Duplicate code is one of the most pungent code smells. A rule that is often used is to re-structure code once it is duplicated in three or more places.
Common duplication problems, and corresponding solutions are:
| 1 | <?php declare(strict_types = 1);  | 
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| 8 | class Expression  | 
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| 9 | { | 
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| 10 | /** @var array */  | 
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| 11 | private $filters;  | 
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| 12 | |||
| 13 | /**  | 
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| 14 | * Expression constructor.  | 
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| 15 | */  | 
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| 16 | 24 | public function __construct()  | 
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| 20 | |||
| 21 | /**  | 
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| 22 | * Adds and filter  | 
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| 23 | *  | 
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| 24 | * @param array|Expression $expression  | 
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| 25 | *  | 
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| 26 | * @return $this|Expression  | 
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| 27 | */  | 
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| 28 | View Code Duplication | public function and($expression): Expression  | 
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| 39 | |||
| 40 | /**  | 
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| 41 | * @param string $operator  | 
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| 42 | */  | 
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| 43 | 19 | private function prepareFilterIndex(string $operator)  | 
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| 49 | |||
| 50 | /**  | 
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| 51 | * @param $expressions  | 
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| 52 | *  | 
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| 53 | * @return array  | 
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| 54 | */  | 
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| 55 | 10 | private function mapExpressions($expressions): array  | 
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| 64 | |||
| 65 | /**  | 
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| 66 | * Returns the filters generated by the expression  | 
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| 67 | *  | 
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| 68 | * @return array  | 
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| 69 | */  | 
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| 70 | 24 | public function getExpressionFilters(): array  | 
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| 74 | |||
| 75 | /**  | 
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| 76 | * Adds or filter  | 
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| 77 | *  | 
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| 78 | * @param array|Expression $expression  | 
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| 79 | *  | 
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| 80 | * @return $this|Expression  | 
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| 81 | */  | 
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| 82 | 3 | View Code Duplication | public function or ($expression): Expression  | 
            
| 93 | |||
| 94 | /**  | 
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| 95 | * Adds eq filter  | 
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| 96 | *  | 
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| 97 | * @param string $field  | 
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| 98 | * @param string $value  | 
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| 99 | *  | 
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| 100 | * @return $this|Expression  | 
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| 101 | */  | 
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| 102 | 2 | public function equal(string $field, $value): Expression  | 
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| 110 | |||
| 111 | /**  | 
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| 112 | * Adds ne filter  | 
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| 113 | *  | 
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| 114 | * @param string $field  | 
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| 115 | * @param string $value  | 
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| 116 | *  | 
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| 117 | * @return $this|Expression  | 
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| 118 | */  | 
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| 119 | 4 | public function notEqual(string $field, $value): Expression  | 
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| 127 | |||
| 128 | /**  | 
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| 129 | * Adds in filter  | 
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| 130 | *  | 
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| 131 | * @param string $field  | 
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| 132 | * @param array $values  | 
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| 133 | *  | 
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| 134 | * @return $this|Expression  | 
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| 135 | */  | 
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| 136 | 2 | public function in(string $field, array $values): Expression  | 
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| 144 | |||
| 145 | /**  | 
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| 146 | * Adds notIn filter  | 
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| 147 | *  | 
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| 148 | * @param string $field  | 
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| 149 | * @param array $values  | 
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| 150 | *  | 
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| 151 | * @return $this|Expression  | 
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| 152 | */  | 
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| 153 | 2 | public function notIn(string $field, array $values): Expression  | 
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| 161 | |||
| 162 | |||
| 163 | /**  | 
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| 164 | * @param string $operation  | 
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| 165 | * @param mixed $value  | 
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| 166 | *  | 
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| 167 | * @return array  | 
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| 168 | */  | 
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| 169 | 10 | private function operationExpression(string $operation, $value): array  | 
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| 173 | }  | 
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| 174 | 
This check looks from parameters that have been defined for a function or method, but which are not used in the method body.