//! Canonical JSON serialization.
//!
//! Writes JSON without whitespace and with lexicographically sorted
//! object keys. This makes the representation deterministic across
//! runs, platforms, and `serde_json` versions, which is what makes it
//! suitable as the pre-image for [`crate::def_hash`] /
//! [`crate::module_hash`].
//!
//! The single entry point is [`to_bytes`]. This module deliberately
//! does **not** parse, validate, or hash — it only emits bytes. It also
//! does not attempt to be a general-purpose canonical-JSON library;
//! number-formatting follows whatever `serde_json::Number::to_string`
//! does, which is fine for the AILang AST (integers and short strings;
//! no floats).
//!
//! # Examples
//!
//! ```ignore
//! use ailang_core::canonical;
//! use serde_json::json;
//!
//! // Object keys are sorted; no whitespace appears in the output.
//! let bytes = canonical::to_bytes(&json!({ "b": 1, "a": 2 }));
//! assert_eq!(std::str::from_utf8(&bytes).unwrap(), r#"{"a":2,"b":1}"#);
//! ```

use std::io::Write;

/// Serialize `value` to canonical JSON bytes.
///
/// Object keys are sorted lexicographically, no whitespace is emitted,
/// and arrays preserve their input order. The output is the byte
/// pre-image used by [`crate::def_hash`] and
/// [`crate::workspace::module_hash`]; if you change what bytes this
/// function emits for a given value, all on-disk hashes change with it.
///
/// # Panics
///
/// Panics if `value` cannot be converted to a `serde_json::Value`
/// (i.e. its `Serialize` impl errors). For the AST types in
/// [`crate::ast`] this never happens.
pub fn to_bytes<T: serde::Serialize>(value: &T) -> Vec<u8> {
    let v = serde_json::to_value(value).expect("serializable");
    let mut out = Vec::new();
    write_value(&v, &mut out).expect("write to Vec");
    out
}

fn write_value(v: &serde_json::Value, out: &mut Vec<u8>) -> std::io::Result<()> {
    use serde_json::Value;
    match v {
        Value::Null => out.write_all(b"null"),
        Value::Bool(true) => out.write_all(b"true"),
        Value::Bool(false) => out.write_all(b"false"),
        Value::Number(n) => out.write_all(n.to_string().as_bytes()),
        Value::String(s) => {
            let escaped = serde_json::to_string(s).expect("string serializable");
            out.write_all(escaped.as_bytes())
        }
        Value::Array(arr) => {
            out.write_all(b"[")?;
            for (i, item) in arr.iter().enumerate() {
                if i > 0 {
                    out.write_all(b",")?;
                }
                write_value(item, out)?;
            }
            out.write_all(b"]")
        }
        Value::Object(map) => {
            let mut keys: Vec<&String> = map.keys().collect();
            keys.sort();
            out.write_all(b"{")?;
            for (i, k) in keys.iter().enumerate() {
                if i > 0 {
                    out.write_all(b",")?;
                }
                let kj = serde_json::to_string(k).expect("key serializable");
                out.write_all(kj.as_bytes())?;
                out.write_all(b":")?;
                write_value(&map[*k], out)?;
            }
            out.write_all(b"}")
        }
    }
}

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

    #[test]
    fn sorts_keys() {
        let v = json!({ "b": 1, "a": 2 });
        let bytes = to_bytes(&v);
        assert_eq!(std::str::from_utf8(&bytes).unwrap(), r#"{"a":2,"b":1}"#);
    }

    #[test]
    fn nested_keys_sorted() {
        let v = json!({ "z": { "y": 1, "x": [3, { "b": 2, "a": 1 }] } });
        let bytes = to_bytes(&v);
        assert_eq!(
            std::str::from_utf8(&bytes).unwrap(),
            r#"{"z":{"x":[3,{"a":1,"b":2}],"y":1}}"#
        );
    }

    #[test]
    fn no_whitespace() {
        let v = json!({ "a": 1, "b": [1, 2, 3] });
        let bytes = to_bytes(&v);
        let s = std::str::from_utf8(&bytes).unwrap();
        assert!(!s.contains(' '));
        assert!(!s.contains('\n'));
    }
}