Complex classes like Collection often do a lot of different things. To break such a class down, we need to identify a cohesive component within that class. A common approach to find such a component is to look for fields/methods that share the same prefixes, or suffixes. You can also have a look at the cohesion graph to spot any un-connected, or weakly-connected components.
Once you have determined the fields that belong together, you can apply the Extract Class refactoring. If the component makes sense as a sub-class, Extract Subclass is also a candidate, and is often faster.
While breaking up the class, it is a good idea to analyze how other classes use Collection, and based on these observations, apply Extract Interface, too.
| 1 | <?php  | 
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| 5 | class Collection implements \Iterator, \ArrayAccess, \Serializable, \Countable  | 
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| 6 | { | 
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| 7 | protected $array = null;  | 
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| 8 | protected $stack = [];  | 
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| 9 | protected $iterator = null;  | 
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| 10 | protected $key = null;  | 
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| 11 | protected $val = null;  | 
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| 12 | |||
| 13 | 4 | protected static function rangeGenerator($low, $high, $step = 1)  | 
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| 14 |     { | 
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| 15 | 4 | $k = -1;  | 
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| 16 | 4 |         for ($i = $low; $i <= $high; $i += $step) { | 
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| 17 | 4 | yield ++$k => $i;  | 
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| 18 | }  | 
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| 19 | 4 | }  | 
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| 20 | /**  | 
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| 21 | * Create a collection based on a range generator  | 
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| 22 | * @param int|float $low start value  | 
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| 23 | * @param int|float $high end value  | 
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| 24 | * @param int|float $step increment  | 
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| 25 | * @return Collection  | 
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| 26 | */  | 
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| 27 | 4 | public static function range($low, $high, $step = 1) : Collection  | 
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| 28 |     { | 
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| 29 | 4 | return new static(static::rangeGenerator($low, $high, $step));  | 
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| 30 | }  | 
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| 31 | /**  | 
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| 32 | * A static alias of the __constructor  | 
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| 33 | * @param mixed $input Anything iterable  | 
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| 34 | * @return Collection  | 
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| 35 | */  | 
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| 36 | 51 | public static function from($input) : Collection  | 
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| 37 |     { | 
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| 38 | 51 | return new static($input);  | 
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| 39 | }  | 
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| 40 | /**  | 
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| 41 | * Create an instance  | 
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| 42 | * @param mixed $input Anything iterable  | 
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| 43 | */  | 
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| 44 | 55 | public function __construct($input = [])  | 
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| 45 |     { | 
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| 46 | 55 |         if (is_object($input)) { | 
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| 47 | 28 |             if ($input instanceof \Iterator) { | 
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| 48 | 5 | $this->array = $input;  | 
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| 49 | 5 | $this->iterator = $input;  | 
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| 50 | 23 |             } else if ($input instanceof self) { | 
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| 51 | $this->array = $input->toArray();  | 
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| 52 |             } else { | 
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| 53 | 23 | $input = get_object_vars($input);  | 
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| 54 | }  | 
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| 55 | }  | 
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| 56 | 55 |         if (is_array($input)) { | 
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| 57 | 51 | $this->array = new \ArrayObject($input);  | 
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| 58 | 51 | $this->iterator = $this->array->getIterator();  | 
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| 59 | }  | 
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| 60 | 55 | }  | 
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| 61 | 1 | public function __clone()  | 
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| 62 |     { | 
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| 63 | 1 | return new static($this->toArray());  | 
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| 64 | }  | 
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| 65 | 1 | public function __toString()  | 
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| 66 |     { | 
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| 67 | 1 |         return implode(', ', $this->toArray()); | 
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| 68 | }  | 
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| 69 | 1 |     public function serialize() { | 
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| 70 | 1 | return serialize($this->toArray());  | 
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| 71 | }  | 
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| 72 | 1 |     public function unserialize($array) { | 
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| 73 | 1 | $this->array = new \ArrayObject(unserialize($array));  | 
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| 74 | 1 | $this->stack = [];  | 
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| 75 | 1 | $this->iterator = $this->array->getIterator();  | 
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| 76 | 1 | }  | 
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| 77 | |||
| 78 | /**  | 
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| 79 | * Applies all pending operations  | 
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| 80 | * @return $this  | 
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| 81 | */  | 
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| 82 | 46 | public function squash() : Collection  | 
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| 83 |     { | 
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| 84 | 46 | $this->array = new \ArrayObject(iterator_to_array($this));  | 
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| 85 | 46 | $this->stack = [];  | 
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| 86 | 46 | $this->iterator = $this->array->getIterator();  | 
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| 87 | 46 | return $this;  | 
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| 88 | }  | 
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| 89 | /**  | 
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| 90 | * Get an actual array from the collection  | 
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| 91 | * @return array  | 
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| 92 | */  | 
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| 93 | 44 | public function toArray() : array  | 
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| 94 |     { | 
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| 95 | 44 | $this->squash();  | 
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| 96 | 44 | return $this->array->getArrayCopy();  | 
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| 97 | }  | 
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| 98 | /**  | 
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| 99 | * Gets the first value in the collection or null if empty  | 
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| 100 | * @return mixed  | 
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| 101 | */  | 
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| 102 | 3 | public function value()  | 
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| 103 |     { | 
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| 104 | 3 |         foreach ($this as $v) { | 
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| 105 | 3 | return $v;  | 
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| 106 | }  | 
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| 107 | 1 | return null;  | 
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| 108 | }  | 
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| 109 | |||
| 110 | // iterator  | 
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| 111 | 52 | public function key()  | 
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| 112 |     { | 
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| 113 | 52 | return $this->key;  | 
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| 114 | }  | 
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| 115 | 55 | public function current()  | 
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| 116 |     { | 
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| 117 | 55 | return $this->val;  | 
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| 118 | }  | 
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| 119 | 55 | public function rewind()  | 
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| 120 |     { | 
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| 121 | 55 | return $this->iterator->rewind();  | 
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| 122 | }  | 
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| 123 | 54 | public function next()  | 
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| 124 |     { | 
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| 125 | 54 | return $this->iterator->next();  | 
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| 126 | }  | 
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| 127 | 55 | public function valid()  | 
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| 128 |     { | 
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| 129 | 55 |         while ($this->iterator->valid()) { | 
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| 130 | 55 | $this->val = $this->iterator->current();  | 
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| 131 | 55 | $this->key = $this->iterator->key();  | 
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| 132 | 55 | $con = false;  | 
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| 133 | 55 |             foreach ($this->stack as $action) { | 
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| 134 | 12 |                 if ($action[0] === 'filter') { | 
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| 135 | 8 |                     if (!call_user_func($action[1], $this->val, $this->key, $this)) { | 
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| 136 | 8 | $con = true;  | 
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| 137 | 8 | break;  | 
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| 138 | }  | 
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| 139 | }  | 
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| 140 | 12 |                 if ($action[0] === 'map') { | 
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| 141 | 5 | $this->val = call_user_func($action[1], $this->val, $this->key, $this);  | 
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| 142 | }  | 
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| 143 | 12 |                 if ($action[0] === 'mapKey') { | 
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| 144 | 12 | $this->key = call_user_func($action[1], $this->val, $this->key, $this);  | 
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| 145 | }  | 
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| 146 | }  | 
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| 147 | 55 |             if ($con) { | 
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| 148 | 8 | $this->iterator->next();  | 
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| 149 | 8 | continue;  | 
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| 150 | }  | 
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| 151 | 55 | return true;  | 
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| 152 | }  | 
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| 153 | 55 | return false;  | 
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| 154 | }  | 
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| 155 | |||
| 156 | // array access  | 
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| 157 | 1 | public function offsetGet($offset)  | 
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| 158 |     { | 
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| 159 | 1 | return $this->squash()->iterator->offsetGet($offset);  | 
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| 160 | }  | 
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| 161 | 1 | public function offsetExists($offset)  | 
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| 162 |     { | 
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| 163 | 1 | return $this->squash()->iterator->offsetExists($offset);  | 
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| 164 | }  | 
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| 165 | 1 | public function offsetUnset($offset)  | 
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| 166 |     { | 
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| 167 | 1 | return $this->squash()->iterator->offsetUnset($offset);  | 
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| 168 | }  | 
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| 169 | 1 | public function offsetSet($offset, $value)  | 
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| 170 |     { | 
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| 171 | 1 | return $this->squash()->iterator->offsetSet($offset, $value);  | 
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| 172 | }  | 
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| 173 | /**  | 
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| 174 | * Get the collection length  | 
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| 175 | * @return int  | 
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| 176 | */  | 
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| 177 | 1 | public function count()  | 
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| 178 |     { | 
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| 179 | 1 | $this->squash();  | 
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| 180 | 1 | return $this->array->count();  | 
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| 181 | }  | 
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| 182 | |||
| 183 | // mutators  | 
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| 184 | /**  | 
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| 185 | * Filter values from the collection based on a predicate. The callback will receive the value, key and collection  | 
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| 186 | * @param callable $iterator the predicate  | 
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| 187 | * @return $this  | 
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| 188 | */  | 
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| 189 | 8 | public function filter(callable $iterator) : Collection  | 
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| 190 |     { | 
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| 191 | 8 | $this->stack[] = [ 'filter', $iterator ];  | 
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| 192 | 8 | return $this;  | 
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| 193 | }  | 
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| 194 | /**  | 
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| 195 | * Pass all values of the collection through a mutator callable, which will receive the value, key and collection  | 
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| 196 | * @param callable $iterator the mutator  | 
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| 197 | * @return $this  | 
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| 198 | */  | 
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| 199 | 5 | public function map(callable $iterator) : Collection  | 
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| 200 |     { | 
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| 201 | 5 | $this->stack[] = [ 'map', $iterator ];  | 
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| 202 | 5 | return $this;  | 
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| 203 | }  | 
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| 204 | /**  | 
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| 205 | * Pass all values of the collection through a key mutator callable, which will receive the value, key and collection  | 
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| 206 | * @param callable $iterator the mutator  | 
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| 207 | * @return $this  | 
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| 208 | */  | 
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| 209 | 1 | public function mapKey(callable $iterator) : Collection  | 
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| 210 |     { | 
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| 211 | 1 | $this->stack[] = [ 'mapKey', $iterator ];  | 
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| 212 | 1 | return $this;  | 
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| 213 | }  | 
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| 214 | /**  | 
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| 215 | * Clone the current collection and return it.  | 
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| 216 | * @return Collection  | 
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| 217 | */  | 
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| 218 | public function clone() : Collection  | 
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| 222 | /**  | 
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| 223 | * Remove all falsy values from the collection (uses filter internally).  | 
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| 224 | * @return $this  | 
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| 225 | */  | 
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| 226 | 1 | public function compact() : Collection  | 
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| 232 | /**  | 
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| 233 | * Exclude all listed values from the collection (uses filter internally).  | 
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| 234 | * @param iterable $values the values to exclude  | 
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| 235 | * @return $this  | 
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| 236 | */  | 
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| 237 | 3 | public function difference($values) : Collection  | 
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| 250 | /**  | 
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| 251 | * Append more values to the collection  | 
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| 252 | * @param iterable $source the values to add  | 
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| 253 | * @return Collection  | 
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| 254 | */  | 
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| 255 | 2 | public function extend($source) : Collection  | 
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| 262 | /**  | 
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| 263 | * Append more values to the collection  | 
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| 264 | * @param iterable $source the values to add  | 
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| 265 | * @return Collection  | 
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| 266 | */  | 
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| 267 | 1 | public function merge($source) : Collection  | 
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| 271 | /**  | 
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| 272 | * Perform a shallow flatten of the collection  | 
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| 273 | * @return Collection  | 
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| 274 | */  | 
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| 275 | 1 | public function flatten() : Collection  | 
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| 284 | /**  | 
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| 285 | * Group by a key (if a callable is used - return the value to group by)  | 
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| 286 | * @param string|callable $iterator the key to group by  | 
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| 287 | * @return Collection  | 
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| 288 | */  | 
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| 289 | 1 | public function groupBy($iterator) : Collection  | 
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| 298 | /**  | 
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| 299 | * Get the first X items from the collection  | 
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| 300 | * @param int $count the number of items to include (defaults to 1)  | 
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| 301 | * @return Collection  | 
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| 302 | */  | 
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| 303 | 1 | public function first(int $count = 1) : Collection  | 
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| 315 | /**  | 
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| 316 | * Get the first X items from the collection  | 
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| 317 | * @param int $count the number of items to include (defaults to 1)  | 
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| 318 | * @return Collection  | 
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| 319 | */  | 
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| 320 | 1 | public function head(int $count = 1) : Collection  | 
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| 324 | /**  | 
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| 325 | * Get the last X items from the collection  | 
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| 326 | * @param int $count the number of items to include (defaults to 1)  | 
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| 327 | * @return Collection  | 
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| 328 | */  | 
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| 329 | 2 | public function last(int $count = 1) : Collection  | 
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| 334 | /**  | 
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| 335 | * Get the first X items from the collection  | 
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| 336 | * @param int $count the number of items to include (defaults to 1)  | 
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| 337 | * @return Collection  | 
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| 338 | */  | 
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| 339 | 1 | public function tail(int $count = 1) : Collection  | 
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| 343 | /**  | 
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| 344 | * Get all but the last X items from the collection  | 
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| 345 | * @param int $count the number of items to exclude (defaults to 1)  | 
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| 346 | * @return Collection  | 
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| 347 | */  | 
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| 348 | 1 | public function initial(int $count = 1) : Collection  | 
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| 353 | /**  | 
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| 354 | * Get all but the first X items from the collection  | 
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| 355 | * @param int $count the number of items to exclude (defaults to 1)  | 
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| 356 | * @return Collection  | 
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| 357 | */  | 
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| 358 | 1 | public function rest(int $count = 1) : Collection  | 
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| 359 |     { | 
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| 360 | 1 | $new = $this->toArray();  | 
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| 361 | 1 | return new static(array_slice($new, $count));  | 
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| 362 | }  | 
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| 363 | /**  | 
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| 364 | * Execute a callable for each item in the collection (does not modify the collection)  | 
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| 365 | * @param callable $iterator the callable to execute  | 
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| 366 | * @return $this  | 
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| 367 | */  | 
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| 368 | 1 | public function each(callable $iterator) : Collection  | 
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| 375 | /**  | 
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| 376 | * Execute a callable for each item in the collection (does not modify the collection)  | 
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| 377 | * @param callable $iterator the callable to execute  | 
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| 378 | * @return $this  | 
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| 379 | */  | 
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| 380 | 1 | public function invoke(callable $iterator) : Collection  | 
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| 384 | /**  | 
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| 385 | * Get all the collection keys  | 
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| 386 | * @return $this  | 
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| 387 | */  | 
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| 388 | public function keys() : Collection  | 
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| 392 | /**  | 
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| 393 | * Pluck a value from each object (uses map internally)  | 
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| 394 | * @param string|int $key the key to extract  | 
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| 395 | * @return $this  | 
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| 396 | */  | 
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| 397 | 2 | public function pluck($key) : Collection  | 
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| 405 | /**  | 
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| 406 | * Intersect the collection with another iterable (uses filter internally)  | 
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| 407 | * @param interable $values the data to intersect with  | 
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| 408 | * @return $this  | 
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| 409 | */  | 
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| 410 | 1 | public function intersection($values) : Collection  | 
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| 423 | /**  | 
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| 424 | * Reject values on a given predicate (opposite of filter)  | 
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| 425 | * @param callable $iterator the predicate  | 
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| 426 | * @return $this  | 
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| 427 | */  | 
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| 428 | 1 | public function reject(callable $iterator) : Collection  | 
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| 434 | /**  | 
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| 435 | * Shuffle the values in the collection  | 
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| 436 | * @return Collection  | 
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| 437 | */  | 
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| 438 | 1 | public function shuffle() : Collection  | 
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| 449 | /**  | 
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| 450 | * Sort the collection using a standard sorting function  | 
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| 451 | * @param callable $iterator the sort function (must return -1, 0 or 1)  | 
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| 452 | * @return Collection  | 
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| 453 | */  | 
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| 454 | 1 | public function sortBy(callable $iterator) : Collection  | 
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| 460 | /**  | 
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| 461 | * Inspect the whole collection (as an array) mid-chain  | 
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| 462 | * @param callable $iterator the callable to execute  | 
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| 463 | * @return $this  | 
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| 464 | */  | 
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| 465 | 1 | public function tap(callable $iterator) : Collection  | 
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| 470 | /**  | 
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| 471 | * Modify the whole collection (as an array) mid-chain  | 
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| 472 | * @param callable $iterator the callable to execute  | 
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| 473 | * @return Collection  | 
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| 474 | */  | 
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| 475 | 1 | public function thru(callable $iterator) : Collection  | 
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| 476 |     { | 
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| 477 | 1 | $temp = $this->toArray();  | 
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| 478 | 1 | $rslt = call_user_func($iterator, $temp);  | 
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| 479 | 1 | return new static($rslt);  | 
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| 480 | }  | 
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| 481 | /**  | 
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| 482 | * Leave only unique items in the collection  | 
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| 483 | * @return Collection  | 
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| 484 | */  | 
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| 485 | 3 | public function unique() : Collection  | 
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| 496 | /**  | 
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| 497 | * Get only the values of the collection  | 
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| 498 | * @return Collection  | 
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| 499 | */  | 
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| 500 | 3 | public function values() : Collection  | 
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| 504 | /**  | 
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| 505 | * Filter items from the collection using key => value pairs  | 
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| 506 | * @param array $properties the key => value to check for in each item  | 
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| 507 | * @param boolean $strict should the comparison be strict  | 
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| 508 | * @return $this  | 
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| 509 | */  | 
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| 510 | public function where(array $properties, $strict = true) : Collection  | 
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| 522 | /**  | 
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| 523 | * Exclude all listed values from the collection (uses filter internally).  | 
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| 524 | * @param iterable $values the values to exclude  | 
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| 525 | * @return $this  | 
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| 526 | */  | 
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| 527 | 2 | public function without($values) : Collection  | 
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| 531 | /**  | 
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| 532 | * Combine all the values from the collection with a key  | 
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| 533 | * @param iterable $keys the keys to use  | 
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| 534 | * @return Collection  | 
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| 535 | */  | 
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| 536 | 2 | public function zip($keys) : Collection  | 
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| 537 |     { | 
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| 538 | 2 |         if (!is_array($keys)) { | 
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| 539 | 1 | $keys = iterator_to_array($keys);  | 
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| 540 | }  | 
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| 541 | 2 | return new static(array_combine($keys, $this->toArray()));  | 
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| 542 | }  | 
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| 543 | /**  | 
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| 544 | * Reverse the collection order  | 
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| 545 | * @return Collection  | 
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| 546 | */  | 
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| 547 | 1 | public function reverse() : Collection  | 
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| 548 |     { | 
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| 549 | 1 | return new static(array_reverse($this->toArray()));  | 
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| 550 | }  | 
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| 551 | |||
| 552 | // accessors  | 
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| 553 | /**  | 
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| 554 | * Do all of the items in the collection match a given criteria  | 
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| 555 | * @param callable $iterator the criteria - should return true / false  | 
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| 556 | * @return bool  | 
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| 557 | */  | 
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| 558 | 1 | public function all(callable $iterator) : bool  | 
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| 567 | /**  | 
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| 568 | * Do any of the items in the collection match a given criteria  | 
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| 569 | * @param callable $iterator the criteria - should return true / false  | 
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| 570 | * @return bool  | 
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| 571 | */  | 
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| 572 | 1 | public function any(callable $iterator) : bool  | 
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| 581 | /**  | 
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| 582 | * Does the collection contain a given value  | 
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| 583 | * @param mixed $needle the value to check for  | 
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| 584 | * @return bool  | 
            ||
| 585 | */  | 
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| 586 | 1 | public function contains($needle) : bool  | 
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| 595 | /**  | 
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| 596 | * Get the first element matching a given criteria (or null)  | 
            ||
| 597 | * @param callable $iterator the filter criteria  | 
            ||
| 598 | * @return mixed  | 
            ||
| 599 | */  | 
            ||
| 600 | 1 | public function find(callable $iterator)  | 
            |
| 609 | /**  | 
            ||
| 610 | * Get all the elements matching a given criteria (with the option to limit the number of results)  | 
            ||
| 611 | * @param callable $iterator the search criteria  | 
            ||
| 612 | * @param int|null $limit optional limit to the number of results (default to null - no limit)  | 
            ||
| 613 | * @return Collection  | 
            ||
| 614 | */  | 
            ||
| 615 | 1 | public function findAll(callable $iterator, int $limit = null) : Collection  | 
            |
| 616 |     { | 
            ||
| 617 | 1 | $res = [];  | 
            |
| 618 | 1 |         foreach ($this as $k => $v) { | 
            |
| 619 | 1 |             if (call_user_func($iterator, $v, $k, $this)) { | 
            |
| 620 | 1 | $res[] = $v;  | 
            |
| 621 | }  | 
            ||
| 622 | 1 |             if ((int)$limit > 0 && count($res) >= $limit) { | 
            |
| 623 | 1 | break;  | 
            |
| 624 | }  | 
            ||
| 625 | }  | 
            ||
| 626 | 1 | return new static($res);  | 
            |
| 627 | }  | 
            ||
| 628 | /**  | 
            ||
| 629 | * Get the key corresponding to a value (or false)  | 
            ||
| 630 | * @param mixed $needle the value to search for  | 
            ||
| 631 | * @return mixed  | 
            ||
| 632 | */  | 
            ||
| 633 | 1 | public function indexOf($needle)  | 
            |
| 634 |     { | 
            ||
| 635 | 1 | return array_search($needle, $this->toArray(), true);  | 
            |
| 636 | }  | 
            ||
| 637 | /**  | 
            ||
| 638 | * Get the last key corresponding to a value (or false)  | 
            ||
| 639 | * @param mixed $needle the value to search for  | 
            ||
| 640 | * @return mixed  | 
            ||
| 641 | */  | 
            ||
| 642 | 1 | public function lastIndexOf($needle)  | 
            |
| 652 | /**  | 
            ||
| 653 | * Get the number of elements in the collection  | 
            ||
| 654 | * @return int  | 
            ||
| 655 | */  | 
            ||
| 656 | 1 | public function size() : int  | 
            |
| 660 | /**  | 
            ||
| 661 | * Get the minimal item in the collection  | 
            ||
| 662 | * @return mixed  | 
            ||
| 663 | */  | 
            ||
| 664 | 1 | public function min()  | 
            |
| 676 | /**  | 
            ||
| 677 | * Get the maximum item in the collection  | 
            ||
| 678 | * @return mixed  | 
            ||
| 679 | */  | 
            ||
| 680 | 1 | public function max()  | 
            |
| 692 | /**  | 
            ||
| 693 | * Does the collection contain a given key  | 
            ||
| 694 | * @param string|int $key the key to check  | 
            ||
| 695 | * @return bool  | 
            ||
| 696 | */  | 
            ||
| 697 | 1 | public function has($key) : bool  | 
            |
| 701 | /**  | 
            ||
| 702 | * Reduce the collection to a single value  | 
            ||
| 703 | * @param callable $iterator the reducer  | 
            ||
| 704 | * @param mixed $initial the initial value  | 
            ||
| 705 | * @return mixed the final value  | 
            ||
| 706 | */  | 
            ||
| 707 | 1 | public function reduce(callable $iterator, $initial = null)  | 
            |
| 711 | /**  | 
            ||
| 712 | * Reduce the collection to a single value, starting from the last element  | 
            ||
| 713 | * @param callable $iterator the reducer  | 
            ||
| 714 | * @param mixed $initial the initial value  | 
            ||
| 715 | * @return mixed the final value  | 
            ||
| 716 | */  | 
            ||
| 717 | 1 | public function reduceRight(callable $iterator, $initial = null)  | 
            |
| 721 | }  | 
            ||
| 722 |