//! # 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};

/// The type of JSON value
#[derive(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,
}

/// 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,
}

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

    /// 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 0 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<JSONValue, ParseError> {
        self.skip_whitespace();

        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())
            .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 value.next() {
            Some(val) => Ok(val),
            None => Err(ParseError::UnexpectedEndOfInput(String::from(
                "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: 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(',')?;
                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;
        }

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

        Ok(Some(JSONValue::Object(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] != ']' {
            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::Array(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 ['"', '\\', '/'].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.increment(1);

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

    /// See if there's a `JSONValue::Number` next in the JSON
    fn parse_number(&mut self) -> Result<Option<JSONValue>, ParseError> {
        // If it doesn't start with 0-9 or a minus sign, it's probably not a number
        if ! (self.chars[self.i].is_ascii_digit() || self.chars[self.i] == '-') {
            return Ok(None);
        }

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

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

        if self.chars[n] == '0' && n < max {
            n += 1;
        } else if self.chars[n] >= '1' && self.chars[n] <= '9' && n < max {
            n += 1;
            loop {
                if n + 1 > max {
                    break;
                }

                if self.chars[n + 1].is_ascii_digit() {
                    n += 1;
                } else {
                    break;
                }
            }
        }

        if self.chars[n] == '.' && n < max {
            n += 1;
            loop {
                if n + 1 > max {
                    break;
                }

                if self.chars[n + 1].is_ascii_digit() {
                    n += 1;
                } else {
                    break;
                }
            }
        }

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

            if self.chars[n] == '-' || self.chars[n] == '+' && n < max {
                n += 1;
            }
            // expect digit
            loop {
                if n + 1 > max {
                    break;
                }

                if self.chars[n + 1].is_ascii_digit() {
                    n += 1;
                } else {
                    break;
                }
            }
        }

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

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

            match str.parse::<f64>() {
                Ok(n) => {
                    self.increment(str.len() - 1);
                    return Ok(Some(JSONValue::Number(n)))
                },
                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(())
    }

    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) -> Result<JSONValue, ParseError> {
        JSON::new(json).parse_value()
    }
}

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

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

        let mut parser = JSON::new("true");
        let res = JSON::parse_keyword(&mut parser, "true", JSONValue::True);
        assert_eq!(res, Ok(Some(JSONValue::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 mut parser = JSON::new(r#""\t""#);
        let res = JSON::parse_string(&mut parser);
        assert_eq!(res, Ok(Some(JSONValue::String(String::from("\t")))));

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

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

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

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

        let mut parser = JSON::new("3.14159");
        let res = JSON::parse_number(&mut parser);
        assert_eq!(res, Ok(Some(JSONValue::Number(3.14159f64))));

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

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

    #[test]
    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
            ]))
        );
    }

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

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

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

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

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