The test is done

.gitignore

@@ -1,6 +1,8 @@
 /node_modules
 /public/hot
 /public/storage
+/public/js
+/public/css
 /storage/*.key
 /vendor
 .idea

app/Http/Controllers/CsvExportController.php

@@ -0,0 +1,51 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Http\Controllers;
+
+use App\Http\Requests\CsvExportRequest;
+use App\Services\Csv\CsvService;
+use App\Services\Csv\Exceptions\InvalidColumnException;
+use App\Services\Csv\Exceptions\InvalidRowException;
+
+class CsvExportController extends Controller
+{
+    /**
+     * @var CsvService
+     */
+    private $service;
+
+    public function __construct(CsvService $service)
+    {
+        $this->service = $service;
+    }
+
+    /**
+     * Converts the user input into a CSV file and streams the file back to the user.
+     */
+    public function convert(CsvExportRequest $request)
+    {
+        $inputDto = $request->getDto();
+
+        try {
+            $csv = $this->service->create($inputDto);
+
+            return response()->streamDownload(function () use ($csv) {
+                echo $csv;
+            }, 'data.csv');
+        } catch (InvalidColumnException $e) {
+            return response()->json([
+                'message' => $e->getMessage(),
+                'code' => $e->getCode(),
+                'row' => $e->getRowIndex(),
+                'column' => $e->getColumnIndex(),
+            ], 400);
+        } catch (InvalidRowException $e) {
+            return response()->json([
+                'message' => $e->getMessage(),
+                'code' => $e->getCode(),
+                'row' => $e->getRowIndex(),
+            ], 400);
+        }
+    }
+}

app/Http/Requests/CsvExportRequest.php

@@ -0,0 +1,38 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Http\Requests;
+
+use App\Services\Csv\InputDto;
+use Illuminate\Foundation\Http\FormRequest;
+
+/**
+ * @property string[] $header
+ * @property string[] $data
+ *
+ * Class CsvExportRequest
+ *
+ * @package App\Http\Requests
+ */
+class CsvExportRequest extends FormRequest
+{
+    /**
+     * @return string[]
+     */
+    public function rules(): array
+    {
+        return [
+            'header' => ['required', 'array'],
+            'header.*' => ['string', 'nullable'],
+
+            'data' => ['required', 'array'],
+            'data.*' => ['array'],
+            'data.*.*' => ['string', 'nullable'],
+        ];
+    }
+
+    public function getDto(): InputDto
+    {
+        return InputDto::create($this->header, $this->data);
+    }
+}

app/Services/Csv/CsvService.php

@@ -0,0 +1,42 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Services\Csv;
+
+final class CsvService
+{
+    /**
+     * @var InputValidator
+     */
+    private $validator;
+
+    public function __construct(InputValidator $validator)
+    {
+        $this->validator = $validator;
+    }
+
+    /**
+     * @throws Exceptions\InvalidColumnException
+     * @throws Exceptions\InvalidRowException
+     */
+    public function create(InputDto $inputDto): string
+    {
+        $this->validator->validate($inputDto);
+
+        $data = $inputDto->getData();
+        array_unshift($data, $inputDto->getHeader());
+
+        return $this->arrayToCsv($data);
+    }
+
+    private function arrayToCsv(array $data): string
+    {
+        $f = fopen('php://memory', 'r+');
+        foreach ($data as $item) {
+            fputcsv($f, $item);
+        }
+        rewind($f);
+
+        return stream_get_contents($f);
+    }
+}

app/Services/Csv/Exceptions/InvalidColumnException.php

@@ -0,0 +1,37 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Services\Csv\Exceptions;
+
+use Exception;
+
+final class InvalidColumnException extends Exception
+{
+    /**
+     * @var int
+     */
+    private $rowIndex;
+
+    /**
+     * @var int
+     */
+    private $columnIndex;
+
+    public function __construct(string $message, int $code, int $rowIndex, int $columnIndex)
+    {
+        parent::__construct($message, $code);
+
+        $this->rowIndex = $rowIndex;
+        $this->columnIndex = $columnIndex;
+    }
+
+    public function getRowIndex(): int
+    {
+        return $this->rowIndex;
+    }
+
+    public function getColumnIndex(): int
+    {
+        return $this->columnIndex;
+    }
+}

app/Services/Csv/Exceptions/InvalidRowException.php

@@ -0,0 +1,26 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Services\Csv\Exceptions;
+
+use Exception;
+
+final class InvalidRowException extends Exception
+{
+    /**
+     * @var int
+     */
+    private $rowIndex;
+
+    public function __construct(string $message, int $code, int $rowIndex)
+    {
+        parent::__construct($message, $code);
+
+        $this->rowIndex = $rowIndex;
+    }
+
+    public function getRowIndex(): int
+    {
+        return $this->rowIndex;
+    }
+}

app/Services/Csv/InputDto.php

@@ -0,0 +1,38 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Services\Csv;
+
+final class InputDto
+{
+    /**
+     * @var array
+     */
+    private $header;
+
+    /**
+     * @var array
+     */
+    private $data;
+
+    private function __construct(array $header, array $data)
+    {
+        $this->header = $header;
+        $this->data = $data;
+    }
+
+    public function getHeader(): array
+    {
+        return $this->header;
+    }
+
+    public function getData(): array
+    {
+        return $this->data;
+    }
+
+    public static function create(array $header, array $data): self
+    {
+        return new self($header, $data);
+    }
+}

app/Services/Csv/InputValidator.php

@@ -0,0 +1,86 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Services\Csv;
+
+use App\Services\Csv\Exceptions\InvalidColumnException;
+use App\Services\Csv\Exceptions\InvalidRowException;
+
+final class InputValidator
+{
+    private const ROW_COLUMNS_COUNT_ERROR_CODE = 1000;
+    private const COLUMN_LENGTH_ERROR_CODE = 2000;
+
+    /**
+     * @var int
+     */
+    private $columnMaxLength;
+
+    public function __construct(int $columnMaxLength = 255)
+    {
+        $this->columnMaxLength = $columnMaxLength;
+    }
+
+    /**
+     * @throws InvalidColumnException
+     * @throws InvalidRowException
+     */
+    public function validate(InputDto $input): void
+    {
+        $this->validateRowsColumnsCount($input);
+        $this->validateColumnsLength($input);
+    }
+
+    /**
+     * @throws InvalidRowException
+     */
+    private function validateRowsColumnsCount(InputDto $input): void
+    {
+        $headerLen = count($input->getHeader());
+
+        $longerRow = collect($input->getData())->search(function (array $data) use ($headerLen) {
+            return count($data) > $headerLen;
+        });
+
+        if ($longerRow !== false) {
+            throw new InvalidRowException(
+                'Invalid row columns count, row must be smaller or equal to header',
+                self::ROW_COLUMNS_COUNT_ERROR_CODE,
+                $longerRow
+            );
+        }
+    }
+
+    /**
+     * @throws InvalidColumnException
+     */
+    private function validateColumnsLength(InputDto $input): void
+    {
+        $data = $input->getData();
+        array_unshift($data, $input->getHeader());
+        $data = array_values($data);
+
+        foreach ($data as $rowIndex => $row) {
+            $invalidLenColumnIndex = collect($row)->search(function ($column) {
+                return $this->columnLength($column) > $this->columnMaxLength;
+            });
+
+            if ($invalidLenColumnIndex !== false) {
+                $message = sprintf(
+                    'Invalid column %d:%d length, current length (%d) more than acceptable (%d)',
+                    $rowIndex,
+                    $invalidLenColumnIndex,
+                    $this->columnLength($row[$invalidLenColumnIndex]),
+                    $this->columnMaxLength
+                );
+
+                throw new InvalidColumnException($message, self::COLUMN_LENGTH_ERROR_CODE, $rowIndex, $invalidLenColumnIndex);
+            }
+        }
+    }
+
+    private function columnLength($column): int
+    {
+        return strlen((string) $column);
+    }
+}