use std::io::{self, Read};

#[derive(Debug, PartialEq)]
enum Json {
    Null,
    Bool(bool),
    Number(String),
    String(String),
    Array(Vec<Json>),
    Object(Vec<(String, Json)>),
}

struct Parser<'a> {
    input: &'a str,
    pos: usize,
}

impl<'a> Parser<'a> {
    fn new(input: &'a str) -> Self {
        Self { input, pos: 0 }
    }

    fn parse(mut self) -> Result<Json, String> {
        let value = self.value()?;
        self.whitespace();

        if self.pos != self.input.len() {
            return Err(self.error("unexpected trailing characters"));
        }

        Ok(value)
    }

    fn value(&mut self) -> Result<Json, String> {
        self.whitespace();

        match self.peek() {
            Some(b'n') => {
                self.keyword("null")?;
                Ok(Json::Null)
            }
            Some(b't') => {
                self.keyword("true")?;
                Ok(Json::Bool(true))
            }
            Some(b'f') => {
                self.keyword("false")?;
                Ok(Json::Bool(false))
            }
            Some(b'"') => Ok(Json::String(self.string()?)),
            Some(b'[') => self.array(),
            Some(b'{') => self.object(),
            Some(b'-' | b'0'..=b'9') => Ok(Json::Number(self.number()?)),
            Some(_) => Err(self.error("expected a JSON value")),
            None => Err(self.error("unexpected end of input")),
        }
    }

    fn array(&mut self) -> Result<Json, String> {
        self.expect(b'[')?;
        self.whitespace();

        let mut values = Vec::new();
        if self.consume(b']') {
            return Ok(Json::Array(values));
        }

        loop {
            values.push(self.value()?);
            self.whitespace();

            if self.consume(b']') {
                break;
            }
            self.expect(b',')?;
        }

        Ok(Json::Array(values))
    }

    fn object(&mut self) -> Result<Json, String> {
        self.expect(b'{')?;
        self.whitespace();

        let mut entries = Vec::new();
        if self.consume(b'}') {
            return Ok(Json::Object(entries));
        }

        loop {
            self.whitespace();
            if self.peek() != Some(b'"') {
                return Err(self.error("expected an object key"));
            }

            let key = self.string()?;
            self.whitespace();
            self.expect(b':')?;
            entries.push((key, self.value()?));
            self.whitespace();

            if self.consume(b'}') {
                break;
            }
            self.expect(b',')?;
        }

        Ok(Json::Object(entries))
    }

    // Strings are kept in their original escaped form.
    fn string(&mut self) -> Result<String, String> {
        let start = self.pos;
        self.expect(b'"')?;

        loop {
            match self.next() {
                Some(b'"') => return Ok(self.input[start..self.pos].to_owned()),
                Some(b'\\') => match self.next() {
                    Some(b'"' | b'\\' | b'/' | b'b' | b'f' | b'n' | b'r' | b't') => {}
                    Some(b'u') => {
                        for _ in 0..4 {
                            match self.next() {
                                Some(b'0'..=b'9' | b'a'..=b'f' | b'A'..=b'F') => {}
                                _ => return Err(self.error("invalid Unicode escape")),
                            }
                        }
                    }
                    _ => return Err(self.error("invalid escape sequence")),
                },
                Some(0x00..=0x1f) => {
                    return Err(self.error("unescaped control character in string"))
                }
                Some(_) => {}
                None => return Err(self.error("unterminated string")),
            }
        }
    }

    fn number(&mut self) -> Result<String, String> {
        let start = self.pos;
        self.consume(b'-');

        if self.consume(b'0') {
            if matches!(self.peek(), Some(b'0'..=b'9')) {
                return Err(self.error("leading zero in number"));
            }
        } else {
            self.digits()?;
        }

        if self.consume(b'.') {
            self.digits()?;
        }

        if matches!(self.peek(), Some(b'e' | b'E')) {
            self.pos += 1;
            if matches!(self.peek(), Some(b'+' | b'-')) {
                self.pos += 1;
            }
            self.digits()?;
        }

        Ok(self.input[start..self.pos].to_owned())
    }

    fn digits(&mut self) -> Result<(), String> {
        let start = self.pos;
        while matches!(self.peek(), Some(b'0'..=b'9')) {
            self.pos += 1;
        }

        if self.pos == start {
            Err(self.error("expected a digit"))
        } else {
            Ok(())
        }
    }

    fn keyword(&mut self, word: &str) -> Result<(), String> {
        if self.input[self.pos..].starts_with(word) {
            self.pos += word.len();
            Ok(())
        } else {
            Err(self.error("invalid keyword"))
        }
    }

    fn whitespace(&mut self) {
        while matches!(self.peek(), Some(b' ' | b'\n' | b'\r' | b'\t')) {
            self.pos += 1;
        }
    }

    fn expect(&mut self, byte: u8) -> Result<(), String> {
        if self.consume(byte) {
            Ok(())
        } else {
            Err(self.error(&format!("expected '{}'", byte as char)))
        }
    }

    fn consume(&mut self, byte: u8) -> bool {
        if self.peek() == Some(byte) {
            self.pos += 1;
            true
        } else {
            false
        }
    }

    fn peek(&self) -> Option<u8> {
        self.input.as_bytes().get(self.pos).copied()
    }

    fn next(&mut self) -> Option<u8> {
        let byte = self.peek()?;
        self.pos += 1;
        Some(byte)
    }

    fn error(&self, message: &str) -> String {
        format!("{message} at byte {}", self.pos)
    }
}

fn write_pretty(value: &Json, output: &mut String, depth: usize) {
    match value {
        Json::Null => output.push_str("null"),
        Json::Bool(value) => output.push_str(if *value { "true" } else { "false" }),
        Json::Number(value) | Json::String(value) => output.push_str(value),
        Json::Array(values) => {
            if values.is_empty() {
                output.push_str("[]");
                return;
            }

            output.push_str("[\n");
            for (index, value) in values.iter().enumerate() {
                indent(output, depth + 1);
                write_pretty(value, output, depth + 1);
                if index + 1 != values.len() {
                    output.push(',');
                }
                output.push('\n');
            }
            indent(output, depth);
            output.push(']');
        }
        Json::Object(entries) => {
            if entries.is_empty() {
                output.push_str("{}");
                return;
            }

            output.push_str("{\n");
            for (index, (key, value)) in entries.iter().enumerate() {
                indent(output, depth + 1);
                output.push_str(key);
                output.push_str(": ");
                write_pretty(value, output, depth + 1);
                if index + 1 != entries.len() {
                    output.push(',');
                }
                output.push('\n');
            }
            indent(output, depth);
            output.push('}');
        }
    }
}

fn indent(output: &mut String, depth: usize) {
    for _ in 0..depth {
        output.push_str("  ");
    }
}

fn pretty_print(input: &str) -> Result<String, String> {
    let value = Parser::new(input).parse()?;
    let mut output = String::new();
    write_pretty(&value, &mut output, 0);
    Ok(output)
}

fn main() {
    let mut input = String::new();

    if let Err(error) = io::stdin().read_to_string(&mut input) {
        eprintln!("failed to read stdin: {error}");
        std::process::exit(1);
    }

    match pretty_print(&input) {
        Ok(output) => println!("{output}"),
        Err(error) => {
            eprintln!("invalid JSON: {error}");
            std::process::exit(1);
        }
    }
}

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

    #[test]
    fn formats_nested_json() {
        let input = r#"{"name":"Ada","values":[1,true,null],"empty":{}}"#;
        let expected = r#"{
  "name": "Ada",
  "values": [
    1,
    true,
    null
  ],
  "empty": {}
}"#;

        assert_eq!(pretty_print(input).unwrap(), expected);
    }

    #[test]
    fn rejects_invalid_json() {
        assert!(pretty_print(r#"{"answer": 42,}"#).is_err());
        assert!(pretty_print("01").is_err());
    }
}