//! HTTP/1.1 over a connected stream: one request, one response, `Connection: close` (M4a spec,
//! section 5). The response head is read a byte at a time, so nothing past it is consumed: the
//! WebSocket handshake reads its frames after it from the same stream.

use std::io::{Read, Write};
use std::time::{Duration, SystemTime, UNIX_EPOCH};

pub const MAX_HEAD: usize = 16 * 1024;
pub const MAX_BODY: usize = 4 * 1024 * 1024;
/// The longest a rate limit is waited out, whatever the server says.
pub const MAX_RATE_WAIT: Duration = Duration::from_secs(60);

#[derive(Debug)]
pub enum HttpError {
    Io(std::io::Error),
    /// The response is not HTTP/1.1 as we read it.
    Protocol(String),
    /// A head or body over its cap.
    TooLarge(&'static str),
}

impl std::fmt::Display for HttpError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            HttpError::Io(e) => write!(f, "{e}"),
            HttpError::Protocol(why) => write!(f, "bad HTTP response: {why}"),
            HttpError::TooLarge(what) => write!(f, "the response {what} is too large"),
        }
    }
}

impl std::error::Error for HttpError {}

impl From<std::io::Error> for HttpError {
    fn from(e: std::io::Error) -> Self {
        HttpError::Io(e)
    }
}

/// A status line and headers.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Head {
    pub status: u16,
    pub headers: Vec<(String, String)>,
}

impl Head {
    /// The first header named `name`, compared case-insensitively.
    pub fn header(&self, name: &str) -> Option<&str> {
        // The value of the first header whose name matches, ignoring ASCII case.
        self.headers
            .iter()
            .find(|(k, _)| k.eq_ignore_ascii_case(name))
            .map(|(_, v)| v.as_str())
    }
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Response {
    pub head: Head,
    pub body: Vec<u8>,
}

/// Write one request. `host` is the `Host` header; `headers` come after it, then
/// `Content-Length` when there is a body, then `Connection: close`.
pub fn write_request(
    stream: &mut dyn Write,
    method: &str,
    host: &str,
    path: &str,
    headers: &[(&str, &str)],
    body: Option<&[u8]>,
) -> Result<(), HttpError> {
    // Exactly: "<method> <path> HTTP/1.1\r\nHost: <host>\r\n", each header as "<k>: <v>\r\n",
    // "Content-Length: <n>\r\n" when there is a body, "Connection: close\r\n\r\n", then the body.
    // Flush.
    let mut head = Vec::new();
    let body_len = body.map(|b| b.len()).unwrap_or(0);
    head.extend_from_slice(format!("{method} {path} HTTP/1.1\r\n").as_bytes());
    head.extend_from_slice(format!("Host: {host}\r\n").as_bytes());
    for (k, v) in headers {
        head.extend_from_slice(format!("{k}: {v}\r\n").as_bytes());
    }
    if body.is_some() {
        head.extend_from_slice(format!("Content-Length: {body_len}\r\n").as_bytes());
    }
    head.extend_from_slice(b"Connection: close\r\n\r\n");
    stream.write_all(&head)?;
    if let Some(b) = body {
        stream.write_all(b)?;
    }
    stream.flush()?;
    Ok(())
}

/// One request and its whole response.
pub fn request(
    stream: &mut (impl Read + Write),
    method: &str,
    host: &str,
    path: &str,
    headers: &[(&str, &str)],
    body: Option<&[u8]>,
) -> Result<Response, HttpError> {
    // `write_request`, then `read_head`, then `read_body`.
    write_request(stream, method, host, path, headers, body)?;
    let head = read_head(stream)?;
    let body = read_body(stream, &head)?;
    Ok(Response { head, body })
}

/// The status line and headers, up to and including the blank line, and not a byte more.
pub fn read_head(stream: &mut dyn Read) -> Result<Head, HttpError> {
    // One byte at a time until CRLF CRLF, never more; over MAX_HEAD is TooLarge("head"); end of
    // stream is Protocol; retry Interrupted. Then: UTF-8; status line "HTTP/1.1" or "HTTP/1.0", a
    // 3-digit status in 100..=599; each header line "name: value" with a non-empty name without
    // spaces, the value trimmed. Anything else is Protocol.
    let mut raw = Vec::new();
    let mut byte = [0u8; 1];
    loop {
        let n = loop {
            match stream.read(&mut byte) {
                Ok(0) => {
                    return Err(HttpError::Protocol(
                        "the head ended before the blank line".to_string(),
                    ));
                }
                Ok(n) => break n,
                Err(e) if e.kind() == std::io::ErrorKind::Interrupted => continue,
                Err(e) => return Err(HttpError::Io(e)),
            }
        };
        debug_assert_eq!(n, 1);
        raw.push(byte[0]);
        if raw.len() > MAX_HEAD {
            return Err(HttpError::TooLarge("head"));
        }
        if raw.ends_with(b"\r\n\r\n") {
            break;
        }
    }

    let text = std::str::from_utf8(&raw)
        .map_err(|_| HttpError::Protocol("the head is not UTF-8".to_string()))?;

    let mut lines = text.split("\r\n");
    let status_line = lines.next().unwrap_or("");
    let (version, code) = parse_status_line(status_line)?;
    let _ = version;

    let mut headers = Vec::new();
    for line in lines {
        if line.is_empty() {
            break;
        }
        let (name, value) = line
            .split_once(':')
            .ok_or_else(|| HttpError::Protocol("a header line has no colon".to_string()))?;
        if name.is_empty() || name.bytes().any(|b| b == b' ') {
            return Err(HttpError::Protocol(
                "a header name is empty or has a space".to_string(),
            ));
        }
        headers.push((name.to_string(), value.trim().to_string()));
    }

    Ok(Head {
        status: code,
        headers,
    })
}

fn parse_status_line(line: &str) -> Result<(&str, u16), HttpError> {
    // "HTTP/1.1 200 OK": a 1.1 or 1.0 version, one space, a 3-digit status in 100..=599, then
    // optionally one space and a free-text reason. Any other spacing is refused.
    let mut parts = line.splitn(3, ' ');
    let version = parts.next().unwrap_or("");
    let code = parts.next().unwrap_or("");
    if version != "HTTP/1.1" && version != "HTTP/1.0" {
        return Err(HttpError::Protocol(
            "not an HTTP/1.1 status line".to_string(),
        ));
    }
    if code.len() != 3 || !code.bytes().all(|b| b.is_ascii_digit()) {
        return Err(HttpError::Protocol(
            "the status code is not three digits".to_string(),
        ));
    }
    let code: u16 = code
        .parse()
        .map_err(|_| HttpError::Protocol("the status code will not parse".to_string()))?;
    if !(100..=599).contains(&code) {
        return Err(HttpError::Protocol(
            "the status code is out of range".to_string(),
        ));
    }
    Ok((version, code))
}

/// The body after `head`: chunked, `Content-Length`, or to the end; at most `MAX_BODY`.
pub fn read_body(stream: &mut dyn Read, head: &Head) -> Result<Vec<u8>, HttpError> {
    // Transfer-Encoding: chunked (any case) -> `read_chunked`. Else Content-Length: parse (else
    // Protocol), over MAX_BODY is TooLarge("body"), then read_exact that many. Else read to the end
    // through `take(MAX_BODY + 1)`; more than MAX_BODY is TooLarge("body").
    if head
        .header("transfer-encoding")
        .is_some_and(|v| v.eq_ignore_ascii_case("chunked"))
    {
        return read_chunked(stream);
    }
    match head.header("content-length") {
        Some(s) => {
            let len = parse_content_length(s)?;
            if len > MAX_BODY {
                return Err(HttpError::TooLarge("body"));
            }
            let mut buf = vec![0u8; len];
            stream.read_exact(&mut buf)?;
            Ok(buf)
        }
        None => {
            let mut buf = Vec::new();
            let limit = u64::try_from(MAX_BODY).map_or(u64::MAX, |n| n.saturating_add(1));
            stream.take(limit).read_to_end(&mut buf)?;
            if buf.len() > MAX_BODY {
                return Err(HttpError::TooLarge("body"));
            }
            Ok(buf)
        }
    }
}

fn parse_content_length(s: &str) -> Result<usize, HttpError> {
    // Its own digits only: a non-number is Protocol, an overflowing one is Protocol too.
    if s.is_empty() || !s.bytes().all(|b| b.is_ascii_digit()) {
        return Err(HttpError::Protocol(
            "the Content-Length is not a number".to_string(),
        ));
    }
    s.parse::<usize>()
        .map_err(|_| HttpError::Protocol("the Content-Length is too large".to_string()))
}

fn read_line(stream: &mut dyn Read, cap: usize) -> Result<String, HttpError> {
    // Bytes up to CRLF (dropped), at most `cap` (else TooLarge("chunk header")); end of stream is
    // Protocol.
    let mut bytes = Vec::new();
    let mut byte = [0u8; 1];
    loop {
        let n = loop {
            match stream.read(&mut byte) {
                Ok(0) => {
                    return Err(HttpError::Protocol(
                        "the line ended before its CRLF".to_string(),
                    ));
                }
                Ok(n) => break n,
                Err(e) if e.kind() == std::io::ErrorKind::Interrupted => continue,
                Err(e) => return Err(HttpError::Io(e)),
            }
        };
        debug_assert_eq!(n, 1);
        match byte[0] {
            b'\n' => {
                // Lines end in CRLF; a bare LF is not accepted.
                if bytes.pop() != Some(b'\r') {
                    return Err(HttpError::Protocol(
                        "a line is not ended by CRLF".to_string(),
                    ));
                }
                return String::from_utf8(bytes)
                    .map_err(|_| HttpError::Protocol("a line is not UTF-8".to_string()));
            }
            other => {
                bytes.push(other);
                if bytes.len() > cap {
                    return Err(HttpError::TooLarge("chunk header"));
                }
            }
        }
    }
}

fn read_chunked(stream: &mut dyn Read) -> Result<Vec<u8>, HttpError> {
    // Loop: a size line (hex, before any ";"), at most 1024 bytes. Size 0: read trailer lines (8
    // KiB each) until an empty one, and return. Otherwise the size must fit in MAX_BODY minus what
    // is already read (else TooLarge("body")), read it, then exactly CRLF (else Protocol).
    let mut out = Vec::new();
    loop {
        let size_line = read_line(stream, 1024)?.trim().to_string();
        let hex = match size_line.split_once(';') {
            Some((h, _)) => h.trim(),
            None => &size_line,
        };
        let size = parse_hex(hex)?;
        if size == 0 {
            loop {
                let trailer = read_line(stream, 8 * 1024)?;
                if trailer.is_empty() {
                    return Ok(out);
                }
            }
        }
        let room = MAX_BODY.saturating_sub(out.len());
        let size = usize::try_from(size)
            .ok()
            .filter(|s| *s <= room)
            .ok_or(HttpError::TooLarge("body"))?;
        let mut chunk = vec![0u8; size];
        stream.read_exact(&mut chunk)?;
        out.extend_from_slice(&chunk);
        let terminator = read_line(stream, 2)?;
        if !terminator.is_empty() {
            return Err(HttpError::Protocol(
                "a chunk is not ended by a blank line".to_string(),
            ));
        }
    }
}

fn parse_hex(s: &str) -> Result<u128, HttpError> {
    // A hex chunk size; an overflow past u128 is refused here, so a size larger than the body cap
    // still comes back as an error to the caller.
    let mut value: u128 = 0;
    for b in s.bytes() {
        let d = match b {
            b'0'..=b'9' => u128::from(b - b'0'),
            b'a'..=b'f' => u128::from(b - b'a' + 10),
            b'A'..=b'F' => u128::from(b - b'A' + 10),
            _ => return Err(HttpError::Protocol("a chunk size is not hex".to_string())),
        };
        value = value
            .checked_mul(16)
            .and_then(|v| v.checked_add(d))
            .ok_or_else(|| HttpError::Protocol("a chunk size is too large".to_string()))?;
    }
    Ok(value)
}

/// How long a 429 asks us to wait, from `X-Ratelimit-Reset`: a Unix time if it is one, else a
/// number of seconds; never more than `MAX_RATE_WAIT`. One second if the header is missing or bad.
pub fn rate_limit_wait(head: &Head, now: SystemTime) -> Duration {
    // X-Ratelimit-Reset as u64: above 1_000_000_000 it is a Unix time (wait = it - now, at least 1
    // s), otherwise seconds (at least 1). Missing or not a number: 1 s. Never more than
    // MAX_RATE_WAIT.
    let reset = match head.header("x-ratelimit-reset") {
        Some(v) => v,
        None => return Duration::from_secs(1),
    };
    let value: u64 = match reset.parse() {
        Ok(v) => v,
        Err(_) => return Duration::from_secs(1),
    };
    let secs = if value > 1_000_000_000 {
        match now.duration_since(UNIX_EPOCH) {
            Ok(elapsed) => value.saturating_sub(elapsed.as_secs()).max(1),
            Err(_) => 1,
        }
    } else {
        value.max(1)
    };
    Duration::from_secs(secs.min(MAX_RATE_WAIT.as_secs()))
}