//! HTTP/1.1, hand-rolled and deliberately narrow: `GET` and `HEAD`, origin-form
//! targets, one response per connection.
//!
//! A reverse proxy in front sends origin-form, so absolute-form is refused rather
//! than half-supported. There is no keep-alive, no chunked encoding and no
//! pipelining; `Connection: close` is always sent so a client knows it.

use std::io::{BufReader, Write};
use std::net::TcpStream;

use anyhow::Result;
use itsybitsy_core::site::{Resolution, Resource};

use crate::proto::{for_log, negotiate, read_line_capped};
use crate::serve::Listener;

const MAX_REQUEST_LINE: usize = 8192;
const MAX_HEADER_LINE: usize = 8192;
const MAX_HEADERS: usize = 64;
const MAX_HEADER_BYTES: usize = 16 * 1024;

pub fn serve(listener: &Listener, mut stream: TcpStream) -> Result<()> {
    let mut reader = BufReader::new(stream.try_clone()?);
    let Some(line) = read_line_capped(&mut reader, MAX_REQUEST_LINE) else {
        return bad_request(&mut stream);
    };

    let parts: Vec<&str> = line.split_whitespace().collect();
    let [method, target, version] = parts.as_slice() else {
        return bad_request(&mut stream);
    };
    if !version.starts_with("HTTP/1.") {
        return bad_request(&mut stream);
    }
    // Origin-form only; the other request-target forms are refused, not
    // half-supported.
    if !target.starts_with('/') {
        return bad_request(&mut stream);
    }

    let mut host = None;
    let mut accept = None;
    let mut total = 0usize;
    for index in 0.. {
        let Some(header) = read_line_capped(&mut reader, MAX_HEADER_LINE) else {
            return bad_request(&mut stream);
        };
        if header.is_empty() {
            break;
        }
        total += header.len();
        if index >= MAX_HEADERS || total > MAX_HEADER_BYTES {
            return bad_request(&mut stream);
        }
        let Some((name, value)) = header.split_once(':') else { continue };
        let value = value.trim().to_string();
        match name.trim().to_ascii_lowercase().as_str() {
            "host" => host = Some(value),
            "accept" => accept = Some(value),
            _ => {}
        }
    }

    let head_only = *method == "HEAD";
    if *method != "GET" && !head_only {
        return respond(
            &mut stream,
            405,
            "Method Not Allowed",
            "text/plain; charset=utf-8",
            b"Method not allowed\n",
            &[("Allow", "GET, HEAD")],
            // Not HEAD: that case does not reach here.
            false,
        );
    }

    // A missing Host on HTTP/1.1 is a malformed request, not a missing resource:
    // 404 would imply the server looked somewhere.
    let Some(host) = host else { return bad_request(&mut stream) };
    let Some(site) = listener.site_for(&host) else {
        log::info!("{} http unknown host {}", listener.name, for_log(&host));
        return respond(
            &mut stream,
            404,
            "Not Found",
            "text/plain; charset=utf-8",
            b"Not found\n",
            &[],
            head_only,
        );
    };

    let (path, query) = target.split_once('?').map_or((*target, ""), |(p, q)| (p, q));
    log::info!("{} http {} {}", listener.name, for_log(&host), for_log(target));

    // `?format=` overrides negotiation, for testing without the hardware.
    let override_format = query
        .split('&')
        .find_map(|pair| pair.strip_prefix("format="))
        .map(|value| value.to_ascii_lowercase());
    let format = match &override_format {
        Some(id) if listener.formats.iter().any(|f| f == id) => id.clone(),
        Some(_) => {
            return respond(
                &mut stream,
                400,
                "Bad Request",
                "text/plain; charset=utf-8",
                b"Unknown format\n",
                &[],
                head_only,
            );
        }
        None => {
            negotiate::choose(&listener.registry, &listener.formats, accept.as_deref()).to_string()
        }
    };

    match site.resolve(path) {
        Ok(Resolution::Found(Resource::Document { page, .. })) => {
            let Some(body) = page.body(&format) else {
                return respond(
                    &mut stream,
                    500,
                    "Internal Server Error",
                    "text/plain; charset=utf-8",
                    b"",
                    &[],
                    head_only,
                );
            };
            let cache = page.settings.max_age.map(|age| format!("max-age={age}"));
            let mut headers: Vec<(&str, &str)> = Vec::new();
            // The response body depends on Accept, so a shared cache must not
            // serve one client's format to another.
            headers.push(("Vary", "Accept"));
            if let Some(cache) = &cache {
                headers.push(("Cache-Control", cache));
            }
            respond(&mut stream, 200, "OK", listener.media_type(&format), body, &headers, head_only)
        }
        Ok(Resolution::Found(Resource::Part { parent_url, slug, page })) => {
            match page.part(&format, &slug) {
                Some(body) => respond(
                    &mut stream,
                    200,
                    "OK",
                    listener.media_type(&format),
                    body,
                    &[("Vary", "Accept")],
                    head_only,
                ),
                // This format does not address that sub-document, so the parent
                // is where the content actually is.
                None => respond(
                    &mut stream,
                    301,
                    "Moved Permanently",
                    "text/plain; charset=utf-8",
                    b"",
                    &[("Location", parent_url.as_str()), ("Vary", "Accept")],
                    head_only,
                ),
            }
        }
        Ok(Resolution::Found(Resource::Raw { path, media_type })) => {
            let meta = std::fs::metadata(&path)?;
            write_head(&mut stream, 200, "OK", media_type, meta.len(), &[])?;
            if !head_only {
                let mut file = std::fs::File::open(&path)?;
                // Streamed, not buffered, so a large file costs the copy buffer
                // rather than its own size on every request.
                std::io::copy(&mut file, &mut stream)?;
            }
            Ok(())
        }
        Ok(Resolution::Redirect(location)) => respond(
            &mut stream,
            301,
            "Moved Permanently",
            "text/plain; charset=utf-8",
            b"",
            &[("Location", location.as_str())],
            head_only,
        ),
        Ok(Resolution::NotFound) => respond(
            &mut stream,
            404,
            "Not Found",
            "text/plain; charset=utf-8",
            b"Not found\n",
            &[],
            head_only,
        ),
        Err(err) => {
            log::warn!("{} http {}: {err}", listener.name, for_log(path));
            respond(
                &mut stream,
                500,
                "Internal Server Error",
                "text/plain; charset=utf-8",
                b"",
                &[],
                head_only,
            )
        }
    }
}

fn bad_request(stream: &mut TcpStream) -> Result<()> {
    respond(stream, 400, "Bad Request", "text/plain; charset=utf-8", b"Bad request\n", &[], false)
}

fn respond(
    stream: &mut TcpStream,
    code: u16,
    reason: &str,
    media_type: &str,
    body: &[u8],
    headers: &[(&str, &str)],
    head_only: bool,
) -> Result<()> {
    write_head(stream, code, reason, media_type, body.len() as u64, headers)?;
    // HEAD sends the headers a GET would, including the length, and no body.
    if !head_only {
        stream.write_all(body)?;
    }
    Ok(())
}

fn write_head(
    stream: &mut TcpStream,
    code: u16,
    reason: &str,
    media_type: &str,
    length: u64,
    headers: &[(&str, &str)],
) -> Result<()> {
    let mut head = format!(
        "HTTP/1.1 {code} {reason}\r\nContent-Type: {media_type}\r\nContent-Length: {length}\r\n"
    );
    for (name, value) in headers {
        head.push_str(&format!("{name}: {value}\r\n"));
    }
    head.push_str("Connection: close\r\n\r\n");
    stream.write_all(head.as_bytes())?;
    Ok(())
}