//! # 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;
use std::{char, u16};

pub type JSONResult = Result<JSONValue, ParseError>;
pub type JSONArray = Vec<JSONValue>;
pub type JSONMap = HashMap<String, JSONValue>;

/// The type of JSON value
#[derive(Clone, Debug, PartialEq)]
pub enum JSONValue {
    /// Object Literal
    Object(HashMap<String, JSONValue>),

    /// Array Literal
    Array(Vec<JSONValue>),

    /// String Literal
    String(String),

    /// Number Literal
    Number(f64),

    /// True Literal
    True,

    /// False Literal
    False,

    /// Null Literal
    Null,
}

impl JSONValue {
    /// Convert the wrapped JSONValue to its simpler rust value
    ///
    /// Example:
    /// ```
    /// use naive_json_parser::JSONValue;
    ///
    /// let str = "Four score and seven years ago...";
    /// let wrapped = JSONValue::from(str);
    ///
    /// // s is now the `String` that was in the `JSONValue` enum
    /// let s: String = wrapped.unwrap();
    ///
    /// # assert_eq!(str, &s);
    /// ```
    pub fn unwrap<T: From<JSONValue>>(self) -> T {
        T::from(self)
    }
}

impl From<JSONValue> for JSONMap {
    /// Extracts the `HashMap` in the `JSONValue` enum, if it exists.
    /// Otherwise, panics.
    fn from(val: JSONValue) -> JSONMap {
        match val {
            JSONValue::Object(o) => o,
            _ => panic!("Invalid type conversion")
        }
    }
}

impl From<JSONValue> for JSONArray{
    /// Extracts the `Vec` in the `JSONValue` enum, if it exists.
    /// Otherwise, panics.
    fn from(val: JSONValue) -> JSONArray {
        match val {
            JSONValue::Array(a) => a,
            _ => panic!("Invalid type conversion")
        }
    }
}

impl From<JSONValue> for f64 {
    /// Extracts the `f64` in the `JSONValue` enum, if it exists.
    /// Otherwise, panics.
    fn from(val: JSONValue) -> f64 {
        match val {
            JSONValue::Number(n) => n,
            _ => panic!("Invalid type conversion")
        }
    }
}

impl From<JSONValue> for String {
    /// Extracts the `String` in the `JSONValue` enum, if it exists.
    /// Otherwise, panics.
    fn from(val: JSONValue) -> String {
        match val {
            JSONValue::String(s) => s,
            _ => panic!("Invalid type conversion")
        }
    }
}

impl From<JSONValue> for bool {
    /// Extracts the `bool` value from the `JSONValue` enum, if it exists.
    /// Otherwise, panics.
    fn from(val: JSONValue) -> bool {
        match val {
            JSONValue::True => true,
            JSONValue::False => false,
            _ => panic!("Invalid type conversion")
        }
    }
}

impl From<JSONValue> for () {
    /// This will just swallow the enum value and return a unit tuple
    fn from(_: JSONValue) -> () { () }
}

impl From<JSONMap> for JSONValue {
    /// Wraps the `HashMap` in the `JSONValue` enum
    fn from(val: JSONMap) -> JSONValue {
        Self::Object(val)
    }
}

impl From<JSONArray> for JSONValue {
    /// Wraps the `Vec` in the `JSONValue` enum
    fn from(val: JSONArray) -> JSONValue {
        Self::Array(val)
    }
}

impl From<bool> for JSONValue {
    /// Sets the `JSONValue` enum to the `True` or `False` value
    fn from(val: bool) -> Self {
        match val {
            true => Self::True,
            false => Self::False,
        }
    }
}

impl From<f64> for JSONValue {
    /// Wraps the `f64` in the `JSONValue` enum
    fn from(n: f64) -> Self {
        Self::Number(n)
    }
}

impl From<()> for JSONValue {
    /// Sets the `JSONValue` enum to the `Null` value
    fn from(_s: ()) -> Self {
        Self::Null
    }
}

impl From<String> for JSONValue {
    /// Wraps the `String` in the `JSONValue` enum
    fn from(s: String) -> Self {
        Self::String(s)
    }
}

impl From<&str> for JSONValue {
    /// Creates a `String` and wraps it in the `JSONValue` enum
    fn from(s: &str) -> Self {
        Self::String(String::from(s))
    }
}

/// 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<char>,

    /// The internal parsing index
    i: usize,
}

/// Cut down the if boilerplate
///
/// Thanks to `uwaterloodudette` on reddit
macro_rules! try_parse {
    ($( $e:expr ),* ) => {
        $(
            if let Some(v) = $e? {
                return Ok(v);
            }
        )*
    };
}

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

    /// Parse a `JSONValue` from the current JSON string
    fn parse_value(&mut self) -> JSONResult {
        self.skip_whitespace();

        // Go through the parser methods, until you find
        // one that doesn't return a `None`
        try_parse!(
            self.parse_string(),
            self.parse_number(),
            self.parse_object(),
            self.parse_array(),
            self.parse_keyword("true", JSONValue::True),
            self.parse_keyword("false", JSONValue::False),
            self.parse_keyword("null", JSONValue::Null)
        );

        // Every parser failed, so the syntax is probably incorrect
        Err(ParseError::UnexpectedEndOfInput(format!("Doesn't seem to be valid JSON")))
    }

    /// See if there's a `JSONValue::Object` next in the JSON
    fn parse_object(&mut self) -> Result<Option<JSONValue>, ParseError> {
        if self.chars[self.i] != '{' {
            return Ok(None);
        }

        self.increment(1);

        self.skip_whitespace();

        let mut result: JSONMap = HashMap::new();

        let mut initial = true;

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

            if initial == false {
                self.eat(',')?;
                self.skip_whitespace();
            }

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

            result.insert(key, value);

            initial = false;

            self.skip_whitespace();
        }

        // Move to the next character: '}'
        self.increment(1);

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

    /// See if there's a `JSONValue::Array` next in the JSON
    fn parse_array(&mut self) -> Result<Option<JSONValue>, ParseError> {
        if self.chars[self.i] != '[' {
            return Ok(None);
        }

        self.increment(1);
        self.skip_whitespace();

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

        while self.chars[self.i] != ']' {
            self.skip_whitespace();

            if initial == false {
                self.eat(',')?;
            }
            let value = self.parse_value()?;
            result.push(value);
            initial = false;
        }

        // move to next character: ']'
        self.increment(1);

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

    /// See if there's a `JSONValue::String` next in the JSON
    fn parse_string(&mut self) -> Result<Option<JSONValue>, ParseError> {
        if self.chars[self.i] != '"' {
            return Ok(None);
        }

        self.increment(1);
        let mut result = String::new();

        while self.chars[self.i] != '"' && self.i < self.chars.len() - 1 {
            // All the escape sequences...
            if self.chars[self.i] == '\\' {
                let ch = self.chars[self.i + 1];
                if ch == '"' {
                    result.push_str("\"");
                    self.increment(1);
                } else if ['\\', '/'].contains(&ch) {
                    let escaped = ch.escape_default().next().unwrap_or(ch);

                    result.push(escaped);
                    self.increment(1);
                } else if ['b', 'f', 'n', 'r', 't'].contains(&ch) {
                    let ch = match ch {
                        'b' => '\u{8}',
                        'f' => '\x0C',
                        'n' => '\n',
                        'r' => '\r',
                        't' => '\t',
                        _ => panic!("Shouldn't be possible!"),
                    };
                    result.push(ch);
                    self.increment(1);
                } else if ch == 'u'
                    && self.chars[self.i + 2].is_ascii_hexdigit()
                    && self.chars[self.i + 3].is_ascii_hexdigit()
                    && self.chars[self.i + 4].is_ascii_hexdigit()
                    && self.chars[self.i + 5].is_ascii_hexdigit()
                {
                    // Blech, parse out a JSON unicode (utf16) escape code. Handles surrogate pairs
                    // by giving you the replacement character...because...yeah
                    let char_str = String::from_iter(&self.chars[self.i + 2..=self.i + 5]);
                    let code = u16::from_str_radix(&char_str, 16)
                        .expect("Failed to parse unicode escape number");
                    let string = String::from_utf16_lossy(&[code]);

                    result.push_str(&string);

                    self.increment(5);
                }
            } else {
                result.push(self.chars[self.i]);
            }
            self.increment(1);
        }

        self.eat('"')?;

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

    /// See if there's a `JSONValue::Number` next in the JSON
    fn parse_number(&mut self) -> Result<Option<JSONValue>, ParseError> {
        let start = self.i;

        // If it doesn't start with 0-9 or a minus sign, it's probably not a number
        if !(self.chars[start].is_ascii_digit() || self.chars[start] == '-') {
            return Ok(None);
        }

        // All this looping basically just counts the number of characters in the number
        let max = self.chars.len() - 1;
        let mut n = start;

        // Minus sign
        if self.chars[n] == '-' && n < max {
            n += 1;
        }

        // Integer Part
        while self.chars[n].is_ascii_digit() && n < max {
            n += 1;
        }

        // Decimal Part
        if self.chars[n] == '.' && n < max {
            n += 1;
            while self.chars[n].is_ascii_digit() && n < max {
                n += 1;
            }
        }

        // Scientific notation part
        if self.chars[n] == 'e' || self.chars[n] == 'E' && n < max {
            n += 1;

            if self.chars[n] == '-' || self.chars[n] == '+' && n < max {
                n += 1;
            }

            // Exponent base
            while self.chars[n].is_ascii_digit() && n < max {
                n += 1;
            }
        }

        // If there are numeric digits attempt to parse the digits as a number
        if n > start {
            let mut end = if n < self.chars.len() { n } else { max };

            // Hack to remove non-number characters
            if !self.chars[end].is_ascii_digit() {
                end -= 1;
            }

            let str = String::from_iter(&self.chars[start..=end]);

            match str.parse::<f64>() {
                Ok(number) => {
                    self.increment(str.len());
                    return Ok(Some(JSONValue::from(number)));
                }
                Err(e) => Err(ParseError::ExpectedDigit(format!("'{}', {:#?}", str, e))),
            }
        } else {
            Ok(None)
        }
    }

    /// 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<Option<JSONValue>, ParseError> {
        let start = self.i;
        let end = if self.i + search.len() > self.chars.len() {
            self.chars.len()
        } else {
            self.i + search.len()
        };

        let slice = &String::from_iter(&self.chars[start..end]);
        if slice == search {
            self.i += search.len();

            return Ok(Some(value));
        }

        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.increment(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!("Expected {}.", ch);
            return Err(ParseError::ExpectedToken(msg));
        }

        self.increment(1);

        Ok(())
    }

    /// Do a checked increment of the internal pointer index
    fn increment(&mut self, amount: usize) {
        let current = self.i;
        if current + amount >= self.chars.len() {
            self.i = self.chars.len() - 1;
        } else {
            self.i += amount;
        }
    }

    /// Convert a `&str` containing JSON into a `Result<JSONValue, ParseError>`
    pub fn parse(json: &str) -> JSONResult {
        JSON::new(json).parse_value()
    }
}

#[cfg(test)]
mod tests {
    use super::JSONValue::{Array, False, Null, Number, Object, True};
    use super::*;

    #[test]
    fn value_conversion() {
        let map: JSONMap = HashMap::new();
        let num = 9.380831539;
        let str = "applesauce";
        let arr: JSONArray = vec![JSONValue::from(map.clone()), JSONValue::from(num), JSONValue::from(str)];

        assert_eq!(map.clone(), JSONMap::from(JSONValue::from(map.clone())));
        assert_eq!(num, f64::from(JSONValue::from(num)));
        assert_eq!(String::from(str), String::from(JSONValue::from(str)));
        assert_eq!(arr.clone(), JSONArray::from(JSONValue::from(arr.clone())));
        assert_eq!(true, bool::from(JSONValue::from(true)));
        assert_eq!(false, bool::from(JSONValue::from(false)));
        assert_eq!((), <()>::from(JSONValue::from(())));
    }

    #[test]
    fn wrap_and_unwrap() {
        let map: JSONMap = HashMap::new();
        let num = 9.380831539;
        let str = "applesauce";
        let arr: JSONArray = vec![JSONValue::from(map.clone()), JSONValue::from(num), JSONValue::from(str)];

        let s: String = JSONValue::from(str).unwrap();
        let a: JSONArray = JSONValue::from(arr.clone()).unwrap();

        assert_eq!(map.clone(), JSONValue::from(map.clone()).unwrap());
        assert_eq!(num, JSONValue::from(num).unwrap());
        assert_eq!(str, &s);
        assert_eq!(arr.clone(), a);
        assert_eq!(true, JSONValue::from(true).unwrap());
        assert_eq!(false, JSONValue::from(false).unwrap());
        assert_eq!((), JSONValue::from(()).unwrap());
    }

    #[test]
    fn parse_keyword() {
        let res = JSON::new(r#""foobarbaz""#).parse_keyword("true", JSONValue::True);
        assert_eq!(res, Ok(None));

        let res = JSON::new("true").parse_keyword("true", JSONValue::True);
        assert_eq!(res, Ok(Some(True)));
    }

    #[test]
    fn skip_whitespace() {
        let mut parser = JSON::new(" \t\r\nx");
        parser.skip_whitespace();
        assert_eq!('x', parser.chars[parser.i]);
    }

    #[test]
    fn parse_string() {
        let res = JSON::new(r#""\t""#).parse_string();
        assert_eq!(res, Ok(Some(JSONValue::from("\t"))));

        let res = JSON::new(r#""\u203d""#).parse_string();
        assert_eq!(res, Ok(Some(JSONValue::from("‽"))));
    }

    #[test]
    fn parse_empty_array() {
        let res = JSON::new("[]").parse_value();
        assert_eq!(res, Ok(Array(vec![])));
    }

    #[test]
    fn parse_number() {
        // This function works like I think, right?
        assert_eq!(','.is_ascii_digit(), false);

        let res = JSON::new(r#""foo""#).parse_number();
        assert_eq!(res, Ok(None));

        let res = JSON::new("3.14159").parse_number();
        assert_eq!(res, Ok(Some(Number(3.14159f64))));

        let res = JSON::new("3e4").parse_number();
        assert_eq!(res, Ok(Some(Number(3e4f64))));

        let res = JSON::new("1.234,").parse_number();
        assert_eq!(res, Ok(Some(Number(1.234f64))));
    }

    #[test]
    fn can_parse_array_of_keywords() {
        let result = JSON::parse("[true,false,null]");

        assert_eq!(result, Ok(Array(vec![True, False, Null])));
    }

    #[test]
    fn parse_object() {
        let result = JSON::new(r#"{"foo": "bar"}"#).parse_object();

        let mut hash_map: JSONMap = HashMap::new();
        hash_map.insert(String::from("foo"), JSONValue::from("bar"));

        assert_eq!(result, Ok(Some(JSONValue::Object(hash_map))));
    }

    #[test]
    fn parse_json_types() {
        // Boolean / Null
        let res = JSON::parse("true");
        assert_eq!(res, Ok(True));
        let res = JSON::parse("false");
        assert_eq!(res, Ok(False));
        let res = JSON::parse("null");
        assert_eq!(res, Ok(Null));

        // Number
        let res = JSON::parse("9.38083151965");
        assert_eq!(res, Ok(Number(9.38083151965)), "Failed to parse number");

        // String
        let res = JSON::parse(r#""/^$/""#);
        assert_eq!(
            res,
            Ok(JSONValue::from("/^$/")),
            "Failed to parse string"
        );

        // Number array
        let res = JSON::parse("[1, 2, 3]");
        assert_eq!(
            res,
            Ok(Array(vec![Number(1f64), Number(2f64), Number(3f64)]))
        );

        // Object array
        let result = JSON::parse("[{}]");
        assert_eq!(result, Ok(JSONValue::Array(vec![Object(HashMap::new())])));
    }

    #[test]
    fn parse_nested_object() {
        let res = JSON::parse(r#"{"a": {"b": []}}"#);
        let mut outermap: JSONMap = HashMap::new();
        let mut innermap: JSONMap = HashMap::new();

        innermap.insert(String::from("b"), Array(vec![]));
        outermap.insert(String::from("a"), Object(innermap));

        let expected = Ok(Object(outermap));

        assert_eq!(res, expected);
    }

    #[test]
    fn parse_object_with_number_values() {
        let result = JSON::parse(r#"[{ "a": 9.38083151965, "b": 4e3 }]"#);
        let mut map: JSONMap = HashMap::new();
        map.insert(String::from("a"), Number(9.38083151965f64));
        map.insert(String::from("b"), Number(4e3f64));

        let expected = Ok(Array(vec![Object(map)]));

        assert_eq!(
            result, expected,
            "Failed on just number values: {:#?}",
            result
        );
    }

    #[test]
    fn parse_weird_character_array() {
        let result =
            JSON::parse(r#"["\"", "\\", "/", "\b", "\f", "\n", "\r", "\t", "\u0001", "\uface"]"#);
        let expected = Ok(Array(vec![
            JSONValue::from("\""),
            JSONValue::from("\\"),
            JSONValue::from("/"),
            JSONValue::from("\u{8}"),
            JSONValue::from("\x0C"),
            JSONValue::from("\n"),
            JSONValue::from("\r"),
            JSONValue::from("\t"),
            JSONValue::from("\u{1}"),
            JSONValue::from("\u{face}"),
        ]));

        assert_eq!(result, expected);
    }

    #[test]
    fn parse_full_json_example() {
        let result = JSON::parse(
            r#"[{
  "a": 9.38083151965,
  "b": 4e3,
  "c": [1, 2, 3],
  "d": "foo",
  "e": {
    "f": {
      "g": {
        "h": null
      }
    }
  },
  "i": ["\"", "\\", "/", "\b", "\f", "\n", "\r", "\t", "\u0001", "\uface"]
}]"#,
        );

        let mut map: JSONMap = HashMap::new();
        let mut emap: JSONMap = HashMap::new();
        let mut fmap: JSONMap = HashMap::new();
        let mut gmap: JSONMap = HashMap::new();

        gmap.insert(String::from("h"), Null);
        fmap.insert(String::from("g"), Object(gmap));
        emap.insert(String::from("f"), Object(fmap));

        map.insert(String::from("a"), Number(9.38083151965f64));
        map.insert(String::from("b"), Number(4e3f64));
        map.insert(
            String::from("c"),
            Array(vec![Number(1f64), Number(2f64), Number(3f64)]),
        );
        map.insert(String::from("d"), JSONValue::from("foo"));
        map.insert(String::from("e"), Object(emap));

        map.insert(
            String::from("i"),
            Array(vec![
                JSONValue::from("\""),
                JSONValue::from("\\"),
                JSONValue::from("/"),
                JSONValue::from("\u{8}"),
                JSONValue::from("\x0C"),
                JSONValue::from("\n"),
                JSONValue::from("\r"),
                JSONValue::from("\t"),
                JSONValue::from("\u{1}"),
                JSONValue::from("\u{face}"),
            ]),
        );

        assert!(result.is_ok(), format!("{:#?}", result));

        let outer_array: Vec<JSONValue> = result.unwrap().unwrap();
        let result_map: JSONMap = outer_array[0].clone().unwrap();

        for (k, v) in &map {
            assert_eq!(
                result_map.get(k).unwrap(),
                v,
                "HashMap Entry Differs: {:#?}, {:#?}",
                result_map.get(k).unwrap(),
                v
            );
        }
    }
}