use std::collections::{HashSet, VecDeque};

use crate::types::OwnedValue;

use super::{convert_json_to_db_type, get_json_value, Val};

/// Represents a single patch operation in the merge queue.
///
/// Used internally by the `merge_patch` function to track the path and value
/// for each pending merge operation.
#[derive(Debug, Clone)]
struct PatchOperation {
    path: Vec<usize>,
    patch: Val,
}

/// The function follows RFC 7386 JSON Merge Patch semantics:
/// * If the patch is null, the target is replaced with null
/// * If the patch contains a scalar value, the target is replaced with that value
/// * If both target and patch are objects, the patch is recursively applied
/// * null values in the patch result in property removal from the target
pub fn json_patch(target: &OwnedValue, patch: &OwnedValue) -> crate::Result<OwnedValue> {
    match (target, patch) {
        (OwnedValue::Blob(_), _) | (_, OwnedValue::Blob(_)) => {
            crate::bail_constraint_error!("blob is not supported!");
        }
        _ => (),
    }

    let mut parsed_target = get_json_value(target)?;
    let parsed_patch = get_json_value(patch)?;

    merge_patch(&mut parsed_target, parsed_patch);

    convert_json_to_db_type(&parsed_target, false)
}

/// # Implementation Notes
///
/// * Processes patches in breadth-first order
/// * Maintains path information for nested updates
/// * Handles object merging with proper null value semantics
/// * Preserves existing values not mentioned in the patch
///
/// Following RFC 7386 rules:
/// * null values remove properties
/// * Objects are merged recursively
/// * Non-object values replace the target completely
fn merge_patch(target: &mut Val, patch: Val) {
    let mut queue = VecDeque::with_capacity(8);
    let mut applied_keys = HashSet::<String>::new();
    queue.push_back(PatchOperation {
        path: Vec::new(),
        patch,
    });
    while let Some(PatchOperation { path, patch }) = queue.pop_front() {
        let mut current: &mut Val = target;

        for (depth, key) in path.iter().enumerate() {
            if let Val::Object(ref mut obj) = current {
                current = &mut obj
                    .get_mut(*key)
                    .unwrap_or_else(|| {
                        panic!("Invalid path at depth {}: key '{}' not found", depth, key)
                    })
                    .1;
            }
        }

        match (current, patch) {
            (current_val, Val::Null) => {
                *current_val = Val::Null;
            }
            (Val::Object(target_map), Val::Object(patch_map)) => {
                applied_keys.clear();
                for (key, patch_val) in patch_map {
                    if applied_keys.insert(key.clone()) == false {
                        continue;
                    }
                    if let Some(pos) = target_map
                        .iter()
                        .position(|(target_key, _)| target_key == &key)
                    {
                        match patch_val {
                            Val::Null => {
                                target_map.remove(pos);
                            }
                            val => {
                                let mut new_path = path.clone();
                                new_path.push(pos);
                                queue.push_back(PatchOperation {
                                    path: new_path,
                                    patch: val,
                                });
                            }
                        };
                    } else {
                        target_map.push((key, patch_val));
                    };
                }
            }
            (current_val, patch_val) => {
                *current_val = patch_val;
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use std::rc::Rc;

    use crate::types::LimboText;

    use super::*;

    fn create_text(s: &str) -> OwnedValue {
        OwnedValue::Text(LimboText::new(Rc::new(s.to_string())))
    }

    fn create_json(s: &str) -> OwnedValue {
        OwnedValue::Text(LimboText::json(Rc::new(s.to_string())))
    }

    #[test]
    fn test_basic_text_replacement() {
        let target = create_text(r#"{"name":"John","age":"30"}"#);
        let patch = create_text(r#"{"age":"31"}"#);

        let result = json_patch(&target, &patch).unwrap();
        assert_eq!(result, create_json(r#"{"name":"John","age":"31"}"#));
    }

    #[test]
    fn test_null_field_removal() {
        let target = create_text(r#"{"name":"John","email":"john@example.com"}"#);
        let patch = create_text(r#"{"email":null}"#);

        let result = json_patch(&target, &patch).unwrap();
        assert_eq!(result, create_json(r#"{"name":"John"}"#));
    }

    #[test]
    fn test_nested_object_merge() {
        let target =
            create_text(r#"{"user":{"name":"John","details":{"age":"30","score":"95.5"}}}"#);

        let patch = create_text(r#"{"user":{"details":{"score":"97.5"}}}"#);

        let result = json_patch(&target, &patch).unwrap();
        assert_eq!(
            result,
            create_json(r#"{"user":{"name":"John","details":{"age":"30","score":"97.5"}}}"#)
        );
    }

    #[test]
    #[should_panic(expected = "blob is not supported!")]
    fn test_blob_not_supported() {
        let target = OwnedValue::Blob(Rc::new(vec![1, 2, 3]));
        let patch = create_text("{}");
        json_patch(&target, &patch).unwrap();
    }

    #[test]
    fn test_deep_null_replacement() {
        let target = create_text(r#"{"level1":{"level2":{"keep":"value","remove":"value"}}}"#);

        let patch = create_text(r#"{"level1":{"level2":{"remove":null}}}"#);

        let result = json_patch(&target, &patch).unwrap();
        assert_eq!(
            result,
            create_json(r#"{"level1":{"level2":{"keep":"value"}}}"#)
        );
    }

    #[test]
    fn test_empty_patch() {
        let target = create_json(r#"{"name":"John","age":"30"}"#);
        let patch = create_text("{}");

        let result = json_patch(&target, &patch).unwrap();
        assert_eq!(result, target);
    }

    #[test]
    fn test_add_new_field() {
        let target = create_text(r#"{"existing":"value"}"#);
        let patch = create_text(r#"{"new":"field"}"#);

        let result = json_patch(&target, &patch).unwrap();
        assert_eq!(result, create_json(r#"{"existing":"value","new":"field"}"#));
    }

    #[test]
    fn test_complete_object_replacement() {
        let target = create_text(r#"{"old":{"nested":"value"}}"#);
        let patch = create_text(r#"{"old":"new_value"}"#);

        let result = json_patch(&target, &patch).unwrap();
        assert_eq!(result, create_json(r#"{"old":"new_value"}"#));
    }
}