json-parser/src/lib.rs

use std::collections::HashMap;
use std::iter::FromIterator;
use crate::ParseError::UnexpectedEndOfInput;

#[derive(Debug, PartialEq)]
pub enum JSONValue {
    Object(HashMap<String, JSONValue>),
    Array(Vec<JSONValue>),
    String(String),
    Number(f64),
    True,
    False,
    Null,
}

#[derive(Debug)]
pub enum ParseError {
    UnexpectedEndOfInput(String),
    ExpectedEndOfInput(String),
    ExpectedObjectKey(String),
    ExpectedToken(String),
    UnexpectedToken(String),
    ExpectedDigit(String),
    ExpectedEscapeChar(String),
    ExpectedUnicodeEscape(String),
}

#[derive(Debug)]
pub struct JSON {
    chars: Vec<char>,
    i: usize
}

pub type JSONResult = Result<JSONValue, ParseError>;
type PartialResult = Result<Option<JSONValue>, ParseError>;

impl JSON {
    fn new(json: &str) -> Self {
       JSON {
           chars: json.chars().collect(),
           i: 0
       }
    }

    fn parse_value(&mut self) -> JSONResult {
        self.skip_whitespace();

        let mut types = self.parse_string()?.into_iter()
            .chain(self.parse_number()?.into_iter())
            .chain(self.parse_object()?.into_iter())
            .chain(self.parse_array()?.into_iter())
            .chain(self.parse_keyword("true", JSONValue::True)?.into_iter())
            .chain(self.parse_keyword("false", JSONValue::False)?.into_iter())
            .chain(self.parse_keyword("null", JSONValue::Null)?.into_iter());

        match types.next() {
            Some(val) => Ok(val),
            None => Err(UnexpectedEndOfInput(String::new()))
        }
    }

    fn parse_object(&mut self) -> PartialResult {
        if self.chars[self.i] != '{' {
            return Ok(None);
        }

        self.i += 1;

        self.skip_whitespace();

        let mut result: HashMap<String, JSONValue> = HashMap::new();

        let mut initial = true;

        // if it is not '}',
        // we take the path of string -> whitespace -> ':' -> value -> ...
        while self.chars[self.i] != '}' {
            if initial == false {
                self.eat_char(',')?;
                self.skip_whitespace();
            }

            let key = match self.parse_string()? {
                Some(value) => match value {
                    JSONValue::String(s) => s,
                    _ => panic!("parse_string returned non-string value"),
                },
                None => String::new()
            };

            self.skip_whitespace();
            self.eat_char(':')?;
            let value = self.parse_value()?;

            result.insert(key, value);

            initial = false;
        }

        // Move to the next character: '}'
        self.i += 1;

        Ok(Some(JSONValue::Object(result)))
    }

    fn parse_array(&mut self) -> PartialResult {
        if self.chars[self.i] != '[' {
            return Ok(None);
        }

        self.i += 1;
        self.skip_whitespace();

        let mut result: Vec<JSONValue> = vec![];
        let mut initial = true;

        while self.chars[self.i] != ']' {
            if initial == false {
                self.eat_char(',')?;
            }
            let value = self.parse_value()?;
            result.push(value);
            initial = false;
        }

        // move to next character: ']'
        self.i += 1;

        Ok(Some(JSONValue::Array(result)))
    }

    fn parse_string(&mut self) -> PartialResult {
        todo!();
    }

    fn parse_number(&mut self) -> PartialResult {
        todo!();
    }

    fn parse_keyword(&mut self, search: &str, value: JSONValue) -> PartialResult {
        let slice = &String::from_iter(&self.chars[self.i..self.i+search.len()]);
        if slice == search {
           self.i += search.len();

            return Ok(Some(value));
        }

        Ok(None)
    }

    fn skip_whitespace(&mut self) {
        while self.chars[self.i].is_ascii_whitespace() {
            self.i += 1;
        }
    }

    fn eat_char(&mut self, ch: char) -> Result<(), ParseError> {
        if self.chars[self.i] != ch {
            let msg = format!(r#"Expected "{}"."#, ch);
            return Err(ParseError::ExpectedToken(msg));
        }

        self.i += 1;

        Ok(())
    }

    pub fn parse(json: &str) -> JSONResult {
        JSON::new(json).parse_value()
    }
}



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

    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}