api: automatically escape fields that contain the comment character

Previously, if data is written with QuoteStyle::Necessary, and the
first field of a row happens to contain a comment character, the row
will be ignored as a comment when later reading it back in.

This change adds a `comment` property to Writer, and automatically
quotes fields that have the provided comment character in them, so they
round-trip correctly.

Closes #283

csv-core/src/writer.rs

@@ -26,6 +26,7 @@ impl WriterBuilder {
  quote: b'"',
  escape: b'\\',
  double_quote: true,
+ comment: None,
  };
  WriterBuilder { wtr: wtr }
  }
@@ -56,6 +57,13 @@ impl WriterBuilder {
  }
  _ => unreachable!(),
  }
+ // If the first field of a row starts with a comment character,
+ // it needs to be quoted, or the row will not be readable later.
+ // As requires_quotes is calculated in advance, we force quotes
+ // when a comment character is encountered anywhere in the field.
+ if let Some(comment) = self.wtr.comment {
+ wtr.requires_quotes[comment as usize] = true;
+ }
  wtr
  }
 
@@ -119,6 +127,17 @@ impl WriterBuilder {
  self.wtr.double_quote = yes;
  self
  }
+
+ /// The comment character that will be used when later reading the file.
+ ///
+ /// If `quote_style` is set to `QuoteStyle::Necessary`, a field will
+ /// be quoted if the comment character is detected anywhere in the field.
+ ///
+ /// The default value is None.
+ pub fn comment(&mut self, comment: Option<u8>) -> &mut WriterBuilder {
+ self.wtr.comment = comment;
+ self
+ }
 }
 
 impl Default for WriterBuilder {
@@ -166,6 +185,7 @@ pub struct Writer {
  quote: u8,
  escape: u8,
  double_quote: bool,
+ comment: Option<u8>,
 }
 
 impl Clone for Writer {
@@ -183,6 +203,7 @@ impl Clone for Writer {
  quote: self.quote,
  escape: self.escape,
  double_quote: self.double_quote,
+ comment: self.comment,
  }
  }
 }
@@ -1044,4 +1065,21 @@ mod tests {
  inp = &inp[1..];
  assert_quote!(inp, out, 1, 2, InputEmpty, r#""""#);
  }
+
+ #[test]
+ fn comment_char_is_automatically_quoted() {
+ let mut wtr = WriterBuilder::new().comment(Some(b'#')).build();
+ let out = &mut [0; 1024];
+
+ assert_field!(
+ wtr,
+ b("# abc"),
+ &mut out[..],
+ 5,
+ 6,
+ InputEmpty,
+ "\"# abc"
+ );
+ assert_write!(wtr, finish, &mut out[..], 1, InputEmpty, "\"");
+ }
 }

src/writer.rs

@@ -470,6 +470,34 @@ impl WriterBuilder {
  self
  }
 
+ /// The comment character that will be used when later reading the file.
+ ///
+ /// If `quote_style` is set to `QuoteStyle::Necessary`, a field will
+ /// be quoted if the comment character is detected anywhere in the field.
+ ///
+ /// The default value is None.
+ ///
+ /// # Example
+ ///
+ /// ```
+ /// use std::error::Error;
+ /// use csv::WriterBuilder;
+ ///
+ /// # fn main() { example().unwrap(); }
+ /// fn example() -> Result<(), Box<dyn Error>> {
+ /// let mut wtr =
+ /// WriterBuilder::new().comment(Some(b'#')).from_writer(Vec::new());
+ /// wtr.write_record(&["# comment", "another"]).unwrap();
+ /// let buf = wtr.into_inner().unwrap();
+ /// assert_eq!(String::from_utf8(buf).unwrap(), "\"# comment\",another\n");
+ /// Ok(())
+ /// }
+ /// ```
+ pub fn comment(&mut self, comment: Option<u8>) -> &mut WriterBuilder {
+ self.builder.comment(comment);
+ self
+ }
+
  /// Set the capacity (in bytes) of the internal buffer used in the CSV
  /// writer. This defaults to a reasonable setting.
  pub fn buffer_capacity(&mut self, capacity: usize) -> &mut WriterBuilder {
@@ -1414,4 +1442,13 @@ mod tests {
  wtr.serialize((true, 1.3, "hi")).unwrap();
  assert_eq!(wtr_as_string(wtr), "true,1.3,hi\n");
  }
+
+ #[test]
+ fn comment_char_is_automatically_quoted() {
+ let mut wtr =
+ WriterBuilder::new().comment(Some(b'#')).from_writer(Vec::new());
+ wtr.write_record(&["# comment", "another"]).unwrap();
+ let buf = wtr.into_inner().unwrap();
+ assert_eq!(String::from_utf8(buf).unwrap(), "\"# comment\",another\n");
+ }
 }