//! A complete hand-rolled JSON value, serializer, and parser (no external
//! crates). The orchestrator's `ci_reports::json` is write-only; the protocol
//! needs to *parse* untrusted client packets too, and parsing must be **total**
//! — any byte sequence yields `Ok` or `Err`, never a panic. That totality is
//! what lets the server treat a malformed packet as a deterministic
//! `ValidationReport` rather than a crash.

use std::collections::BTreeMap;
use std::fmt;

/// A parsed JSON value. Objects use a `BTreeMap` so key order is canonical,
/// which keeps re-serialization stable and hashable.
#[derive(Clone, PartialEq, Debug)]
pub enum Json {
    Null,
    Bool(bool),
    /// All numbers are carried as `f64`; integer accessors round-trip exact
    /// values within the safe integer range, which is all the protocol uses.
    Num(f64),
    Str(String),
    Arr(Vec<Json>),
    Obj(BTreeMap<String, Json>),
}

impl Json {
    pub fn s(v: impl Into<String>) -> Json {
        Json::Str(v.into())
    }
    pub fn i(v: i64) -> Json {
        Json::Num(v as f64)
    }
    pub fn u(v: u64) -> Json {
        Json::Num(v as f64)
    }
    pub fn obj(fields: Vec<(&str, Json)>) -> Json {
        let mut m = BTreeMap::new();
        for (k, v) in fields {
            m.insert(k.to_string(), v);
        }
        Json::Obj(m)
    }

    // ---- typed accessors (all fallible, none panic) ----

    pub fn get(&self, key: &str) -> Option<&Json> {
        match self {
            Json::Obj(m) => m.get(key),
            _ => None,
        }
    }
    pub fn as_str(&self) -> Option<&str> {
        match self {
            Json::Str(s) => Some(s),
            _ => None,
        }
    }
    pub fn as_f64(&self) -> Option<f64> {
        match self {
            Json::Num(n) => Some(*n),
            _ => None,
        }
    }
    pub fn as_i64(&self) -> Option<i64> {
        match self {
            Json::Num(n) if n.is_finite() => Some(*n as i64),
            _ => None,
        }
    }
    pub fn as_u64(&self) -> Option<u64> {
        match self {
            Json::Num(n) if n.is_finite() && *n >= 0.0 => Some(*n as u64),
            _ => None,
        }
    }
    pub fn as_u8(&self) -> Option<u8> {
        self.as_u64().and_then(|v| u8::try_from(v).ok())
    }
    pub fn as_bool(&self) -> Option<bool> {
        match self {
            Json::Bool(b) => Some(*b),
            _ => None,
        }
    }
    pub fn as_arr(&self) -> Option<&[Json]> {
        match self {
            Json::Arr(a) => Some(a),
            _ => None,
        }
    }

    /// Convenience: required field accessors that produce a descriptive error.
    pub fn field<'a>(&'a self, key: &str) -> Result<&'a Json, JsonError> {
        self.get(key).ok_or_else(|| JsonError::Field(key.to_string()))
    }
    pub fn str_field(&self, key: &str) -> Result<String, JsonError> {
        self.field(key)?
            .as_str()
            .map(|s| s.to_string())
            .ok_or_else(|| JsonError::Type(key.to_string(), "string"))
    }
    pub fn u64_field(&self, key: &str) -> Result<u64, JsonError> {
        self.field(key)?
            .as_u64()
            .ok_or_else(|| JsonError::Type(key.to_string(), "u64"))
    }
    pub fn i64_field(&self, key: &str) -> Result<i64, JsonError> {
        self.field(key)?
            .as_i64()
            .ok_or_else(|| JsonError::Type(key.to_string(), "i64"))
    }
    pub fn arr_field<'a>(&'a self, key: &str) -> Result<&'a [Json], JsonError> {
        self.field(key)?
            .as_arr()
            .ok_or_else(|| JsonError::Type(key.to_string(), "array"))
    }

    // ---- serialization ----

    /// Compact canonical serialization (no whitespace). Deterministic because
    /// object keys are stored sorted.
    pub fn to_compact(&self) -> String {
        let mut out = String::new();
        self.write(&mut out);
        out
    }

    fn write(&self, out: &mut String) {
        match self {
            Json::Null => out.push_str("null"),
            Json::Bool(b) => out.push_str(if *b { "true" } else { "false" }),
            Json::Num(n) => {
                if !n.is_finite() {
                    out.push_str("null");
                } else if *n == n.trunc() && n.abs() < 9_007_199_254_740_992.0 {
                    // Exact integer: print without a decimal point.
                    out.push_str(&(*n as i64).to_string());
                } else {
                    out.push_str(&format!("{}", n));
                }
            }
            Json::Str(s) => write_str(out, s),
            Json::Arr(items) => {
                out.push('[');
                for (i, it) in items.iter().enumerate() {
                    if i > 0 {
                        out.push(',');
                    }
                    it.write(out);
                }
                out.push(']');
            }
            Json::Obj(m) => {
                out.push('{');
                for (i, (k, v)) in m.iter().enumerate() {
                    if i > 0 {
                        out.push(',');
                    }
                    write_str(out, k);
                    out.push(':');
                    v.write(out);
                }
                out.push('}');
            }
        }
    }
}

fn write_str(out: &mut String, s: &str) {
    out.push('"');
    for c in s.chars() {
        match c {
            '"' => out.push_str("\\\""),
            '\\' => out.push_str("\\\\"),
            '\n' => out.push_str("\\n"),
            '\r' => out.push_str("\\r"),
            '\t' => out.push_str("\\t"),
            c if (c as u32) < 0x20 => out.push_str(&format!("\\u{:04x}", c as u32)),
            c => out.push(c),
        }
    }
    out.push('"');
}

/// A JSON parse / shape error. Carrying a message keeps decode failures
/// diagnosable without ever unwinding.
#[derive(Clone, PartialEq, Eq, Debug)]
pub enum JsonError {
    Parse(String),
    Field(String),
    Type(String, &'static str),
}

impl fmt::Display for JsonError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            JsonError::Parse(m) => write!(f, "json parse error: {m}"),
            JsonError::Field(k) => write!(f, "missing field: {k}"),
            JsonError::Type(k, t) => write!(f, "field {k} is not a {t}"),
        }
    }
}

impl std::error::Error for JsonError {}

/// Parse a JSON document. Total: never panics on any input.
pub fn parse(input: &str) -> Result<Json, JsonError> {
    let bytes = input.as_bytes();
    let mut p = Parser { bytes, pos: 0, depth: 0 };
    p.skip_ws();
    let v = p.value()?;
    p.skip_ws();
    if p.pos != bytes.len() {
        return Err(JsonError::Parse("trailing characters".into()));
    }
    Ok(v)
}

/// Maximum nesting depth. Bounds recursion so a deeply-nested adversarial
/// packet returns `Err` instead of overflowing the stack.
const MAX_DEPTH: usize = 64;

struct Parser<'a> {
    bytes: &'a [u8],
    pos: usize,
    depth: usize,
}

impl<'a> Parser<'a> {
    fn peek(&self) -> Option<u8> {
        self.bytes.get(self.pos).copied()
    }

    fn skip_ws(&mut self) {
        while let Some(b) = self.peek() {
            if b == b' ' || b == b'\t' || b == b'\n' || b == b'\r' {
                self.pos += 1;
            } else {
                break;
            }
        }
    }

    fn value(&mut self) -> Result<Json, JsonError> {
        self.depth += 1;
        if self.depth > MAX_DEPTH {
            return Err(JsonError::Parse("max depth exceeded".into()));
        }
        let r = match self.peek() {
            Some(b'{') => self.object(),
            Some(b'[') => self.array(),
            Some(b'"') => Ok(Json::Str(self.string()?)),
            Some(b't') | Some(b'f') => self.boolean(),
            Some(b'n') => self.null(),
            Some(b'-') | Some(b'0'..=b'9') => self.number(),
            Some(c) => Err(JsonError::Parse(format!("unexpected byte '{}'", c as char))),
            None => Err(JsonError::Parse("unexpected end".into())),
        };
        self.depth -= 1;
        r
    }

    fn expect(&mut self, b: u8) -> Result<(), JsonError> {
        if self.peek() == Some(b) {
            self.pos += 1;
            Ok(())
        } else {
            Err(JsonError::Parse(format!("expected '{}'", b as char)))
        }
    }

    fn object(&mut self) -> Result<Json, JsonError> {
        self.expect(b'{')?;
        let mut m = BTreeMap::new();
        self.skip_ws();
        if self.peek() == Some(b'}') {
            self.pos += 1;
            return Ok(Json::Obj(m));
        }
        loop {
            self.skip_ws();
            let key = self.string()?;
            self.skip_ws();
            self.expect(b':')?;
            self.skip_ws();
            let val = self.value()?;
            m.insert(key, val);
            self.skip_ws();
            match self.peek() {
                Some(b',') => {
                    self.pos += 1;
                    continue;
                }
                Some(b'}') => {
                    self.pos += 1;
                    break;
                }
                _ => return Err(JsonError::Parse("expected ',' or '}'".into())),
            }
        }
        Ok(Json::Obj(m))
    }

    fn array(&mut self) -> Result<Json, JsonError> {
        self.expect(b'[')?;
        let mut a = Vec::new();
        self.skip_ws();
        if self.peek() == Some(b']') {
            self.pos += 1;
            return Ok(Json::Arr(a));
        }
        loop {
            self.skip_ws();
            a.push(self.value()?);
            self.skip_ws();
            match self.peek() {
                Some(b',') => {
                    self.pos += 1;
                    continue;
                }
                Some(b']') => {
                    self.pos += 1;
                    break;
                }
                _ => return Err(JsonError::Parse("expected ',' or ']'".into())),
            }
        }
        Ok(Json::Arr(a))
    }

    fn string(&mut self) -> Result<String, JsonError> {
        self.expect(b'"')?;
        let mut s = String::new();
        loop {
            match self.peek() {
                None => return Err(JsonError::Parse("unterminated string".into())),
                Some(b'"') => {
                    self.pos += 1;
                    break;
                }
                Some(b'\\') => {
                    self.pos += 1;
                    match self.peek() {
                        Some(b'"') => s.push('"'),
                        Some(b'\\') => s.push('\\'),
                        Some(b'/') => s.push('/'),
                        Some(b'n') => s.push('\n'),
                        Some(b'r') => s.push('\r'),
                        Some(b't') => s.push('\t'),
                        Some(b'b') => s.push('\u{0008}'),
                        Some(b'f') => s.push('\u{000c}'),
                        Some(b'u') => {
                            let cp = self.hex4()?;
                            // Handle surrogate pairs.
                            if (0xD800..=0xDBFF).contains(&cp) {
                                if self.peek() == Some(b'\\') {
                                    self.pos += 1;
                                    if self.peek() == Some(b'u') {
                                        let lo = self.hex4()?;
                                        if (0xDC00..=0xDFFF).contains(&lo) {
                                            let c = 0x10000
                                                + ((cp - 0xD800) << 10)
                                                + (lo - 0xDC00);
                                            if let Some(ch) = char::from_u32(c) {
                                                s.push(ch);
                                            } else {
                                                s.push('\u{FFFD}');
                                            }
                                            continue;
                                        }
                                    }
                                }
                                s.push('\u{FFFD}');
                            } else if let Some(ch) = char::from_u32(cp) {
                                s.push(ch);
                            } else {
                                s.push('\u{FFFD}');
                            }
                            continue;
                        }
                        _ => return Err(JsonError::Parse("bad escape".into())),
                    }
                    self.pos += 1;
                }
                Some(_) => {
                    // Copy one UTF-8 codepoint from the source.
                    let start = self.pos;
                    let len = utf8_len(self.bytes[start]);
                    if start + len > self.bytes.len() {
                        return Err(JsonError::Parse("bad utf8".into()));
                    }
                    match std::str::from_utf8(&self.bytes[start..start + len]) {
                        Ok(chunk) => s.push_str(chunk),
                        Err(_) => return Err(JsonError::Parse("bad utf8".into())),
                    }
                    self.pos += len;
                }
            }
        }
        Ok(s)
    }

    fn hex4(&mut self) -> Result<u32, JsonError> {
        // assumes the 'u' has been consumed
        self.pos += 1;
        let mut v: u32 = 0;
        for _ in 0..4 {
            let d = self
                .peek()
                .and_then(|b| (b as char).to_digit(16))
                .ok_or_else(|| JsonError::Parse("bad \\u".into()))?;
            v = v * 16 + d;
            self.pos += 1;
        }
        Ok(v)
    }

    fn boolean(&mut self) -> Result<Json, JsonError> {
        if self.bytes[self.pos..].starts_with(b"true") {
            self.pos += 4;
            Ok(Json::Bool(true))
        } else if self.bytes[self.pos..].starts_with(b"false") {
            self.pos += 5;
            Ok(Json::Bool(false))
        } else {
            Err(JsonError::Parse("bad literal".into()))
        }
    }

    fn null(&mut self) -> Result<Json, JsonError> {
        if self.bytes[self.pos..].starts_with(b"null") {
            self.pos += 4;
            Ok(Json::Null)
        } else {
            Err(JsonError::Parse("bad literal".into()))
        }
    }

    fn number(&mut self) -> Result<Json, JsonError> {
        let start = self.pos;
        if self.peek() == Some(b'-') {
            self.pos += 1;
        }
        while let Some(b'0'..=b'9') = self.peek() {
            self.pos += 1;
        }
        if self.peek() == Some(b'.') {
            self.pos += 1;
            while let Some(b'0'..=b'9') = self.peek() {
                self.pos += 1;
            }
        }
        if let Some(b'e') | Some(b'E') = self.peek() {
            self.pos += 1;
            if let Some(b'+') | Some(b'-') = self.peek() {
                self.pos += 1;
            }
            while let Some(b'0'..=b'9') = self.peek() {
                self.pos += 1;
            }
        }
        let slice = std::str::from_utf8(&self.bytes[start..self.pos])
            .map_err(|_| JsonError::Parse("bad number".into()))?;
        slice
            .parse::<f64>()
            .map(Json::Num)
            .map_err(|_| JsonError::Parse("bad number".into()))
    }
}

fn utf8_len(b: u8) -> usize {
    if b < 0x80 {
        1
    } else if b >> 5 == 0b110 {
        2
    } else if b >> 4 == 0b1110 {
        3
    } else if b >> 3 == 0b11110 {
        4
    } else {
        1
    }
}

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

    #[test]
    fn roundtrip_basic() {
        let v = Json::obj(vec![
            ("a", Json::i(42)),
            ("b", Json::Arr(vec![Json::Bool(true), Json::Null, Json::s("x")])),
            ("c", Json::Num(1.5)),
        ]);
        let s = v.to_compact();
        let back = parse(&s).unwrap();
        assert_eq!(v, back);
    }

    #[test]
    fn parse_never_panics_on_garbage() {
        let deep = "{".repeat(1000);
        let cases = [
            "", "{", "[", "\"", "nul", "{\"a\":}", "[1,2,", "tru", "12.3.4",
            "{\"a\"1}", "\\", "\"\\u00\"", deep.as_str(),
        ];
        for c in cases {
            // Must return without panicking; value is irrelevant.
            let _ = parse(c);
        }
    }

    #[test]
    fn deep_nesting_is_rejected_not_overflowed() {
        let deep = "[".repeat(10_000);
        assert!(parse(&deep).is_err());
    }

    #[test]
    fn integers_roundtrip_exact() {
        let v = Json::i(-1234567890123);
        assert_eq!(parse(&v.to_compact()).unwrap().as_i64(), Some(-1234567890123));
    }

    #[test]
    fn escapes_roundtrip() {
        let v = Json::s("line\ntab\tquote\"slash\\end");
        let s = v.to_compact();
        assert_eq!(parse(&s).unwrap(), v);
    }
}