use std::collections::BTreeMap;
use std::fmt::{Display, Formatter};
use std::fs;
use std::path::{Path, PathBuf};

use crate::json::JsonValue;

pub const CLAUDE_CODE_SETTINGS_SCHEMA_NAME: &str = "SettingsSchema";

#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
pub enum ConfigSource {
    User,
    Project,
    Local,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ConfigEntry {
    pub source: ConfigSource,
    pub path: PathBuf,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct RuntimeConfig {
    merged: BTreeMap<String, JsonValue>,
    loaded_entries: Vec<ConfigEntry>,
}

#[derive(Debug)]
pub enum ConfigError {
    Io(std::io::Error),
    Parse(String),
}

impl Display for ConfigError {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Io(error) => write!(f, "{error}"),
            Self::Parse(error) => write!(f, "{error}"),
        }
    }
}

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

impl From<std::io::Error> for ConfigError {
    fn from(value: std::io::Error) -> Self {
        Self::Io(value)
    }
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ConfigLoader {
    cwd: PathBuf,
    config_home: PathBuf,
}

impl ConfigLoader {
    #[must_use]
    pub fn new(cwd: impl Into<PathBuf>, config_home: impl Into<PathBuf>) -> Self {
        Self {
            cwd: cwd.into(),
            config_home: config_home.into(),
        }
    }

    #[must_use]
    pub fn default_for(cwd: impl Into<PathBuf>) -> Self {
        let cwd = cwd.into();
        let config_home = std::env::var_os("CLAUDE_CONFIG_HOME")
            .map(PathBuf::from)
            .or_else(|| std::env::var_os("HOME").map(|home| PathBuf::from(home).join(".claude")))
            .unwrap_or_else(|| PathBuf::from(".claude"));
        Self { cwd, config_home }
    }

    #[must_use]
    pub fn discover(&self) -> Vec<ConfigEntry> {
        vec![
            ConfigEntry {
                source: ConfigSource::User,
                path: self.config_home.join("settings.json"),
            },
            ConfigEntry {
                source: ConfigSource::Project,
                path: self.cwd.join(".claude").join("settings.json"),
            },
            ConfigEntry {
                source: ConfigSource::Local,
                path: self.cwd.join(".claude").join("settings.local.json"),
            },
        ]
    }

    pub fn load(&self) -> Result<RuntimeConfig, ConfigError> {
        let mut merged = BTreeMap::new();
        let mut loaded_entries = Vec::new();

        for entry in self.discover() {
            let Some(value) = read_optional_json_object(&entry.path)? else {
                continue;
            };
            deep_merge_objects(&mut merged, &value);
            loaded_entries.push(entry);
        }

        Ok(RuntimeConfig {
            merged,
            loaded_entries,
        })
    }
}

impl RuntimeConfig {
    #[must_use]
    pub fn empty() -> Self {
        Self {
            merged: BTreeMap::new(),
            loaded_entries: Vec::new(),
        }
    }

    #[must_use]
    pub fn merged(&self) -> &BTreeMap<String, JsonValue> {
        &self.merged
    }

    #[must_use]
    pub fn loaded_entries(&self) -> &[ConfigEntry] {
        &self.loaded_entries
    }

    #[must_use]
    pub fn get(&self, key: &str) -> Option<&JsonValue> {
        self.merged.get(key)
    }

    #[must_use]
    pub fn as_json(&self) -> JsonValue {
        JsonValue::Object(self.merged.clone())
    }
}

fn read_optional_json_object(
    path: &Path,
) -> Result<Option<BTreeMap<String, JsonValue>>, ConfigError> {
    let contents = match fs::read_to_string(path) {
        Ok(contents) => contents,
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => return Ok(None),
        Err(error) => return Err(ConfigError::Io(error)),
    };

    if contents.trim().is_empty() {
        return Ok(Some(BTreeMap::new()));
    }

    let parsed = JsonValue::parse(&contents)
        .map_err(|error| ConfigError::Parse(format!("{}: {error}", path.display())))?;
    let object = parsed.as_object().ok_or_else(|| {
        ConfigError::Parse(format!(
            "{}: top-level settings value must be a JSON object",
            path.display()
        ))
    })?;
    Ok(Some(object.clone()))
}

fn deep_merge_objects(
    target: &mut BTreeMap<String, JsonValue>,
    source: &BTreeMap<String, JsonValue>,
) {
    for (key, value) in source {
        match (target.get_mut(key), value) {
            (Some(JsonValue::Object(existing)), JsonValue::Object(incoming)) => {
                deep_merge_objects(existing, incoming);
            }
            _ => {
                target.insert(key.clone(), value.clone());
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::{ConfigLoader, ConfigSource, CLAUDE_CODE_SETTINGS_SCHEMA_NAME};
    use crate::json::JsonValue;
    use std::fs;
    use std::time::{SystemTime, UNIX_EPOCH};

    fn temp_dir() -> std::path::PathBuf {
        let nanos = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .expect("time should be after epoch")
            .as_nanos();
        std::env::temp_dir().join(format!("runtime-config-{nanos}"))
    }

    #[test]
    fn rejects_non_object_settings_files() {
        let root = temp_dir();
        let cwd = root.join("project");
        let home = root.join("home").join(".claude");
        fs::create_dir_all(&home).expect("home config dir");
        fs::create_dir_all(&cwd).expect("project dir");
        fs::write(home.join("settings.json"), "[]").expect("write bad settings");

        let error = ConfigLoader::new(&cwd, &home)
            .load()
            .expect_err("config should fail");
        assert!(error
            .to_string()
            .contains("top-level settings value must be a JSON object"));

        fs::remove_dir_all(root).expect("cleanup temp dir");
    }

    #[test]
    fn loads_and_merges_claude_code_config_files_by_precedence() {
        let root = temp_dir();
        let cwd = root.join("project");
        let home = root.join("home").join(".claude");
        fs::create_dir_all(cwd.join(".claude")).expect("project config dir");
        fs::create_dir_all(&home).expect("home config dir");

        fs::write(
            home.join("settings.json"),
            r#"{"model":"sonnet","env":{"A":"1"},"hooks":{"PreToolUse":["base"]}}"#,
        )
        .expect("write user settings");
        fs::write(
            cwd.join(".claude").join("settings.json"),
            r#"{"env":{"B":"2"},"hooks":{"PostToolUse":["project"]}}"#,
        )
        .expect("write project settings");
        fs::write(
            cwd.join(".claude").join("settings.local.json"),
            r#"{"model":"opus","permissionMode":"acceptEdits"}"#,
        )
        .expect("write local settings");

        let loaded = ConfigLoader::new(&cwd, &home)
            .load()
            .expect("config should load");

        assert_eq!(CLAUDE_CODE_SETTINGS_SCHEMA_NAME, "SettingsSchema");
        assert_eq!(loaded.loaded_entries().len(), 3);
        assert_eq!(loaded.loaded_entries()[0].source, ConfigSource::User);
        assert_eq!(
            loaded.get("model"),
            Some(&JsonValue::String("opus".to_string()))
        );
        assert_eq!(
            loaded
                .get("env")
                .and_then(JsonValue::as_object)
                .expect("env object")
                .len(),
            2
        );
        assert!(loaded
            .get("hooks")
            .and_then(JsonValue::as_object)
            .expect("hooks object")
            .contains_key("PreToolUse"));

        fs::remove_dir_all(root).expect("cleanup temp dir");
    }
}