// Golden file comparison for test-client.
// Compares the final ObserverSnapshot (as JSON) against a golden file.
// Reports field-by-field differences with JSON paths.

use serde_json::Value;
use settled_reach_server::bridge::types::ObserverSnapshot;
use std::path::Path;

/// Compare an ObserverSnapshot against a golden JSON file.
/// Returns a list of difference descriptions (empty = match).
pub fn compare_golden(
    golden_path: &Path,
    snapshot: &ObserverSnapshot,
) -> Result<Vec<String>, String> {
    let golden_str = std::fs::read_to_string(golden_path).map_err(|e| {
        format!(
            "failed to read golden file {}: {}",
            golden_path.display(),
            e
        )
    })?;
    let golden: Value = serde_json::from_str(&golden_str)
        .map_err(|e| format!("failed to parse golden file: {}", e))?;
    let actual: Value = serde_json::to_value(snapshot)
        .map_err(|e| format!("failed to serialize snapshot: {}", e))?;

    let mut diffs = Vec::new();
    diff_values("", &golden, &actual, &mut diffs);
    Ok(diffs)
}

fn diff_values(path: &str, expected: &Value, actual: &Value, diffs: &mut Vec<String>) {
    match (expected, actual) {
        (Value::Object(e), Value::Object(a)) => {
            for key in e.keys() {
                let child_path = if path.is_empty() {
                    format!(".{}", key)
                } else {
                    format!("{}.{}", path, key)
                };
                match a.get(key) {
                    Some(av) => diff_values(&child_path, &e[key], av, diffs),
                    None => diffs.push(format!(
                        "{}: expected {}, got <missing>",
                        child_path,
                        format_value(&e[key])
                    )),
                }
            }
            for key in a.keys() {
                if !e.contains_key(key) {
                    let child_path = if path.is_empty() {
                        format!(".{}", key)
                    } else {
                        format!("{}.{}", path, key)
                    };
                    diffs.push(format!(
                        "{}: expected <missing>, got {}",
                        child_path,
                        format_value(&a[key])
                    ));
                }
            }
        }
        (Value::Array(e), Value::Array(a)) => {
            let max_len = e.len().max(a.len());
            for i in 0..max_len {
                let child_path = format!("{}[{}]", path, i);
                match (e.get(i), a.get(i)) {
                    (Some(ev), Some(av)) => diff_values(&child_path, ev, av, diffs),
                    (Some(ev), None) => diffs.push(format!(
                        "{}: expected {}, got <missing>",
                        child_path,
                        format_value(ev)
                    )),
                    (None, Some(av)) => diffs.push(format!(
                        "{}: expected <missing>, got {}",
                        child_path,
                        format_value(av)
                    )),
                    (None, None) => unreachable!(),
                }
            }
        }
        _ => {
            if expected != actual {
                diffs.push(format!(
                    "{}: expected {}, got {}",
                    path,
                    format_value(expected),
                    format_value(actual)
                ));
            }
        }
    }
}

fn format_value(v: &Value) -> String {
    match v {
        Value::String(s) => format!("{:?}", s),
        Value::Null => "null".to_string(),
        other => other.to_string(),
    }
}