Add findKey() method to return index of first element matching a callable

.gitattributes

@@ -18,3 +18,4 @@
 /phpunit.xml.dist export-ignore
 /psalm.xml export-ignore
 /renovate.json export-ignore
+/AGENTS.md export-ignore

AGENTS.md

@@ -0,0 +1,182 @@
+# AGENTS.md — Copilot Agent Knowledge Base for voku/Arrayy
+
+This file documents project conventions, architecture, and findings from agent interactions. It is intended to help AI coding agents work effectively in this repository.
+
+---
+
+## Project Overview
+
+- **Package**: `voku/arrayy`
+- **Description**: Array manipulation library for PHP
+- **License**: MIT
+- **Maintainer**: Lars Moelleken
+- **PHP requirement**: `>=8.0.0`
+- **Autoload namespace**: `Arrayy\` → `src/`
+- **Helper function namespace**: `src/Create.php` (defines global `a()` helper)
+
+---
+
+## Directory Structure
+
+```
+src/
+  Arrayy.php                          # Main class (~8000+ lines), all array methods live here
+  ArrayyStrict.php                    # Strict-typed subclass of Arrayy
+  ArrayyMeta.php                      # Meta/magic property helper (~67 lines)
+  ArrayyIterator.php                  # Custom iterator
+  ArrayyRewindableGenerator.php       # Generator-backed iteration
+  ArrayyRewindableExtendedGenerator.php
+  Create.php                          # Global helper functions (a(), etc.)
+  StaticArrayy.php                    # Static façade using __callStatic
+  Collection/                         # Collection base classes
+  Type/                               # Typed collections (StringCollection, ArrayCollection, etc.)
+  TypeCheck/                          # Type-checking utilities
+  Mapper/                             # JSON mapper
+
+tests/
+  ArrayyTest.php                      # Main test file (~6500+ lines), all tests in one class
+
+build/
+  generate_docs.php                   # Regenerates README.md from src/Arrayy.php + build/docs/base.md
+  composer.json                       # Requires voku/php-readme-helper for doc generation
+  docs/base.md                        # Base template for README generation
+
+examples/
+  JsonResponseDataExample.php
+```
+
+---
+
+## Running Tests
+
+```bash
+php vendor/bin/phpunit --no-coverage
+```
+
+- Tests use PHPUnit (supports `~6.0 || ~7.0 || ~9.0`)
+- All tests are in a single class `Arrayy\tests\ArrayyTest` in `tests/ArrayyTest.php`
+- Known pre-existing failures (unrelated to feature work): 2 errors (`array_sum` on strings) + 1 failure (sigma case)
+- Run targeted tests with `--filter "testMethodName"` to speed up iteration
+
+---
+
+## Regenerating README.md
+
+The README is **auto-generated** — do not edit it manually.
+
+```bash
+# Install build dependencies first (only needed once):
+cd build && composer install && cd ..
+
+# Also install main project dependencies (needed by generate_docs.php):
+composer install --no-dev
+
+# Generate README:
+php build/generate_docs.php
+```
+
+This reads `src/Arrayy.php` docblocks and `build/docs/base.md`, then writes `README.md`.
+
+---
+
+## Adding New Methods
+
+All public methods in `src/Arrayy.php` follow this pattern:
+
+1. **PHPDoc block** with:
+   - Description
+   - `EXAMPLE: <code>...</code>` block (used in README generation)
+   - `@param` and `@return` tags
+   - `@phpstan-param` / `@phpstan-return` for generics (`TKey`, `T`)
+   - `@psalm-mutation-free` if the method is immutable
+
+2. **Method signature** — use `self` return type for immutable methods, `$this` for mutable
+
+3. **Implementation** — prefer `$this->getGenerator()` for lazy iteration
+
+4. **Test** in `tests/ArrayyTest.php`:
+   - Add a `{method}Provider()` data provider method
+   - Add a `test{Method}()` test method with `@dataProvider`
+   - Place tests alphabetically relative to similar methods
+
+5. **Rebuild README** by running `php build/generate_docs.php`
+
+---
+
+## Key Patterns
+
+### Immutable vs Mutable
+- Immutable methods return `static` and create a new instance via `static::create()`
+- Mutable methods return `$this` and modify `$this->array` directly
+
+### Generator-backed iteration
+- Use `$this->getGenerator()` instead of `$this->array` to support lazy/generator-backed arrays
+- Call `$this->generatorToArray()` first if you need direct array access
+
+### Template types
+```php
+/**
+ * @template TKey of array-key
+ * @template T
+ */
+class Arrayy ...
+```
+Use `@phpstan-param T $value` and `@phpstan-return TKey|false` etc. for PHPStan generics.
+
+### StaticArrayy
+`StaticArrayy` uses `__callStatic` to delegate all calls to an `Arrayy` instance — no changes needed there when adding instance methods.
+
+---
+
+## Findings from Task: `findKey()` (Issue #139)
+
+**Problem**: No method existed to find the *key/index* of an array element by a predicate callable. The existing methods were:
+- `find(\Closure $closure)` — returns the matching **value**, or `false`
+- `searchIndex($value)` — returns the key for an exact **value** match
+- `indexOf($value)` — alias for `searchIndex()`
+
+**Solution**: Added `findKey(\Closure $closure)` in `src/Arrayy.php` (after `find()`):
+- Accepts `\Closure` with signature `($value, $key): bool`
+- Iterates via `$this->getGenerator()` for generator compatibility
+- Returns the key of the first matching element, or `false`
+
+**Usage examples**:
+```php
+// Find index of minimum value
+$minIdx = a([3, 1, 4, 1, 5, 9])->findKey(fn($v) => $v === 1); // 1
+
+// Find by key
+a(['a' => 10, 'b' => 20])->findKey(fn($v, $k) => $k === 'b'); // 'b'
+
+// Returns false when no match
+a([1, 2, 3])->findKey(fn($v) => $v === 99); // false
+```
+
+**Test coverage added**:
+- `findKeyProvider()` — 11 data-driven cases covering empty arrays, booleans, integers, floats, strings, string keys, and no-match
+- `testFindKey()` — value-based matching via data provider
+- `testFindKeyWithKeyParameter()` — verifies `$key` is correctly forwarded to the closure
+
+---
+
+## .gitattributes
+
+Files excluded from Composer distribution (`export-ignore`):
+- `/build`, `/examples`, `/tests`
+- Config files: `.editorconfig`, `.scrutinizer.yml`, `.styleci.yml`, etc.
+- `AGENTS.md` (this file)
+
+---
+
+## Dependencies
+
+### Runtime
+- `symfony/polyfill-mbstring` ~1.0
+- `phpdocumentor/reflection-docblock` ~6.0
+
+### Dev
+- `phpunit/phpunit` ~6.0 || ~7.0 || ~9.0
+- `phpstan/phpstan` ^2.0
+
+### Build (doc generation only, in `build/`)
+- `voku/php-readme-helper` ~0.6