jsonformat/src/lib.rs

//!
//! jsonformat is a library for formatting json.
//!
//! It does not do anything more than that, which makes it so fast.

use std::{
    io,
    io::{Read, Write},
};

/// Set the indentation used for the formatting.
///
/// Note: It is *not* recommended to set indentation to anything oder than some spaces or some tabs,
/// but nothing is stopping you from doing that.
#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
pub enum Indentation<'a> {
    /// Fast path for two spaces
    TwoSpace,
    /// Fast path for four spaces
    FourSpace,
    /// Fast path for tab
    Tab,
    /// Use a custom indentation String
    Custom(&'a str),
}

impl Default for Indentation<'_> {
    fn default() -> Self {
        Self::TwoSpace
    }
}

/// # Formats a json string
///
/// The indentation can be set to any value using [`Indentation`]
/// The default value is two spaces
/// The default indentation is faster than a custom one
pub fn format(json: &str, indentation: Indentation) -> String {
    let mut reader = json.as_bytes();
    let mut writer = Vec::with_capacity(json.len());

    format_reader_writer(&mut reader, &mut writer, indentation).unwrap();
    String::from_utf8(writer).unwrap()
}

/// # Formats a json string
///
/// The indentation can be set to any value using [`Indentation`]
/// The default value is two spaces
/// The default indentation is faster than a custom one
pub fn format_reader_writer<R, W>(
    reader: R,
    mut writer: W,
    indentation: Indentation,
) -> io::Result<()>
where
    R: Read,
    W: Write,
{
    let mut escaped = false;
    let mut in_string = false;
    let mut indent_level = 0usize;
    let mut newline_requested = false; // invalidated if next character is ] or }

    for char in reader.bytes() {
        let char = char?;
        if in_string {
            let mut escape_here = false;
            match char {
                b'"' => {
                    if !escaped {
                        in_string = false;
                    }
                }
                b'\\' => {
                    if !escaped {
                        escape_here = true;
                    }
                }
                _ => {}
            }
            writer.write_all(&[char])?;
            escaped = escape_here;
        } else {
            let mut auto_push = true;
            let mut request_newline = false;
            let old_level = indent_level;

            match char {
                b'"' => in_string = true,
                b' ' | b'\n' | b'\t' => continue,
                b'[' => {
                    indent_level += 1;
                    request_newline = true;
                }
                b'{' => {
                    indent_level += 1;
                    request_newline = true;
                }
                b'}' | b']' => {
                    indent_level = indent_level.saturating_sub(1);
                    if !newline_requested {
                        // see comment below about newline_requested
                        writer.write_all(b"\n")?;
                        indent(&mut writer, indent_level, indentation)?;
                    }
                }
                b':' => {
                    auto_push = false;
                    writer.write_all(&[char])?;
                    writer.write_all(&[b' '])?;
                }
                b',' => {
                    request_newline = true;
                }
                _ => {}
            }
            if newline_requested && char != b']' && char != b'}' {
                // newline only happens after { [ and ,
                // this means we can safely assume that it being followed up by } or ]
                // means an empty object/array
                writer.write_all(b"\n")?;
                indent(&mut writer, old_level, indentation)?;
            }

            if auto_push {
                writer.write_all(&[char])?;
            }

            newline_requested = request_newline;
        }
    }

    // trailing newline
    writer.write_all(b"\n")?;

    Ok(())
}

fn indent<W>(writer: &mut W, level: usize, indent_str: Indentation) -> io::Result<()>
where
    W: Write,
{
    for _ in 0..level {
        match indent_str {
            Indentation::TwoSpace => {
                writer.write_all(b"  ")?;
            }
            Indentation::FourSpace => {
                writer.write_all(b"    ")?;
            }
            Indentation::Tab => {
                writer.write_all(b"\t")?;
            }
            Indentation::Custom(indent) => {
                writer.write_all(indent.as_bytes())?;
            }
        }
    }

    Ok(())
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn echoes_primitive() {
        let json = "1.35\n";
        assert_eq!(json, format(json, Indentation::TwoSpace));
    }

    #[test]
    fn ignore_whitespace_in_string() {
        let json = "\" hallo \"\n";
        assert_eq!(json, format(json, Indentation::TwoSpace));
    }

    #[test]
    fn remove_leading_whitespace() {
        let json = "   0";
        let expected = "0\n";
        assert_eq!(expected, format(json, Indentation::TwoSpace));
    }

    #[test]
    fn handle_escaped_strings() {
        let json = "  \" hallo \\\" \" ";
        let expected = "\" hallo \\\" \"\n";
        assert_eq!(expected, format(json, Indentation::TwoSpace));
    }

    #[test]
    fn simple_object() {
        let json = "{\"a\":0}";
        let expected = "{
  \"a\": 0
}
";
        assert_eq!(expected, format(json, Indentation::TwoSpace));
    }

    #[test]
    fn simple_array() {
        let json = "[1,2,null]";
        let expected = "[
  1,
  2,
  null
]
";
        assert_eq!(expected, format(json, Indentation::TwoSpace));
    }

    #[test]
    fn array_of_object() {
        let json = "[{\"a\": 0}, {}, {\"a\": null}]";
        let expected = "[
  {
    \"a\": 0
  },
  {},
  {
    \"a\": null
  }
]
";

        assert_eq!(expected, format(json, Indentation::TwoSpace));
    }

    #[test]
    fn already_formatted() {
        let expected = "[
  {
    \"a\": 0
  },
  {},
  {
    \"a\": null
  }
]
";

        assert_eq!(expected, format(expected, Indentation::TwoSpace));
    }
}