From 7e49c40e984fadee6b39c23a06cee5a89ae3a58e Mon Sep 17 00:00:00 2001 From: "Timothy J. Warren" Date: Fri, 7 Feb 2020 21:27:46 -0500 Subject: [PATCH] Add comments --- src/lib.rs | 136 +++++++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 111 insertions(+), 25 deletions(-) diff --git a/src/lib.rs b/src/lib.rs index 496314d..5ed0c0c 100644 --- a/src/lib.rs +++ b/src/lib.rs @@ -1,51 +1,102 @@ //! # Naive JSON Parser +//! +//! Based on [JSON Parser with JavaScript](https://lihautan.com/json-parser-with-javascript/) +#![forbid(unsafe_code)] use std::collections::HashMap; use std::iter::FromIterator; +/// The type of JSON value #[derive(Debug, PartialEq)] pub enum JSONValue { + /// Object Literal Object(HashMap), + + /// Array Literal Array(Vec), + + /// String Literal String(String), + + /// Number Literal Number(f64), + + /// True Literal True, + + /// False Literal False, + + /// Null Literal Null, } +/// The type of error returned by the parser #[derive(Debug, PartialEq)] pub enum ParseError { + /// The input looks like JSON, but seems to end UnexpectedEndOfInput(String), + + /// Looks like JSON, but seems to have characters after it should ExpectedEndOfInput(String), + + /// Wasn't this supposed to be an object literal? ExpectedObjectKey(String), + + /// Hey, wasn't there supposed to be...? ExpectedToken(String), + + /// What's this character? UnexpectedToken(String), + + /// Shouldn't this be a numeral? ExpectedDigit(String), + + /// There's a backslash...were you going somewhere with that? ExpectedEscapeChar(String), + + /// Should be a unicode escape character...missing a few digits? ExpectedUnicodeEscape(String), } +/// This struct holds a little state for parsing #[derive(Debug, PartialEq)] pub struct JSON { + /// The input JSON String as a character array chars: Vec, - i: usize + + /// The internal parsing index + i: usize, } -pub type JSONResult = Result; -type PartialResult = Result, ParseError>; - impl JSON { + /// Private constructor fn new(json: &str) -> Self { - JSON { - chars: json.chars().collect(), - i: 0 - } + JSON { + chars: json.chars().collect(), + i: 0, + } } - fn parse_value(&mut self) -> JSONResult { + /// Parse a `JSONValue` from the current JSON string + /// + /// This is probably an abuse of iterators...but it's still much better than the alternative + /// of nested matches. + /// + /// In order to determine the type of JSON value, each parse method is tried, until one + /// matches, or a parse error happens. + /// + /// * `Option`s implement IntoIterator, which returns an iterator of -1 or 1 items: the + /// transferred (not borrowed) Some() value. + /// * The `chain` method of iterators allows you to link iterators together, + /// to act as one iterator + /// * The first result from the iterator is the first parse method with a non-empty value, + /// and should be the value wanted + fn parse_value(&mut self) -> Result { self.skip_whitespace(); - let mut types = self.parse_string()?.into_iter() + let mut value = self + .parse_string()? + .into_iter() .chain(self.parse_number()?.into_iter()) .chain(self.parse_object()?.into_iter()) .chain(self.parse_array()?.into_iter()) @@ -53,13 +104,16 @@ impl JSON { .chain(self.parse_keyword("false", JSONValue::False)?.into_iter()) .chain(self.parse_keyword("null", JSONValue::Null)?.into_iter()); - match types.next() { + match value.next() { Some(val) => Ok(val), - None => Err(ParseError::UnexpectedEndOfInput(String::new())) + None => Err(ParseError::UnexpectedEndOfInput(String::from( + "Doesn't seem to be valid JSON", + ))), } } - fn parse_object(&mut self) -> PartialResult { + /// See if there's a `JSONValue::Object` next in the JSON + fn parse_object(&mut self) -> Result, ParseError> { if self.chars[self.i] != '{' { return Ok(None); } @@ -85,7 +139,7 @@ impl JSON { JSONValue::String(s) => s, _ => panic!("parse_string returned non-string value"), }, - None => String::new() + None => String::new(), }; self.skip_whitespace(); @@ -103,7 +157,8 @@ impl JSON { Ok(Some(JSONValue::Object(result))) } - fn parse_array(&mut self) -> PartialResult { + /// See if there's a `JSONValue::Array` next in the JSON + fn parse_array(&mut self) -> Result, ParseError> { if self.chars[self.i] != '[' { return Ok(None); } @@ -129,15 +184,35 @@ impl JSON { Ok(Some(JSONValue::Array(result))) } - fn parse_string(&mut self) -> PartialResult { - Ok(None) + /// See if there's a `JSONValue::String` next in the JSON + fn parse_string(&mut self) -> Result, ParseError> { + if self.chars[self.i] != '"' { + return Ok(None); + } + + self.i += 1; + let mut result = String::new(); + + while self.chars[self.i] != '"' { + if self.chars[i] == '\\' { + + } + } + + Ok(None) } - fn parse_number(&mut self) -> PartialResult { - Ok(None) + /// See if there's a `JSONValue::Number` next in the JSON + fn parse_number(&mut self) -> Result, ParseError> { + Ok(None) } - fn parse_keyword(&mut self, search: &str, value: JSONValue) -> PartialResult { + /// See if there's a `JSONValue::True`, `JSONValue::False`, or a `JSONValue::Null` next in the JSON + fn parse_keyword( + &mut self, + search: &str, + value: JSONValue, + ) -> Result, ParseError> { let start = self.i; let end = if self.i + search.len() > self.chars.len() { self.chars.len() @@ -147,7 +222,7 @@ impl JSON { let slice = &String::from_iter(&self.chars[start..end]); if slice == search { - self.i += search.len(); + self.i += search.len(); return Ok(Some(value)); } @@ -155,12 +230,18 @@ impl JSON { Ok(None) } + /// Increment the internal index until the next character is not a whitespace character fn skip_whitespace(&mut self) { while self.chars[self.i].is_ascii_whitespace() { self.i += 1; } } + /// 'Eat' the specified character + /// + /// * If the next `char` matches the one passed, the internal index is incremented + /// * If the next `char` does not match the one passed, a `ParseError::ExpectedToken` + /// error is returned fn eat(&mut self, ch: char) -> Result<(), ParseError> { if self.chars[self.i] != ch { let msg = format!(r#"Expected "{}"."#, ch); @@ -173,13 +254,11 @@ impl JSON { } /// Convert a `&str` containing JSON into a `Result` - pub fn parse(json: &str) -> JSONResult { + pub fn parse(json: &str) -> Result { JSON::new(json).parse_value() } } - - #[cfg(test)] mod tests { use super::*; @@ -213,6 +292,13 @@ mod tests { fn can_parse_array_of_keywords() { let result = JSON::parse("[true,false,null]"); - assert_eq!(result, Ok(JSONValue::Array(vec![JSONValue::True, JSONValue::False, JSONValue::Null]))); + assert_eq!( + result, + Ok(JSONValue::Array(vec![ + JSONValue::True, + JSONValue::False, + JSONValue::Null + ])) + ); } }