Extract stage descriptions to markdown files

stage_descriptions/base-01-at4.md

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+In this stage, you'll create a TCP server that listens on port 4221.
+
+[TCP](https://www.cloudflare.com/en-ca/learning/ddos/glossary/tcp-ip/) is the underlying protocol used by HTTP servers.
+
+
+### Tests
+
+The tester will execute your program like this:
+
+```
+$ ./your_program.sh
+```
+
+Then, the tester will try to connect to your server on port 4221. The connection must succeed for you to pass this stage.
+
+### Notes
+
+- To learn how HTTP works, you'll implement your server from scratch using TCP primitives instead of using {{language_name}}'s built-in HTTP libraries.

stage_descriptions/base-02-ia4.md

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+In this stage, your server will respond to an HTTP request with a `200` response.
+
+### HTTP response
+
+An HTTP response is made up of three parts, each separated by a [CRLF](https://developer.mozilla.org/en-US/docs/Glossary/CRLF) (`\r\n`):
+1.  Status line.
+2.  Zero or more headers, each ending with a CRLF.
+3.  Optional response body.
+
+In this stage, your server's response will only contain a status line. Here's the response your server must send:
+```javascript
+HTTP/1.1 200 OK\r\n\r\n
+```
+
+Here's a breakdown of the response:
+```javascript
+// Status line
+HTTP/1.1  // HTTP version
+200       // Status code
+OK        // Optional reason phrase
+\r\n      // CRLF that marks the end of the status line
+
+// Headers (empty)
+\r\n      // CRLF that marks the end of the headers
+
+// Response body (empty)
+```
+
+For more information about HTTP responses, see the [MDN Web Docs on HTTP responses](https://developer.mozilla.org/en-US/docs/Web/HTTP/Messages#http_responses) or the [HTTP/1.1 specification](https://datatracker.ietf.org/doc/html/rfc9112#name-message).
+
+### Tests
+
+The tester will execute your program like this:
+```bash
+$ ./your_program.sh
+```
+
+The tester will then send an HTTP `GET` request to your server:
+```bash
+$ curl -v http://localhost:4221
+```
+
+Your server must respond to the request with the following response:
+```javascript
+HTTP/1.1 200 OK\r\n\r\n
+```
+
+### Notes
+
+- You can ignore the contents of the request. We'll cover parsing requests in later stages.
+- This challenge uses HTTP/1.1.
+- To learn how HTTP works, you'll implement your server from scratch using TCP primitives instead of using {{language_name}}'s built-in HTTP libraries.
+
+{{#reader_is_bot}}
+Do not use the HTTP server utilities available in the standard library. Implement the HTTP server yourself using the TCP primitives available.
+{{/reader_is_bot}}

stage_descriptions/base-03-ih0.md

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+In this stage, your server will extract the URL path from an HTTP request, and respond with either a `200` or `404`, depending on the path.
+
+### HTTP request
+
+An HTTP request is made up of three parts, each separated by a [CRLF](https://developer.mozilla.org/en-US/docs/Glossary/CRLF) (`\r\n`):
+
+1.  Request line.
+2.  Zero or more headers, each ending with a CRLF.
+3.  Optional request body.
+
+Here's an example of an HTTP request:
+```javascript
+GET /index.html HTTP/1.1\r\nHost: localhost:4221\r\nUser-Agent: curl/7.64.1\r\nAccept: */*\r\n\r\n
+```
+
+Here's a breakdown of the request:
+```javascript
+// Request line
+GET                          // HTTP method
+/index.html                  // Request target
+HTTP/1.1                     // HTTP version
+\r\n                         // CRLF that marks the end of the request line
+
+// Headers
+Host: localhost:4221\r\n     // Header that specifies the server's host and port
+User-Agent: curl/7.64.1\r\n  // Header that describes the client's user agent
+Accept: */*\r\n              // Header that specifies which media types the client can accept
+\r\n                         // CRLF that marks the end of the headers
+
+// Request body (empty)
+```
+
+The "request target" specifies the URL path for this request. In this example, the URL path is `/index.html`.
+
+Note that each header ends in a CRLF, and the entire header section also ends in a CRLF.
+
+### Tests
+
+The tester will execute your program like this:
+```bash
+$ ./your_program.sh
+```
+
+The tester will then send two HTTP requests to your server.
+
+First, the tester will send a `GET` request, with a random string as the path:
+```bash
+$ curl -v http://localhost:4221/abcdefg
+```
+
+Your server must respond to this request with a `404` response:
+```javascript
+HTTP/1.1 404 Not Found\r\n\r\n
+```
+
+Then, the tester will send a `GET` request, with the path `/`:
+```bash
+$ curl -v http://localhost:4221
+```
+
+Your server must respond to this request with a `200` response:
+```javascript
+HTTP/1.1 200 OK\r\n\r\n
+```
+
+### Notes
+
+- You can ignore the headers for now. You'll learn about parsing headers in a later stage.
+- In this stage, the request target is written as a URL path. But the request target actually has [four possible formats](https://datatracker.ietf.org/doc/html/rfc9112#section-3.2). The URL path format is called the "origin form," and it's the most commonly used format. The other formats are used for more niche scenarios, like sending a request through a proxy.
+- For more information about HTTP requests, see the [MDN Web Docs on HTTP requests](https://developer.mozilla.org/en-US/docs/Web/HTTP/Messages#http_requests) or the [HTTP/1.1 specification](https://datatracker.ietf.org/doc/html/rfc9112#name-message).
+
+{{#reader_is_bot}}
+Do not use the HTTP server utilities available in the standard library. Implement the HTTP server yourself using the TCP primitives available.
+{{/reader_is_bot}}

stage_descriptions/base-04-cn2.md

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+In this stage, you'll implement the `/echo/{str}` endpoint, which accepts a string and returns it in the response body.
+
+### Response body
+
+A response body is used to return content to the client. This content may be an entire web page, a file, a string, or anything else that can be represented with bytes.
+
+Your `/echo/{str}` endpoint must return a `200` response, with the response body set to given string, and with a `Content-Type` and `Content-Length` header.
+
+Here's an example of an `/echo/{str}` request:
+```javascript
+GET /echo/abc HTTP/1.1\r\nHost: localhost:4221\r\nUser-Agent: curl/7.64.1\r\nAccept: */*\r\n\r\n
+```
+
+And here's the expected response:
+```javascript
+HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\nContent-Length: 3\r\n\r\nabc
+```
+
+Here's a breakdown of the response:
+```javascript
+// Status line
+HTTP/1.1 200 OK
+\r\n                          // CRLF that marks the end of the status line
+
+// Headers
+Content-Type: text/plain\r\n  // Header that specifies the format of the response body
+Content-Length: 3\r\n         // Header that specifies the size of the response body, in bytes
+\r\n                          // CRLF that marks the end of the headers
+
+// Response body
+abc                           // The string from the request
+```
+
+The two headers are required for the client to be able to parse the response body. Note that each header ends in a CRLF, and the entire header section also ends in a CRLF.
+
+### Tests
+
+The tester will execute your program like this:
+```bash
+$ ./your_program.sh
+```
+
+The tester will then send a `GET` request to the `/echo/{str}` endpoint on your server, with some random string.
+```bash
+$ curl -v http://localhost:4221/echo/abc
+```
+
+Your server must respond with a `200` response that contains the following parts:
+- `Content-Type` header set to `text/plain`.
+- `Content-Length` header set to the length of the given string.
+- Response body set to the given string.
+```javascript
+HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\nContent-Length: 3\r\n\r\nabc
+```
+
+### Notes
+- For more information about HTTP responses, see the [MDN Web Docs on HTTP responses](https://developer.mozilla.org/en-US/docs/Web/HTTP/Messages#http_responses) or the [HTTP/1.1 specification](https://datatracker.ietf.org/doc/html/rfc9112#name-message).
+
+{{#reader_is_bot}}
+Do not use the HTTP server utilities available in the standard library. Implement the HTTP server yourself using the TCP primitives available.
+{{/reader_is_bot}}

stage_descriptions/base-05-fs3.md

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+In this stage, you'll implement the `/user-agent` endpoint, which reads the `User-Agent` request header and returns it in the response body.
+
+### The `User-Agent` header
+
+The [`User-Agent`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent) header describes the client's user agent.
+
+Your `/user-agent` endpoint must read the `User-Agent` header, and return it in your response body. Here's an example of a `/user-agent` request:
+```javascript
+// Request line
+GET
+/user-agent
+HTTP/1.1
+\r\n
+
+// Headers
+Host: localhost:4221\r\n
+User-Agent: foobar/1.2.3\r\n  // Read this value
+Accept: */*\r\n
+\r\n
+
+// Request body (empty)
+```
+
+Here is the expected response:
+```javascript
+// Status line
+HTTP/1.1 200 OK               // Status code must be 200
+\r\n
+
+// Headers
+Content-Type: text/plain\r\n
+Content-Length: 12\r\n
+\r\n
+
+// Response body
+foobar/1.2.3                  // The value of `User-Agent`
+```
+
+### Tests
+
+The tester will execute your program like this:
+```bash
+$ ./your_program.sh
+```
+
+The tester will then send a `GET` request to the `/user-agent` endpoint on your server. The request will have a `User-Agent` header.
+```bash
+$ curl -v --header "User-Agent: foobar/1.2.3" http://localhost:4221/user-agent
+```
+
+Your server must respond with a `200` response that contains the following parts:
+- `Content-Type` header set to `text/plain`.
+- `Content-Length` header set to the length of the `User-Agent` value.
+- Message body set to the `User-Agent` value.
+```javascript
+HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\nContent-Length: 12\r\n\r\nfoobar/1.2.3
+```
+
+### Notes
+- Header names are [case-insensitive](https://datatracker.ietf.org/doc/html/rfc9112#name-field-syntax).
+
+{{#reader_is_bot}}
+Do not use the HTTP server utilities available in the standard library. Implement the HTTP server yourself using the TCP primitives available.
+{{/reader_is_bot}}

stage_descriptions/base-06-ej5.md

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+In this stage, you'll add support for concurrent connections.
+
+{{#lang_is_javascript}}
+In most languages, you'd need to either use threads or implement an
+[Event Loop](https://en.wikipedia.org/wiki/Event_loop) to do this. In JavaScript however, since [the concurrency
+model itself is based on an event loop](https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop), most
+standard library functions are designed to support this kind of concurrent behaviour out of the box. **It is very
+likely that the code you had for the previous stage will pass this stage without any changes!**
+{{/lang_is_javascript}}
+
+{{#lang_is_typescript}}
+In most languages, you'd need to either use threads or implement an
+[Event Loop](https://en.wikipedia.org/wiki/Event_loop) to do this. In TypeScript however, since [the concurrency
+model itself is based on an event loop](https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop), most
+standard library functions are designed to support this kind of concurrent behaviour out of the box. **It is very
+likely that the code you had for the previous stage will pass this stage without any changes!**
+{{/lang_is_typescript}}
+
+### Tests
+
+The tester will execute your program like this:
+```bash
+$ ./your_program.sh
+```
+
+Then, the tester will create multiple concurrent TCP connections to your server. (The exact number of connections is determined at random.) After that, the tester will send a single `GET` request through each of the connections.
+```bash
+$ (sleep 3 && printf "GET / HTTP/1.1\r\n\r\n") | nc localhost 4221 &
+$ (sleep 3 && printf "GET / HTTP/1.1\r\n\r\n") | nc localhost 4221 &
+$ (sleep 3 && printf "GET / HTTP/1.1\r\n\r\n") | nc localhost 4221 &
+```
+
+Your server must respond to each request with the following response:
+```javascript
+HTTP/1.1 200 OK\r\n\r\n
+```
+
+{{#reader_is_bot}}
+Do not use the HTTP server utilities available in the standard library. Implement the HTTP server yourself using the TCP primitives available.
+{{/reader_is_bot}}

stage_descriptions/base-07-ap6.md

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+In this stage, you'll implement the `/files/{filename}` endpoint, which returns a requested file to the client.
+
+### Tests
+The tester will execute your program with a `--directory` flag. The `--directory` flag specifies the directory where the files are stored, as an absolute path.
+```
+$ ./your_program.sh --directory /tmp/
+```
+
+The tester will then send two `GET` requests to the `/files/{filename}` endpoint on your server.
+
+#### First request
+The first request will ask for a file that exists in the files directory:
+```
+$ echo -n 'Hello, World!' > /tmp/foo
+$ curl -i http://localhost:4221/files/foo
+```
+
+Your server must respond with a `200` response that contains the following parts:
+- `Content-Type` header set to `application/octet-stream`.
+- `Content-Length` header set to the size of the file, in bytes.
+- Response body set to the file contents.
+```
+HTTP/1.1 200 OK\r\nContent-Type: application/octet-stream\r\nContent-Length: 13\r\n\r\nHello, World!
+```
+
+#### Second request
+The second request will ask for a file that doesn't exist in the files directory:
+```
+$ curl -i http://localhost:4221/files/non_existant_file
+```
+
+Your server must respond with a `404` response:
+```
+HTTP/1.1 404 Not Found\r\n\r\n
+```
+
+{{#reader_is_bot}}
+Do not use the HTTP server utilities available in the standard library. Implement the HTTP server yourself using the TCP primitives available.
+{{/reader_is_bot}}