanKasan\Knapsack\Collection, EmptyIterator, File_Iterator, FilesystemIterator, FilterIterator, Generator, GlobIterator, HttpMessage, HttpRequestPool, Imagick, ImagickPixelIterator, InfiniteIterator, Issue523, IteratorIterator, LimitIterator, MongoCommandCursor, MongoCursor, MongoGridFSCursor, MultipleIterator, NoRewindIterator, PDepend\Input\Iterator, PDepend\Metrics\AnalyzerIterator, PDepend\Source\AST\ASTArtifactList, PDepend\Source\AST\ASTClassOrInterfaceReferenceIterator, PHPUnit_Runner_Filter_GroupFilterIterator, PHPUnit_Runner_Filter_Group_Exclude, PHPUnit_Runner_Filter_Group_Include, PHPUnit_Runner_Filter_Test, PHPUnit_Util_TestSuiteIterator, PHP_CodeCoverage_Report_Node_Iterator, PHP_Token_Stream, ParentIterator, Phar, PharData, RecursiveArrayIterator, RecursiveCachingIterator, RecursiveCallbackFilterIterator, RecursiveDirectoryIterator, RecursiveFilterIterator, RecursiveIteratorIterator, RecursiveRegexIterator, RecursiveTreeIterator, RegexIterator, SQLiteResult, SimpleXMLIterator, SplDoublyLinkedList, SplFileObject, SplFixedArray, SplHeap, SplMaxHeap, SplMinHeap, SplObjectStorage, SplPriorityQueue, SplQueue, SplStack, SplTempFileObject, Symfony\Component\Finder\Iterator\CustomFilterIterator, Symfony\Component\Finder\Iterator\DateRangeFilterIterator, Symfony\Component\Finder\Iterator\DepthRangeFilterIterator, Symfony\Component\Finder\Iterator\ExcludeDirectoryFilterIterator, Symfony\Component\Finder\Iterator\FileTypeFilterIterator, Symfony\Component\Finder\Iterator\FilecontentFilterIterator, Symfony\Component\Finder\Iterator\FilenameFilterIterator, Symfony\Component\Finder\Iterator\FilterIterator, Symfony\Component\Finder\Iterator\MultiplePcreFilterIterator, Symfony\Component\Finder\Iterator\PathFilterIterator, Symfony\Component\Finder\Iterator\RecursiveDirectoryIterator, Symfony\Component\Finder\Iterator\SizeRangeFilterIterator, Symfony\Component\Finder\Tests\Iterator\InnerNameIterator, Symfony\Component\Finder\Tests\Iterator\InnerSizeIterator, Symfony\Component\Finder\Tests\Iterator\InnerTypeIterator, Symfony\Component\Finder\Tests\Iterator\Iterator, Symfony\Component\Finder\Tests\Iterator\MockFileListIterator, Symfony\Component\Finder\Tests\Iterator\TestMultiplePcreFilterIterator, TestIterator, TestIterator2.

            throw new ItemNotFound;
        }
    };

    $collection = new Collection($generatorFactory);

    if ($strict) {
        $collection = realize($collection);
    }

    return $collection;
}

/**
 * Returns a lazy collection without the items associated to any of the keys from $keys.
 *
 * @param array|Traversable $collection
 * @param array|Traversable $keys
 * @return Collection
 */
function except($collection, $keys)
{
    $keys = toArray(values($keys));

    return reject(
        $collection,
        function ($value, $key) use ($keys) {
            return in_array($key, $keys);
        }
    );
}

/**
 * Returns a lazy collection of items associated to any of the keys from $keys.
 *
 * @param array|Traversable $collection
 * @param array|Traversable $keys
 * @return Collection
 */
function only($collection, $keys)
{
    $keys = toArray(values($keys));

    return filter(
        $collection,
        function ($value, $key) use ($keys) {
            return in_array($key, $keys);
        }
    );
}

/**
 * Returns a lazy collection of items that are in $collection but are not in any of the other arguments. Note that the
 * ...$collections are iterated non-lazily.
 *
 * @param array|Traversable $collection
 * @param array|Traversable ...$collections
 * @return Collection
 */
function difference($collection, ...$collections)
{
    $valuesToCompare = toArray(values(concat(...$collections)));
    $generatorFactory = function () use ($collection, $valuesToCompare) {
        foreach ($collection as $key => $value) {
            if (!in_array($value, $valuesToCompare)) {
                yield $key => $value;
            }
        }
    };

    return new Collection($generatorFactory);
}

/**
 * Returns a lazy collection where keys and values are flipped.
 *
 * @param array|Traversable $collection
 * @return Collection
 */
function flip($collection)
{
    $generatorFactory = function () use ($collection) {
        foreach ($collection as $key => $value) {
            yield $value => $key;
        }
    };

    return new Collection($generatorFactory);
}

/**
 * @param array|Traversable $collection
 * @param mixed $key
 * @return bool
 */
function has($collection, $key)

The parameter $key is not used and could be removed.

{
    try {
        get($collection, $key);
        return true;
    } catch (ItemNotFound $key) {
        return false;
    }
}