Efficient parsing of JSON record sets in Rust

use crate::Error; use async_std::io::SeekFrom; use byteorder::ReadBytesExt; use serde_json::{Deserializer, Map, Value}; use std::io::{Read, Seek}; type Record = Map<String, Value>; pub struct JsonRecords<'de, R> where R: Read + Seek, { reader: &'de mut R, is_array_start: bool, } impl<'de, R> JsonRecords<'de, R> where R: Read + Seek, { #[inline] pub fn new(reader: &'de mut R) -> Self { Self { reader, is_array_start: true, } } #[inline] fn skip_array_start(&mut self) -> Result<(), Error> { self.skip_char(b'[')?; self.is_array_start = false; Ok(()) } fn skip_char(&mut self, val: u8) -> Result<(), Error> { self.skip_ws()?; let c = self.reader.read_u8()?; if c != val { return Err(Error::Parse(format!( "expect '{}', but was '{}'", val, c ))); } Ok(()) } fn skip_ws(&mut self) -> Result<(), Error> { loop { let c = self.peek()?; match c { b' ' | b'\n' | b'\r' => { self.discard(); continue; } _ => break, } } Ok(()) } #[inline] fn peek(&mut self) -> Result<u8, Error> { let result = self.reader.read_u8()?; self.reader.seek(SeekFrom::Current(-1))?; Ok(result) } #[inline] fn discard(&mut self) { self.reader.read_u8().unwrap(); } #[inline] fn read_record(&mut self) -> Option<Result<Record, Error>> { let de = Deserializer::from_reader(&mut self.reader); let mut stream = de.into_iter(); match stream.next() { None => None, Some(t) => match t { Ok(v) => Some(Ok(v)), Err(e) => Some(Err(e.into())), }, } } } impl<'de, R> Iterator for JsonRecords<'de, R> where R: Read + Seek, { type Item = Result<Record, Error>; fn next(&mut self) -> Option<Self::Item> { if self.is_array_start { match self.skip_array_start() { Err(e) => return Some(Err(e)), Ok(()) => {} } } match self.skip_ws() { Err(e) => return Some(Err(e)), Ok(()) => {} } loop { match self.peek() { Err(e) => return Some(Err(e)), Ok(c) => match c { b']' => { self.discard(); let _ = self.skip_ws(); return None; } b',' => { self.discard(); self.skip_ws().unwrap(); continue; } b'{' => return self.read_record(), _ => { return Some(Err(Error::Parse(format!( "expected '{{' or ']', but got '{}'", c as char )))) } }, } } } } #[cfg(test)] mod tests { use crate::json_records::JsonRecords; use crate::Error; use std::io::Cursor; #[test] fn test_json_array() -> Result<(), Error> { let json = "[{\"a\": 1}, {\"a\": 2}]"; let mut cursor = Cursor::new(json.as_bytes().to_vec()); let array = JsonRecords::new(&mut cursor); for rec in array { println!("{:?}", rec?); } Ok(()) } }