Merge pull request #18 from Innmind/optimize-takeEnd

Optimize the lazy Sequence::takeEnd()

Author: Baptouuuu

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Changed
+
+- A lazy `Sequence::takeEnd()` no longer loads the whole sequence in memory, only the number of elements taken + 1.
+
 ## 5.4.0 - 2024-05-29
 
 ### Added

src/Sequence/Lazy.php

@@ -469,9 +469,27 @@ static function(RegisterCleanup $register) use ($values, $size): \Generator {
      */
     public function takeEnd(int $size): Implementation
     {
-        // this cannot be optimised as the whole generator needs to be loaded
-        // in order to know the elements to drop
-        return $this->load()->takeEnd($size);
+        $values = $this->values;
+
+        return new self(
+            static function(RegisterCleanup $register) use ($values, $size): \Generator {
+                $buffer = [];
+                $count = 0;
+
+                foreach ($values($register) as $value) {
+                    $buffer[] = $value;
+                    ++$count;
+
+                    if ($count > $size) {
+                        \array_shift($buffer);
+                    }
+                }
+
+                foreach ($buffer as $value) {
+                    yield $value;
+                }
+            },
+        );
     }
 
     /**