lossy_string.rs

/// A wrapper type for `String` that has a serde::Deserialize implementation
/// that will deserialize lossily (i.e. using replacement characters for
/// invalid UTF-8 sequences).
pub struct LossyString(String);

static ESCAPE: [bool; 256] = {
  const CT: bool = true; // control character \x00..=\x1F
  const QU: bool = true; // quote \x22
  const BS: bool = true; // backslash \x5C
  const __: bool = false; // allow unescaped
  [
    //   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
    CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, // 0
    CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, // 1
    __, __, QU, __, __, __, __, __, __, __, __, __, __, __, __, __, // 2
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 3
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 4
    __, __, __, __, __, __, __, __, __, __, __, __, BS, __, __, __, // 5
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 6
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 7
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 8
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 9
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // A
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // B
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // C
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // D
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // E
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // F
  ]
};

impl<'de> serde::Deserialize<'de> for LossyString {
  fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
  where
    D: serde::Deserializer<'de>,
  {
    macro_rules! tri {
      ($opt:expr) => {
        match $opt {
          Some(v) => v,
          None => {
            return Err(serde::de::Error::custom("Unexpected end of string"))
          }
        }
      };
    }

    let val = Box::<RawValue>::deserialize(deserializer)?;
    let mut chars = val.get().chars().peekable();

    if tri!(chars.next()) != '"' {
      return Err(serde::de::Error::custom("Expected string"));
    }

    let mut str = String::new();

    loop {
      let ch = tri!(chars.next());
      if !ESCAPE[ch as usize] {
        str.push(ch);
        continue;
      }
      match ch {
        '"' => {
          break;
        }
        '\\' => match parse_escape(&mut chars, &mut str) {
          Ok(_) => {}
          Err(err) => return Err(serde::de::Error::custom(err)),
        },
        _ => {
          return Err(serde::de::Error::custom("Constrol character in string"));
        }
      }
    }

    if chars.next() != None {
      return Err(serde::de::Error::custom("Trailing characters in string"));
    }

    Ok(LossyString(str))
  }
}

static HEX: [u8; 256] = {
  const __: u8 = 255; // not a hex digit
  [
    //   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 0
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 1
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 2
    00, 01, 02, 03, 04, 05, 06, 07, 08, 09, __, __, __, __, __, __, // 3
    __, 10, 11, 12, 13, 14, 15, __, __, __, __, __, __, __, __, __, // 4
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 5
    __, 10, 11, 12, 13, 14, 15, __, __, __, __, __, __, __, __, __, // 6
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 7
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 8
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 9
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // A
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // B
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // C
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // D
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // E
    __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // F
  ]
};

fn decode_hex_val(val: char) -> Option<u16> {
  let n = HEX[val as usize] as u16;
  if n == 255 {
    None
  } else {
    Some(n)
  }
}

fn decode_hex_escape(
  chars: &mut Peekable<Chars<'_>>,
) -> Result<u16, &'static str> {
  let mut n = 0;
  for _ in 0..4 {
    let ch = match chars.next() {
      Some(ch) => ch,
      None => return Err("Unexpected end of string"),
    };
    match decode_hex_val(ch) {
      None => return Err("Invalid hex escape"),
      Some(val) => {
        n = (n << 4) + val;
      }
    }
  }
  Ok(n)
}

fn parse_escape(
  chars: &mut Peekable<Chars<'_>>,
  str: &mut String,
) -> Result<(), &'static str> {
  macro_rules! tri {
    ($opt:expr) => {
      match $opt {
        Some(v) => v,
        None => return Err("Unexpected end of string"),
      }
    };
  }

  let ch = tri!(chars.next());

  match ch {
    '"' => str.push('"'),
    '\\' => str.push('\\'),
    '/' => str.push('/'),
    'b' => str.push('\x08'),
    'f' => str.push('\x0c'),
    'n' => str.push('\n'),
    'r' => str.push('\r'),
    't' => str.push('\t'),
    'u' => {
      let c = match decode_hex_escape(chars)? {
        0xDC00..=0xDFFF => {
          str.push('\u{FFFD}');
          return Ok(());
        }

        // Non-BMP characters are encoded as a sequence of two hex
        // escapes, representing UTF-16 surrogates. If deserializing a
        // utf-8 string the surrogates are required to be paired,
        // whereas deserializing a byte string accepts lone surrogates.
        n1 @ 0xD800..=0xDBFF => {
          if *tri!(chars.peek()) == '\\' {
            chars.next();
          } else {
            str.push('\u{FFFD}');
            return Ok(());
          }

          if *tri!(chars.peek()) == 'u' {
            chars.next();
          } else {
            str.push('\u{FFFD}');
            return parse_escape(chars, str);
          }

          let n2 = decode_hex_escape(chars)?;

          if n2 < 0xDC00 || n2 > 0xDFFF {
            str.push('\u{FFFD}');
          }

          let n =
            (((n1 - 0xD800) as u32) << 10 | (n2 - 0xDC00) as u32) + 0x1_0000;

          match char::from_u32(n) {
            Some(c) => c,
            None => {
              str.push('\u{FFFD}');
              return Ok(());
            }
          }
        }

        n => match char::from_u32(n as u32) {
          Some(c) => c,
          None => {
            str.push('\u{FFFD}');
            return Ok(());
          }
        },
      };

      str.push(c);
    }
    _ => {
      return Err("Invalid escape sequence");
    }
  }

  Ok(())
}