//! # Naive JSON Parser
//!
//! Based on [JSON Parser with JavaScript](https://lihautan.com/json-parser-with-javascript/)
//!
//! This project is only concerned with parsing JSON. Serializing the [JSONValue](enum.JSONValue.html) into
//! a native rust object is out of scope.
//!
//! Basic usage:
//! ```rust
//! use std::convert::TryFrom;
//! use naive_json_parser::{JSON, JSONArray, JSONValue};
//!
//! // Convert the JSON string to a `JSONValue`
//! let result = JSON::parse("[0, 1, 2]");
//! # assert!(&result.is_ok());
//!
//! // Let's assume you know the JSON is valid
//! let result = result.unwrap();
//!
//! // If you want the value inside of the top `JSONValue`, you
//! // may use the `unwrap` method
//! let array: JSONArray = result.clone().unwrap();
//!
//! // You may also try the type conversion directly, so you can handle a potential error
//! let array = match JSONArray::try_from(result.clone()) {
//!     Ok(a) => a,
//!     Err(_) => todo!(),
//! };
//!
//! // If you want to create a `JSONValue` from one of its wrapped types, you
//! // may use the `from` or `into` methods
//! let json_array = JSONValue::from(array.clone()); // or
//! let json_array: JSONValue = array.clone().into();
//! ```
#![forbid(unsafe_code)]
use std::collections::HashMap;
use std::convert::TryFrom;
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
///
/// The `From` trait is implemented for all the
/// types of values wrapped in the `JSONValue` enum
///
/// Additionally, `()` will convert to `JSONValue::Null`
#[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
    ///
    /// This is a convenience method that calls the `try_from` method
    /// for the appropriate type. This will panic if the output type of
    /// the unwrap does not match the type of value in the `JSONValue` struct.
    ///
    /// 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);
    /// ```
    ///
    /// Panicing example:
    /// ```should_panic
    /// # use naive_json_parser::JSONValue;
    /// // Don't try to unwrap one type as another
    /// let json = JSONValue::from(4.5);
    ///
    /// // json currently has a `JSONValue::Number` value
    /// // trying to unwrap the value as a string will result in a
    /// // panic
    /// let s: String = json.unwrap(); // This panics
    /// ```
    pub fn unwrap<T: TryFrom<JSONValue>>(self) -> T {
        match T::try_from(self) {
            Ok(val) => val,
            Err(_) => panic!("Tried to unwrap an empty value")
        }
    }
}

impl TryFrom<JSONValue> for JSONMap {
    type Error = &'static str;

    /// Extracts the `HashMap` in the `JSONValue` enum, if it exists.
    ///
    /// Returns an error if `v` is not a `JSONValue::Object`.
    fn try_from(v: JSONValue) -> Result<Self, Self::Error> {
        match v {
            JSONValue::Object(o) => Ok(o),
            _ => Err("Invalid type conversion")
        }
    }
}

impl TryFrom<JSONValue> for JSONArray {
    type Error = &'static str;

    /// Extracts the `Vec` in the `JSONValue` enum, if it exists.
    ///
    /// Returns an error if `v` is not a `JSONValue::Array`.
    fn try_from(v: JSONValue) -> Result<Self, Self::Error> {
        match v {
            JSONValue::Array(a) => Ok(a),
            _ => Err("Invalid type conversion")
        }
    }
}

impl TryFrom<JSONValue> for f64 {
    type Error = &'static str;

    /// Extracts the `f64` in the `JSONValue` enum, if it exists.
    ///
    /// Returns an error if `v` is not a `JSONValue::Number`.
    fn try_from(v: JSONValue) -> Result<Self, Self::Error> {
        match v {
            JSONValue::Number(n) => Ok(n),
            _ => Err("Invalid type conversion")
        }
    }
}

impl TryFrom<JSONValue> for String {
    type Error = &'static str;

    /// Extracts the `String` in the `JSONValue` enum, if it exists.
    ///
    /// Returns an error if `v` is not a `JSONValue::String`.
    fn try_from(v: JSONValue) -> Result<Self, Self::Error> {
        match v {
            JSONValue::String(s) => Ok(s),
            _ => Err("Invalid type conversion")
        }
    }
}

impl TryFrom<JSONValue> for bool {
    type Error = &'static str;

    /// Extracts the `bool` in the `JSONValue` enum, if it exists.
    ///
    /// Returns an error if `v` is not a `JSONValue::True` or `JSONValue::False`.
    fn try_from(v: JSONValue) -> Result<Self, Self::Error> {
        match v {
            JSONValue::True => Ok(true),
            JSONValue::False => Ok(false),
            _ => Err("Invalid type conversion")
        }
    }
}

impl From<JSONValue> for () {
    /// This will just swallow the enum value and return a unit tuple.
    ///
    /// This impl only exists to mirror the `()` to `JSONValue::Null`
    /// conversion.
    fn from(_: JSONValue) -> () {
        ()
    }
}

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

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

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

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

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

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<()> for JSONValue {
    /// Sets the `JSONValue` enum to the `Null` value
    fn from(_s: ()) -> Self {
        Self::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 is the bulk of the parser
///
/// The members of the struct are private, as they
/// are implementation details of the parser
#[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();
            } else {
                self.skip_whitespace();
            }

            let maybe_key = self.parse_string()?;
            if maybe_key.is_none() {
                return Err(ParseError::ExpectedObjectKey(format!(
                    "Expected an object key. Does the object have a trailing comma?"
                )));
            }

            self.skip_whitespace();
            self.eat(':')?;

            let key = maybe_key.unwrap().unwrap();
            let value = self.parse_value()?;

            result.insert(key, value);

            initial = false;

            self.skip_whitespace();
        }

        self.expect_not_end('}')?;
        // 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;
        }

        self.expect_not_end(']')?;
        // 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',
                        _ => unreachable!(),
                    };
                    result.push(ch);
                    self.increment(1);
                } else if ch == 'u' {
                    if 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 {
                        return Err(ParseError::ExpectedUnicodeEscape(format!(
                            "Expected a unicode escape sequence"
                        )));
                    }
                } else {
                    return Err(ParseError::ExpectedEscapeChar(format!(
                        "Expected an escape sequence"
                    )));
                }
            } else {
                result.push(self.chars[self.i]);
            }
            self.increment(1);
        }

        self.expect_not_end('"')?;
        self.increment(1);

        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;
            self.expect_digit(start, n)?;
        }

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

        // Decimal Part
        if self.chars[n] == '.' && n < max {
            n += 1;
            self.expect_digit(start, n)?;
            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
            self.expect_digit(start, n)?;
            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;
        }
    }

    /// Check that the next character is a digit. If not, return ParseError.
    fn expect_digit(&mut self, start: usize, end: usize) -> Result<(), ParseError> {
        let current = String::from_iter(&self.chars[start..end]);
        if !self.chars[end].is_ascii_digit() {
            Err(ParseError::ExpectedDigit(format!(
                "Expected a digit, received '{}' after numeric '{}'",
                self.chars[end], current
            )))
        } else {
            Ok(())
        }
    }

    /// Verify that we are not at the end of the input string
    fn expect_not_end(&mut self, ch: char) -> Result<(), ParseError> {
        if self.i == self.chars.len() {
            Err(ParseError::UnexpectedEndOfInput(format!(
                "Unexpected end of input. Expected '{}'",
                ch
            )))
        } else {
            Ok(())
        }
    }

    /// Convert a `&str` containing JSON into a `Result<JSONValue, ParseError>`
    ///
    /// Example:
    /// ```rust
    /// use naive_json_parser::JSON;
    ///
    /// let json = r#"[1, 2.0, 3e4, "foo", {}, [], true, false, null]"#;
    ///
    /// // If valid JSON, this should now be a set of nested `JSONValue` enums containing the
    /// // parsed values. If invalid, a `ParseError` is returned.
    /// let parse_result = JSON::parse(json);
    /// # assert!(parse_result.is_ok(), "Parse method example failed");
    /// ```
    pub fn parse(json: &str) -> JSONResult {
        JSON::new(json).parse_value()
    }
}


#[cfg(test)]
#[cfg(not(tarpaulin_include))]
mod tests {
    use super::JSONValue::{Array, Number, True};
    use super::*;

    #[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("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 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))));
    }
}