parse-net 0.9.2

//! RFC 2045 section 5.1 parameters with RFC 2231 continuations and
//! extended values.

use super::cfws;
use crate::{
    class,
    error::{ErrorKind, ParseError, Result, WriteError, err},
    ext_value::{Charset, ExtValue, is_mime_attribute_char},
    limits::{Limits, MAX_RFC2231_SECTIONS},
    param::{MAX_CHECKED_PARAMS, Names},
    scan::control_kind,
    text::{Word, parse_quoted},
    write::{CountSink, Sink, reserve},
};

/// Limits for walking text that was already validated.
const RELAXED: Limits = Limits::DEFAULT
    .max_params(u16::MAX)
    .max_comment_depth(u8::MAX);

/// How a parameter name takes part in RFC 2231.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum Section {
    /// `name`.
    Plain,
    /// `name*`: one extended value.
    Ext,
    /// `name*N` or `name*N*`.
    Part { n: usize, ext: bool },
}

/// Splits `name*N*` into the base name and its section.
fn split_name(name: &[u8]) -> (&[u8], Section) {
    let (body, ext) = match name.split_last() {
        Some((b'*', body)) => (body, true),
        _ => (name, false),
    };
    if let Some(star) = body.iter().rposition(|&b| b == b'*') {
        let digits = body.get(star + 1..).unwrap_or(&[]);
        let canonical = !digits.is_empty()
            && digits.iter().all(u8::is_ascii_digit)
            && (digits.len() == 1 || digits.first() != Some(&b'0'));
        if canonical {
            // Saturating: any number this large is past the section limit.
            let n = digits.iter().fold(0usize, |a, &d| {
                a.saturating_mul(10).saturating_add(usize::from(d - b'0'))
            });
            return (body.get(..star).unwrap_or(&[]), Section::Part { n, ext });
        }
    }
    if ext {
        (body, Section::Ext)
    } else {
        (name, Section::Plain)
    }
}

fn refused(v: &[u8], at: usize, kind: ErrorKind) -> ParseError {
    match v.get(at) {
        Some(&b) if crate::scan::is_control(b) => ParseError::new(control_kind(b), at),
        _ => ParseError::new(kind, at),
    }
}

/// One `; attribute = value` at `v[i]` (the `;`); returns the name, value,
/// where the value starts, and the index after it.
fn param_at<'a>(
    v: &'a [u8],
    i: usize,
    limits: &Limits,
) -> Result<(&'a [u8], Word<'a>, usize, usize)> {
    let j = cfws(v, i + 1, limits)?;
    let n = v
        .get(j..)
        .unwrap_or(&[])
        .iter()
        .take_while(|&&b| class::is_mime_token_char(b))
        .count();
    if n == 0 {
        return Err(refused(v, j, ErrorKind::NotToken));
    }
    let name = v.get(j..j + n).unwrap_or(&[]);
    let k = cfws(v, j + n, limits)?;
    if v.get(k) != Some(&b'=') {
        return Err(refused(v, k, ErrorKind::MissingValue));
    }
    let at = cfws(v, k + 1, limits)?;
    let rest = v.get(at..).unwrap_or(&[]);
    if rest.first() == Some(&b'"') {
        let (q, used) = parse_quoted(rest).map_err(|e| e.shifted(at))?;
        return Ok((name, Word::Quoted(q), at, at + used));
    }
    let t = rest
        .iter()
        .take_while(|&&b| class::is_mime_token_char(b))
        .count();
    if t == 0 {
        return Err(refused(v, at, ErrorKind::MissingValue));
    }
    Ok((name, Word::Token(rest.get(..t).unwrap_or(&[])), at, at + t))
}

/// Whether `t` is RFC 2231 `extended-other-values`: attr-chars and `%XX`.
fn pct_chars(t: &[u8]) -> Option<usize> {
    let mut i = 0usize;
    while let Some(&b) = t.get(i) {
        if b == b'%' {
            match (t.get(i + 1), t.get(i + 2)) {
                (Some(&h), Some(&l)) if class::is_hexdig(h) && class::is_hexdig(l) => i += 3,
                _ => return Some(i),
            }
        } else if is_mime_attribute_char(b) {
            i += 1;
        } else {
            return Some(i);
        }
    }
    None
}

/// Parses `*( ";" attribute "=" value )` (RFC 2045 section 5.1) with the MIME
/// token class, CFWS between the parts, and case-insensitive names, and
/// validates every RFC 2231 extended value and continuation. Returns the
/// parameters and the bytes used (parsing stops at the first byte that does
/// not start another parameter).
///
/// # Errors
///
/// [`ErrorKind::NotToken`], [`ErrorKind::MissingValue`],
/// [`ErrorKind::DuplicateParameter`], [`ErrorKind::UnterminatedQuote`],
/// [`ErrorKind::TooManyMembers`] beyond `max_params` or
/// [`MAX_CHECKED_PARAMS`](crate::param::MAX_CHECKED_PARAMS); for RFC 2231:
/// [`ErrorKind::SectionGap`], [`ErrorKind::TooManySections`] beyond
/// `max_rfc2231_sections`, the [`ExtValue::parse`] errors (a charset outside
/// section 0 is [`ErrorKind::InvalidPercentEncoding`] at its `'`),
/// [`ErrorKind::InvalidUtf8`], [`ErrorKind::ControlInDecoded`];
/// [`ErrorKind::UnbalancedComment`], [`ErrorKind::CommentTooDeep`].
///
/// # Examples
///
/// ```
/// use parse_net::{mail, Limits};
///
/// let (p, used) = mail::parse_mime_params(b"; charset=\"utf-8\" (comment); format=flowed rest", &Limits::DEFAULT)?;
/// assert_eq!((p.len(), used), (2, 42));
/// let mut buf = [0u8; 8];
/// assert_eq!(p.decode_into("CHARSET", &mut buf)?, Some("utf-8"));
/// # Ok::<(), parse_net::ParseError>(())
/// ```
pub fn parse_mime_params<'a>(s: &'a [u8], limits: &Limits) -> Result<(MimeParams<'a>, usize)> {
    let (mut i, mut count) = (0usize, 0usize);
    let max = usize::from(limits.params).min(MAX_CHECKED_PARAMS);
    let mut names = Names::new();
    loop {
        let j = cfws(s, i, limits)?;
        if s.get(j) != Some(&b';') {
            break;
        }
        let (name, value, at, end) = param_at(s, j, limits)?;
        count += 1;
        if count > max {
            return err(ErrorKind::TooManyMembers, j);
        }
        let name_at = (name.as_ptr() as usize).saturating_sub(s.as_ptr() as usize);
        if names.repeats(s, name_at, name, class::is_mime_token_char) {
            return err(ErrorKind::DuplicateParameter, j);
        }
        match (split_name(name).1, value) {
            (Section::Ext | Section::Part { n: 0, ext: true }, Word::Token(t)) => {
                let ext = ExtValue::parse_mime(t).map_err(|e| e.shifted(at))?;
                if split_name(name).1 == Section::Ext {
                    ext.check_text()
                        .map_err(|e| ParseError::new(e.kind(), at))?;
                }
            }
            (Section::Part { ext: true, .. }, Word::Token(t)) => {
                if let Some(bad) = pct_chars(t) {
                    return err(ErrorKind::InvalidPercentEncoding, at + bad);
                }
            }
            (Section::Ext | Section::Part { ext: true, .. }, Word::Quoted(_)) => {
                // Extended values are tokens (RFC 2231 section 7).
                return err(ErrorKind::InvalidCharset, at);
            }
            _ => {}
        }
        i = end;
    }
    let params = MimeParams {
        text: s.get(..i).unwrap_or(&[]),
        count,
    };
    // Every continuation must be whole and decodable.
    for (name, _) in params.iter() {
        if let (base, Section::Part { n: 0, .. }) = split_name(name) {
            let c = Continuation { params, name: base };
            let _ = c.walk(limits, &mut |_| {})?;
        } else if let (base, Section::Part { .. }) = split_name(name) {
            let has_zero = params
                .iter()
                .any(|(n, _)| matches!(split_name(n), (b, Section::Part { n: 0, .. }) if b.eq_ignore_ascii_case(base)));
            if !has_zero {
                let at = (name.as_ptr() as usize).saturating_sub(s.as_ptr() as usize);
                return err(ErrorKind::SectionGap, at);
            }
        }
    }
    Ok((params, i))
}

/// Validated MIME parameters.
///
/// # Examples
///
/// ```
/// use parse_net::{mail, Limits};
///
/// let (p, _) = mail::parse_mime_params(b"; charset=us-ascii", &Limits::DEFAULT)?;
/// let mut buf = [0u8; 16];
/// assert_eq!(p.decode_into("charset", &mut buf)?, Some("us-ascii"));
/// # Ok::<(), parse_net::ParseError>(())
/// ```
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct MimeParams<'a> {
    text: &'a [u8],
    count: usize,
}

impl<'a> MimeParams<'a> {
    /// The value of `name` (case-insensitive): a continuation when
    /// `name*0` or `name*0*` is present, else the extended `name*`, else the
    /// plain `name`.
    ///
    /// # Examples
    ///
    /// ```
    /// use parse_net::{mail::{self, MimeValue}, Limits};
    ///
    /// let (p, _) = mail::parse_mime_params(b"; name=plain.txt; name*=UTF-8''ext.txt", &Limits::DEFAULT)?;
    /// assert!(matches!(p.get("NAME"), Some(MimeValue::Extended(e)) if e.encoded() == b"ext.txt"));
    /// assert!(p.get("other").is_none());
    /// # Ok::<(), parse_net::ParseError>(())
    /// ```
    #[must_use]
    pub fn get(&self, name: &str) -> Option<MimeValue<'a>> {
        let want = name.as_bytes();
        let mut ext = None;
        let mut plain = None;
        for (n, w) in self.iter() {
            match split_name(n) {
                (b, Section::Part { n: 0, .. }) if b.eq_ignore_ascii_case(want) => {
                    return Some(MimeValue::Continued(Continuation {
                        params: *self,
                        name: b,
                    }));
                }
                (b, Section::Ext) if b.eq_ignore_ascii_case(want) => {
                    if let Word::Token(t) = w {
                        ext = ExtValue::parse_mime(t).ok();
                    }
                }
                (b, Section::Plain) if b.eq_ignore_ascii_case(want) => plain = Some(w),
                _ => {}
            }
        }
        ext.map(MimeValue::Extended).or(plain.map(MimeValue::Plain))
    }

    /// Decodes the value of `name` into `out` (as [`get`](Self::get)
    /// chooses it); `Ok(None)` when absent.
    ///
    /// # Errors
    ///
    /// [`ErrorKind::BufferTooSmall`]; [`ErrorKind::InvalidUtf8`] for a plain
    /// value that is not UTF-8. Nothing is written on error.
    ///
    /// # Examples
    ///
    /// ```
    /// use parse_net::{mail, ErrorKind, Limits};
    ///
    /// let (p, _) = mail::parse_mime_params(b"; boundary=\"simple boundary\"", &Limits::DEFAULT)?;
    /// let mut buf = [0u8; 32];
    /// assert_eq!(p.decode_into("boundary", &mut buf)?, Some("simple boundary"));
    /// assert_eq!(p.decode_into("charset", &mut buf)?, None);
    /// assert_eq!(p.decode_into("boundary", &mut [0u8; 4]).unwrap_err().kind(), ErrorKind::BufferTooSmall);
    /// # Ok::<(), parse_net::ParseError>(())
    /// ```
    pub fn decode_into<'b>(&self, name: &str, out: &'b mut [u8]) -> Result<Option<&'b str>> {
        match self.get(name) {
            None => Ok(None),
            Some(MimeValue::Continued(c)) => c.decode_into(out).map(Some),
            Some(MimeValue::Extended(e)) => e.decode_into(out).map(Some),
            Some(MimeValue::Plain(w)) => {
                let Some(dst) = out.get_mut(..w.len()) else {
                    return err(ErrorKind::BufferTooSmall, 0);
                };
                // Checked before anything is written.
                let (mut utf8, text) = (crate::utf8::Utf8::new(), self.value_text(&w));
                let valid = match w {
                    Word::Token(t) => t.iter().all(|&b| utf8.push(b)),
                    Word::Quoted(q) => q.bytes().all(|b| utf8.push(b)),
                };
                if !valid || !utf8.is_complete() {
                    return err(ErrorKind::InvalidUtf8, text);
                }
                core::str::from_utf8(w.unescape_into(dst)?)
                    .map(Some)
                    .map_err(|e| ParseError::new(ErrorKind::InvalidUtf8, e.valid_up_to()))
            }
        }
    }

    /// The offset of a value in the text.
    fn value_text(&self, w: &Word<'_>) -> usize {
        let raw = match w {
            Word::Token(t) => t,
            Word::Quoted(q) => q.raw(),
        };
        (raw.as_ptr() as usize).saturating_sub(self.text.as_ptr() as usize)
    }

    /// The parameters as sent, sections included, in order.
    ///
    /// # Examples
    ///
    /// ```
    /// use parse_net::{mail, Limits};
    ///
    /// let (p, _) = mail::parse_mime_params(b"; a*0=x; a*1=y", &Limits::DEFAULT)?;
    /// let names: Vec<&[u8]> = p.iter().map(|(name, _)| name).collect();
    /// assert_eq!(names, [&b"a*0"[..], b"a*1"]);
    /// # Ok::<(), parse_net::ParseError>(())
    /// ```
    #[must_use]
    pub fn iter(&self) -> MimeParamIter<'a> {
        MimeParamIter { s: self.text, i: 0 }
    }

    /// The number of parameters, sections counted one by one.
    ///
    /// # Examples
    ///
    /// ```
    /// use parse_net::{mail, Limits};
    ///
    /// let (p, _) = mail::parse_mime_params(b"; a*0=x; a*1=y; b=z", &Limits::DEFAULT)?;
    /// assert_eq!(p.len(), 3);
    /// # Ok::<(), parse_net::ParseError>(())
    /// ```
    #[must_use]
    pub const fn len(&self) -> usize {
        self.count
    }

    /// Whether there are no parameters.
    ///
    /// # Examples
    ///
    /// ```
    /// use parse_net::{mail, Limits};
    ///
    /// let (p, used) = mail::parse_mime_params(b" no params", &Limits::DEFAULT)?;
    /// assert!(p.is_empty() && used == 0);
    /// # Ok::<(), parse_net::ParseError>(())
    /// ```
    #[must_use]
    pub const fn is_empty(&self) -> bool {
        self.count == 0
    }
}

/// The parameters of a validated [`MimeParams`].
///
/// # Examples
///
/// ```
/// use parse_net::{mail, Limits};
///
/// let (p, _) = mail::parse_mime_params(b"; format=flowed", &Limits::DEFAULT)?;
/// let mut it = p.iter();
/// let (name, value) = it.next().expect("one parameter");
/// assert!(name == b"format" && value.eq_bytes(b"flowed"));
/// assert!(it.next().is_none());
/// # Ok::<(), parse_net::ParseError>(())
/// ```
#[derive(Clone, Debug)]
pub struct MimeParamIter<'a> {
    s: &'a [u8],
    i: usize,
}

impl<'a> Iterator for MimeParamIter<'a> {
    type Item = (&'a [u8], Word<'a>);

    fn next(&mut self) -> Option<Self::Item> {
        let j = cfws(self.s, self.i, &RELAXED).ok()?;
        if self.s.get(j) != Some(&b';') {
            return None;
        }
        let (name, value, _, end) = param_at(self.s, j, &RELAXED).ok()?;
        self.i = end;
        Some((name, value))
    }
}

/// A MIME parameter value.
///
/// # Examples
///
/// ```
/// use parse_net::{mail::{self, MimeValue}, Limits};
///
/// let (p, _) = mail::parse_mime_params(b"; size=10; t*0=a; t*1=b", &Limits::DEFAULT)?;
/// assert!(matches!(p.get("size"), Some(MimeValue::Plain(w)) if w.eq_bytes(b"10")));
/// assert!(matches!(p.get("t"), Some(MimeValue::Continued(_))));
/// # Ok::<(), parse_net::ParseError>(())
/// ```
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum MimeValue<'a> {
    /// `name=value`.
    Plain(Word<'a>),
    /// `name*=charset'lang'value`.
    Extended(ExtValue<'a>),
    /// `name*0`, `name*1*`, ...: RFC 2231 continuations.
    Continued(Continuation<'a>),
}

/// The sections of an RFC 2231 continued value, validated at parse.
///
/// # Examples
///
/// ```
/// use parse_net::{mail::{self, MimeValue}, Limits};
///
/// let (p, _) = mail::parse_mime_params(b"; url*0=\"http://a.example/\"; url*1=\"b\"", &Limits::DEFAULT)?;
/// let Some(MimeValue::Continued(c)) = p.get("url") else { unreachable!() };
/// let mut buf = [0u8; 32];
/// assert_eq!(c.decode_into(&mut buf)?, "http://a.example/b");
/// # Ok::<(), parse_net::ParseError>(())
/// ```
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct Continuation<'a> {
    params: MimeParams<'a>,
    name: &'a [u8],
}

impl Continuation<'_> {
    /// The number of sections.
    ///
    /// # Examples
    ///
    /// ```
    /// use parse_net::{mail::{self, MimeValue}, Limits};
    ///
    /// let (p, _) = mail::parse_mime_params(b"; n*0=a; n*1=b; n*2=c", &Limits::DEFAULT)?;
    /// let Some(MimeValue::Continued(c)) = p.get("n") else { unreachable!() };
    /// assert_eq!(c.sections(), 3);
    /// # Ok::<(), parse_net::ParseError>(())
    /// ```
    #[must_use]
    pub fn sections(&self) -> usize {
        self.params
            .iter()
            .filter(|(n, _)| matches!(split_name(n), (b, Section::Part { .. }) if b.eq_ignore_ascii_case(self.name)))
            .count()
    }

    /// An upper bound on the decoded length (twice the raw length, for
    /// ISO-8859-1 transcoded to UTF-8).
    ///
    /// # Examples
    ///
    /// ```
    /// use parse_net::{mail::{self, MimeValue}, Limits};
    ///
    /// let (p, _) = mail::parse_mime_params(b"; t*0=abc; t*1=de", &Limits::DEFAULT)?;
    /// let Some(MimeValue::Continued(c)) = p.get("t") else { unreachable!() };
    /// assert_eq!(c.max_decoded_len(), 10);
    /// let mut buf = [0u8; 10];
    /// assert_eq!(c.decode_into(&mut buf)?, "abcde");
    /// # Ok::<(), parse_net::ParseError>(())
    /// ```
    #[must_use]
    pub fn max_decoded_len(&self) -> usize {
        self.params
            .iter()
            .filter(|(n, _)| matches!(split_name(n), (b, Section::Part { .. }) if b.eq_ignore_ascii_case(self.name)))
            .map(|(_, w)| w.len().saturating_mul(2))
            .sum()
    }

    /// Decodes the sections in order into `out`: `%XX` in extended sections,
    /// the charset of section 0 (UTF-8 or ISO-8859-1, transcoded).
    ///
    /// # Errors
    ///
    /// [`ErrorKind::BufferTooSmall`]; nothing is written on error.
    ///
    /// # Examples
    ///
    /// ```
    /// use parse_net::{mail::{self, MimeValue}, Limits};
    ///
    /// let (p, _) = mail::parse_mime_params(b"; title*0*=iso-8859-1'en'%A3; title*1=\" rates\"", &Limits::DEFAULT)?;
    /// let Some(MimeValue::Continued(c)) = p.get("title") else { unreachable!() };
    /// let mut buf = [0u8; 16];
    /// assert_eq!((c.sections(), c.decode_into(&mut buf)?), (2, "\u{a3} rates"));
    /// # Ok::<(), parse_net::ParseError>(())
    /// ```
    pub fn decode_into<'b>(&self, out: &'b mut [u8]) -> Result<&'b str> {
        let len = self.walk(&RELAXED, &mut |_| {})?;
        let Some(dst) = out.get_mut(..len) else {
            return err(ErrorKind::BufferTooSmall, 0);
        };
        let mut n = 0usize;
        let _ = self.walk(&RELAXED, &mut |b: u8| {
            if let Some(d) = dst.get_mut(n) {
                *d = b;
            }
            n += 1;
        })?;
        core::str::from_utf8(dst)
            .map_err(|e| ParseError::new(ErrorKind::InvalidUtf8, e.valid_up_to()))
    }

    /// Orders the sections, checks them, and emits the decoded UTF-8 bytes;
    /// returns their count. A section given twice is refused, so the value
    /// cannot depend on the order of the parameters.
    fn walk(&self, limits: &Limits, emit: &mut impl FnMut(u8)) -> Result<usize> {
        let base = self.params.text.as_ptr() as usize;
        let at = |s: &[u8]| (s.as_ptr() as usize).saturating_sub(base);
        let mut slots: [Option<(bool, Word<'_>)>; MAX_RFC2231_SECTIONS as usize] =
            [None; MAX_RFC2231_SECTIONS as usize];
        let max = usize::from(limits.rfc2231_sections.min(MAX_RFC2231_SECTIONS));
        let mut highest = 0usize;
        for (name, w) in self.params.iter() {
            if let (b, Section::Part { n, ext }) = split_name(name) {
                if !b.eq_ignore_ascii_case(self.name) {
                    continue;
                }
                if n >= max {
                    return err(ErrorKind::TooManySections, at(name));
                }
                match slots.get_mut(n) {
                    Some(Some(_)) => return err(ErrorKind::SectionGap, at(name)),
                    Some(slot) => *slot = Some((ext, w)),
                    None => {}
                }
                highest = highest.max(n);
            }
        }
        let used = slots.get(..=highest).unwrap_or(&[]);
        if let Some(gap) = used.iter().position(Option::is_none) {
            // The section after the gap is the first that cannot be accepted.
            let after = self.params.iter().find(|(name, _)| {
                matches!(split_name(name), (b, Section::Part { n, .. })
                    if b.eq_ignore_ascii_case(self.name) && n > gap)
            });
            let offset = after.map_or(0, |(name, _)| at(name));
            return err(ErrorKind::SectionGap, offset);
        }
        let mut charset = Charset::Utf8;
        let mut utf8 = crate::utf8::Utf8::new();
        let (mut count, mut prev) = (0usize, 0u8);
        // Every section passes through here, so what spans two sections is
        // seen whole: `strict` also refuses HTAB, for decoded bytes.
        let mut put = |b: u8, charset: Charset, strict: bool, off: usize| -> Result<()> {
            if (b < 0x20 && (strict || b != b'\t')) || b == 0x7f {
                return err(ErrorKind::ControlInDecoded, off);
            }
            if charset == Charset::UsAscii && b >= 0x80 {
                return err(ErrorKind::InvalidByte, off);
            }
            let c1 = if charset == Charset::Iso8859_1 {
                (0x80..=0x9f).contains(&b)
            } else {
                prev == 0xc2 && (0x80..=0x9f).contains(&b)
            };
            if c1 {
                return err(ErrorKind::ControlInDecoded, off);
            }
            prev = b;
            let decoded = [0xc0 | (b >> 6), 0x80 | (b & 0x3f)];
            let bytes: &[u8] = if charset == Charset::Iso8859_1 && b >= 0x80 {
                &decoded
            } else {
                core::slice::from_ref(&b)
            };
            for &x in bytes {
                if !utf8.push(x) {
                    return err(ErrorKind::InvalidUtf8, off);
                }
                emit(x);
                count += 1;
            }
            Ok(())
        };
        for (k, slot) in used.iter().enumerate() {
            let Some((ext, w)) = *slot else { continue };
            let raw: &[u8] = match (k, ext, w) {
                (0, true, Word::Token(t)) => {
                    let e = ExtValue::parse_mime(t)?;
                    charset = e.charset();
                    e.encoded()
                }
                (_, true, Word::Token(t)) => t,
                _ => &[],
            };
            if ext {
                let mut i = 0usize;
                while let Some(&b) = raw.get(i) {
                    let start = i;
                    let byte = if b == b'%' {
                        let hi = raw
                            .get(i + 1)
                            .copied()
                            .and_then(class::hex_value)
                            .unwrap_or(0);
                        let lo = raw
                            .get(i + 2)
                            .copied()
                            .and_then(class::hex_value)
                            .unwrap_or(0);
                        i += 3;
                        (hi << 4) | lo
                    } else {
                        i += 1;
                        b
                    };
                    put(byte, charset, true, at(raw) + start)?;
                }
            } else {
                match w {
                    Word::Token(t) => {
                        for (i, &byte) in t.iter().enumerate() {
                            put(byte, charset, false, at(t) + i)?;
                        }
                    }
                    Word::Quoted(q) => {
                        for byte in q.bytes() {
                            put(byte, charset, false, at(q.raw()))?;
                        }
                    }
                }
            }
        }
        if !utf8.is_complete() {
            return err(ErrorKind::InvalidUtf8, self.params.text.len());
        }
        Ok(count)
    }
}

/// The bytes a parameter name may hold: MIME token characters except `*`,
/// `'`, and `%`, which RFC 2231 gives meaning to.
fn is_attribute(name: &[u8]) -> bool {
    !name.is_empty()
        && name
            .iter()
            .all(|&b| class::is_mime_token_char(b) && !matches!(b, b'*' | b'\'' | b'%'))
}

/// The line length writers keep to.
const LINE: usize = 78;

/// The length of `b` in an extended value: 1 for an attr-char, else 3.
const fn unit_len(b: u8) -> usize {
    if class::is_attr_char(b) { 1 } else { 3 }
}

/// Writes `b` in an extended value: itself or `%XX`.
fn put_unit(b: u8, out: &mut impl Sink) -> core::result::Result<(), WriteError> {
    const HEX: &[u8; 16] = b"0123456789ABCDEF";
    if class::is_attr_char(b) {
        out.put(&[b])?;
    } else {
        let hi = HEX.get(usize::from(b >> 4)).copied().unwrap_or(b'0');
        let lo = HEX.get(usize::from(b & 0xf)).copied().unwrap_or(b'0');
        out.put(&[b'%', hi, lo])?;
    }
    Ok(())
}

/// The one-line forms of a parameter value.
#[derive(Clone, Copy)]
enum Form {
    Token,
    Quoted,
    Extended,
}

/// The one-line form `value` takes, and the length of `; name=value` in it.
fn form(n: &[u8], v: &[u8]) -> (Form, usize) {
    // Each test stops at the first byte that fails it; the printable test
    // and the count of escapes are simple enough to vectorize.
    if !v.is_empty() && v.iter().all(|&b| class::is_mime_token_char(b)) {
        return (Form::Token, 3 + n.len() + v.len());
    }
    if v.iter().all(|&b| (0x20..=0x7e).contains(&b)) {
        let escaped = v.iter().filter(|&&b| b == b'"' || b == b'\\').count();
        return (Form::Quoted, 5 + n.len() + v.len() + escaped);
    }
    let encoded: usize = v.iter().map(|&b| unit_len(b)).sum();
    (Form::Extended, 11 + n.len() + encoded)
}

/// Writes `name=value` in `form`, without the `; ` before it.
fn put_form(
    form: Form,
    n: &[u8],
    v: &[u8],
    out: &mut impl Sink,
) -> core::result::Result<(), WriteError> {
    out.put(n)?;
    match form {
        Form::Token => {
            out.put(b"=")?;
            out.put(v)?;
        }
        Form::Quoted => {
            out.put(b"=\"")?;
            // The runs between the bytes that need a `\` go out whole.
            let mut rest = v;
            while let Some(p) = rest.iter().position(|&b| b == b'"' || b == b'\\') {
                out.put(rest.get(..p).unwrap_or(&[]))?;
                out.put(b"\\")?;
                out.put(rest.get(p..p + 1).unwrap_or(&[]))?;
                rest = rest.get(p + 1..).unwrap_or(&[]);
            }
            out.put(rest)?;
            out.put(b"\"")?;
        }
        Form::Extended => {
            out.put(b"*=UTF-8''")?;
            let mut rest = v;
            while let Some(p) = rest.iter().position(|&b| !class::is_attr_char(b)) {
                out.put(rest.get(..p).unwrap_or(&[]))?;
                put_unit(rest.get(p).copied().unwrap_or(0), out)?;
                rest = rest.get(p + 1..).unwrap_or(&[]);
            }
            out.put(rest)?;
        }
    }
    Ok(())
}

/// Writes the parameter, whose one-line form is `f` of `len` bytes, where
/// `room` columns are left on the line, and returns the columns left on the
/// line it ends on.
fn put_param(
    n: &[u8],
    v: &[u8],
    (f, len): (Form, usize),
    room: usize,
    out: &mut impl Sink,
) -> core::result::Result<usize, WriteError> {
    if len <= room {
        out.put(b"; ")?;
        put_form(f, n, v, out)?;
        return Ok(room - len);
    }
    // On a line of its own, ` name=value` keeps one column for the `;` that
    // may follow, as the sections do.
    if len <= LINE {
        out.put(b";\r\n ")?;
        put_form(f, n, v, out)?;
        return Ok(LINE - (len - 1));
    }
    // Sections, one per line: `;CRLF name*k*=...`, each line within 78
    // columns with its trailing `;`. A name that leaves no room for one
    // encoded byte after `*kk*=UTF-8''` cannot be written.
    if 1 + n.len() + 12 + 3 + 1 > LINE {
        return Err(WriteError::OutOfRange);
    }
    out.put(b";")?;
    let mut rest = v;
    let mut k = 0usize;
    let mut col = 0usize;
    while !rest.is_empty() || k == 0 {
        if k >= usize::from(MAX_RFC2231_SECTIONS) {
            return Err(WriteError::OutOfRange);
        }
        let mut digits = [0u8; 2];
        let dl = if k < 10 {
            digits[0] = b'0' + k as u8;
            1
        } else {
            digits[0] = b'0' + (k / 10) as u8;
            digits[1] = b'0' + (k % 10) as u8;
            2
        };
        if k > 0 {
            out.put(b";")?;
        }
        out.put(b"\r\n ")?;
        out.put(n)?;
        out.put(b"*")?;
        out.put(digits.get(..dl).unwrap_or(&[]))?;
        out.put(b"*=")?;
        col = 1 + n.len() + 1 + dl + 2;
        if k == 0 {
            out.put(b"UTF-8''")?;
            col += 7;
        }
        let mut took = 0usize;
        for &b in rest {
            let u = unit_len(b);
            // One column stays for the `;` that may follow.
            if col + u + 1 > LINE && took > 0 {
                break;
            }
            put_unit(b, out)?;
            col += u;
            took += 1;
        }
        rest = rest.get(took..).unwrap_or(&[]);
        k += 1;
    }
    Ok(LINE.saturating_sub(col))
}

/// Checks the name and value, then writes the parameter with nothing written
/// when the sink reports too little room.
fn write_param(
    name: &str,
    value: &str,
    room: usize,
    out: &mut impl Sink,
) -> core::result::Result<usize, WriteError> {
    if !is_attribute(name.as_bytes()) {
        return Err(WriteError::NotToken);
    }
    // ASCII text holds no C1 control, so its bytes are tested alone.
    let control = if value.is_ascii() {
        value.bytes().position(|b| b < 0x20 || b == 0x7f)
    } else {
        value
            .char_indices()
            .find(|&(_, c)| c.is_control())
            .map(|(at, _)| at)
    };
    if let Some(at) = control {
        return Err(WriteError::forbidden(at));
    }
    let (n, v) = (name.as_bytes(), value.as_bytes());
    let f = form(n, v);
    let mut count = CountSink::new();
    let _ = put_param(n, v, f, room, &mut count)?;
    reserve(out, count.count())?;
    put_param(n, v, f, room, out)
}

/// Writes `; name=value`: as a token, as a quoted string, or, for non-ASCII
/// or long values, as `; name*=UTF-8''...`, split into `name*0*`,
/// `name*1*` sections on folded lines when one line would pass 78 columns.
/// The parameter starts a line of its own: use [`write_mime_param_at`]
/// after other text on the line. The output parses back with
/// [`parse_mime_params`] to the same value.
///
/// # Errors
///
/// [`WriteError::NotToken`] for a name that is not a MIME token or holds `*`,
/// `'`, or `%`; [`WriteError::Forbidden`] for a control character in
/// `value`; [`WriteError::OutOfRange`] for a value needing more than 32
/// sections; [`WriteError::Full`] (nothing written when the sink reports its
/// room).
///
/// # Examples
///
/// ```
/// use parse_net::{mail, SliceSink};
///
/// let mut buf = [0u8; 64];
/// let mut out = SliceSink::new(&mut buf);
/// mail::write_mime_param("filename", "caf\u{e9}.txt", &mut out)?;
/// assert_eq!(out.written(), b"; filename*=UTF-8''caf%C3%A9.txt");
/// # Ok::<(), parse_net::WriteError>(())
/// ```
pub fn write_mime_param(
    name: &str,
    value: &str,
    out: &mut impl Sink,
) -> core::result::Result<(), WriteError> {
    write_param(name, value, LINE, out).map(|_| ())
}

/// Writes `; name=value` as [`write_mime_param`] does, where
/// `room_first_line` columns are left on the current line (78 less what the
/// line holds), and returns the columns left on the line the parameter ends
/// on, for the next call. A one-line form that does not fit in the room goes
/// on a new line, `;CRLF name=value`, before the value is split into
/// sections; the `;` always goes on the current line. With a
/// `room_first_line` of 78 the output is that of [`write_mime_param`].
///
/// # Errors
///
/// As [`write_mime_param`].
///
/// # Examples
///
/// ```
/// use parse_net::{mail, SliceSink};
///
/// let mut buf = [0u8; 128];
/// let mut out = SliceSink::new(&mut buf);
/// // After `Content-Disposition: attachment`, 47 columns are left.
/// let room = mail::write_mime_param_at("filename", "quarterly report for the northern region.pdf", 47, &mut out)?;
/// assert_eq!(out.written(), &b";\r\n filename=\"quarterly report for the northern region.pdf\""[..]);
/// assert_eq!(room, 22);
/// let room = mail::write_mime_param_at("size", "1024", room, &mut out)?;
/// assert_eq!(room, 22 - 11);
/// # Ok::<(), parse_net::WriteError>(())
/// ```
pub fn write_mime_param_at(
    name: &str,
    value: &str,
    room_first_line: usize,
    out: &mut impl Sink,
) -> core::result::Result<usize, WriteError> {
    write_param(name, value, room_first_line, out)
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::{format, string::String, vec, vec::Vec};

    fn p(v: &[u8]) -> MimeParams<'_> {
        parse_mime_params(v, &Limits::DEFAULT).unwrap().0
    }

    fn k(v: &[u8]) -> ErrorKind {
        parse_mime_params(v, &Limits::DEFAULT).unwrap_err().kind()
    }

    fn get(v: &[u8], name: &str) -> String {
        let mut buf = vec![0u8; 512];
        String::from(p(v).decode_into(name, &mut buf).unwrap().unwrap())
    }

    #[test]
    fn test_mime_params_basic() {
        assert_eq!(get(b"; a=b; c=\"d e\"", "C"), "d e");
        assert_eq!(get(b"; boundary={x}", "boundary"), "{x}");
        assert_eq!(get(b" ; a = (c) b (d)", "a"), "b");
        assert_eq!(get(b";\r\n a=b", "a"), "b");
        assert!(
            p(b"; a=b")
                .decode_into("z", &mut [0u8; 4])
                .unwrap()
                .is_none()
        );
        assert_eq!(parse_mime_params(b"", &Limits::DEFAULT).unwrap().1, 0);
        assert_eq!(
            parse_mime_params(b"; a=b x", &Limits::DEFAULT).unwrap().1,
            5
        );
        assert_eq!(k(b"; =b"), ErrorKind::NotToken);
        assert_eq!(k(b"; a"), ErrorKind::MissingValue);
        assert_eq!(k(b"; a="), ErrorKind::MissingValue);
        assert_eq!(k(b"; a=\"b"), ErrorKind::UnterminatedQuote);
        assert_eq!(k(b"; a=1; A=2"), ErrorKind::DuplicateParameter);
        assert_eq!(k(b"; a=1 (x"), ErrorKind::UnbalancedComment);
        let l = Limits::DEFAULT.max_params(2);
        assert!(parse_mime_params(b"; a=1; b=2", &l).is_ok());
        assert_eq!(
            parse_mime_params(b"; a=1; b=2; c=3", &l)
                .unwrap_err()
                .kind(),
            ErrorKind::TooManyMembers
        );
    }

    #[test]
    fn test_mime_params_rfc2231() {
        assert_eq!(
            get(
                b"; t*=us-ascii'en-us'This%20is%20%2A%2A%2Afun%2A%2A%2A",
                "t"
            ),
            "This is ***fun***"
        );
        assert_eq!(get(b"; t*0=\"foo\"; t*1=bar", "t"), "foobar");
        assert_eq!(get(b"; t*1=bar; t*0=\"foo\"", "t"), "foobar");
        assert_eq!(get(b"; t*0*=UTF-8''%E2%82; t*1*=%AC", "t"), "\u{20ac}");
        assert_eq!(get(b"; t=plain; t*=UTF-8''ext", "t"), "ext");
        assert_eq!(get(b"; t=plain; t*0=cont", "t"), "cont");
        assert_eq!(get(b"; t*00=x; t=plain", "t"), "plain");
        assert_eq!(k(b"; t*0=a; t*2=c"), ErrorKind::SectionGap);
        assert_eq!(k(b"; t*1=a"), ErrorKind::SectionGap);
        assert_eq!(k(b"; t*0=a; t*0=b"), ErrorKind::DuplicateParameter);
        assert_eq!(
            k(b"; t*0*=UTF-8''a; t*1*=UTF-8''b"),
            ErrorKind::InvalidPercentEncoding
        );
        assert_eq!(k(b"; t*0*=UTF-8''%C3"), ErrorKind::InvalidUtf8);
        assert_eq!(k(b"; t*0*=UTF-8''%0D"), ErrorKind::ControlInDecoded);
        assert_eq!(k(b"; t*=\"UTF-8''x\""), ErrorKind::InvalidCharset);
        let mut v = Vec::new();
        for i in 0..32 {
            v.extend_from_slice(format!("; t*{i}=x").as_bytes());
        }
        assert_eq!(
            p(&v)
                .get("t")
                .map(|m| matches!(m, MimeValue::Continued(c) if c.sections() == 32)),
            Some(true)
        );
        v.extend_from_slice(b"; t*32=x");
        assert_eq!(k(&v), ErrorKind::TooManySections);
        let l = Limits::DEFAULT.max_rfc2231_sections(2);
        assert!(parse_mime_params(b"; t*0=a; t*1=b", &l).is_ok());
        assert_eq!(
            parse_mime_params(b"; t*0=a; t*1=b; t*2=c", &l)
                .unwrap_err()
                .kind(),
            ErrorKind::TooManySections
        );
    }

    fn written(name: &str, value: &str) -> Vec<u8> {
        let mut buf = vec![0u8; 4096];
        let mut out = crate::SliceSink::new(&mut buf);
        write_mime_param(name, value, &mut out).unwrap();
        out.written().to_vec()
    }

    #[test]
    fn test_write_mime_param_forms_and_round_trip() {
        assert_eq!(written("charset", "utf-8"), b"; charset=utf-8");
        assert_eq!(written("name", "a \"b\""), b"; name=\"a \\\"b\\\"\"");
        assert_eq!(written("name", ""), b"; name=\"\"");
        let long = "x".repeat(200);
        let nonascii = "\u{e9}".repeat(60);
        for value in [
            String::from("plain"),
            long,
            nonascii,
            String::from("caf\u{e9} au lait.txt"),
        ] {
            let w = written("filename", &value);
            for line in w.split(|&b| b == b'\n') {
                assert!(line.len() <= 79, "{}", String::from_utf8_lossy(line));
            }
            let (params, used) = parse_mime_params(&w, &Limits::DEFAULT).unwrap();
            assert_eq!(used, w.len());
            let mut buf = vec![0u8; 1024];
            assert_eq!(
                params.decode_into("filename", &mut buf).unwrap(),
                Some(value.as_str())
            );
        }
        let mut buf = [0u8; 8];
        let mut out = crate::SliceSink::new(&mut buf);
        assert_eq!(
            write_mime_param("a*b", "x", &mut out),
            Err(WriteError::NotToken)
        );
        assert_eq!(
            write_mime_param("a", "x\ny", &mut out),
            Err(WriteError::Forbidden { at: 1 })
        );
        assert_eq!(
            write_mime_param("a", "toolongvalue", &mut out),
            Err(WriteError::Full)
        );
        assert!(out.is_empty());
        let mut big = vec![0u8; 1 << 16];
        let mut out = crate::SliceSink::new(&mut big);
        assert_eq!(
            write_mime_param("f", &"\u{e9}".repeat(1000), &mut out),
            Err(WriteError::OutOfRange)
        );
    }

    fn written_at(name: &str, value: &str, room: usize) -> (String, usize) {
        let mut buf = vec![0u8; 4096];
        let mut out = crate::SliceSink::new(&mut buf);
        let left = write_mime_param_at(name, value, room, &mut out).unwrap();
        (String::from_utf8(out.written().to_vec()).unwrap(), left)
    }

    #[test]
    fn test_write_mime_param_at_room() {
        // `; charset=utf-8` is 15 columns: it fits in 15, not in 14.
        assert_eq!(
            written_at("charset", "utf-8", 15),
            (String::from("; charset=utf-8"), 0)
        );
        assert_eq!(
            written_at("charset", "utf-8", 14),
            (String::from(";\r\n charset=utf-8"), 64)
        );
        assert_eq!(
            written_at("charset", "utf-8", 0),
            (String::from(";\r\n charset=utf-8"), 64)
        );
        assert_eq!(written_at("n", "a b", 9), (String::from("; n=\"a b\""), 0));
        assert_eq!(
            written_at("n", "a b", 8),
            (String::from(";\r\n n=\"a b\""), 70)
        );
        assert_eq!(
            written_at("f", "caf\u{e9}", 21),
            (String::from("; f*=UTF-8''caf%C3%A9"), 0)
        );
        assert_eq!(
            written_at("f", "caf\u{e9}", 20),
            (String::from(";\r\n f*=UTF-8''caf%C3%A9"), 58)
        );
        // A one-line form of 78 columns goes on a line of its own, where its
        // 77 leave the column for a `;`.
        let v = "x".repeat(78 - 3 - 1);
        let (w, left) = written_at("a", &v, 78);
        assert_eq!((w.len(), left), (78, 0));
        let (w, left) = written_at("a", &v, 10);
        assert_eq!((w.len(), left), (80, 1));
        // Too long for one line at all: sections, as `write_mime_param`.
        let long = "x".repeat(200);
        for room in [0, 10, 78] {
            let (w, left) = written_at("filename", &long, room);
            assert!(w.starts_with(";\r\n filename*0*=UTF-8''"), "{w}");
            let last = w.rsplit("\r\n").next().unwrap();
            assert_eq!(left, 78 - last.len());
        }
        // Calls chain on what is left.
        let mut buf = vec![0u8; 256];
        let mut out = crate::SliceSink::new(&mut buf);
        let mut room = 78 - "Content-Type: text/plain".len();
        for (n, v) in [
            ("charset", "utf-8"),
            ("format", "flowed"),
            ("delsp", "yes"),
            ("name", "a long attachment name.txt"),
        ] {
            room = write_mime_param_at(n, v, room, &mut out).unwrap();
        }
        assert_eq!(
            out.written(),
            b"; charset=utf-8; format=flowed; delsp=yes;\r\n name=\"a long attachment name.txt\""
        );
        assert_eq!(room, 78 - 34);
        assert_eq!(
            write_mime_param_at("a*", "x", 78, &mut crate::SliceSink::new(&mut [0u8; 8])),
            Err(WriteError::NotToken)
        );
    }
}