use builtin_macros::builtins;
use smol_str::SmolStr;

use std::{
    collections::BTreeMap,
    env,
    rc::{Rc, Weak},
    time::{SystemTime, UNIX_EPOCH},
};

use crate::{
    compiler::GlobalsMap,
    errors::ErrorKind,
    io::FileType,
    observer::NoOpObserver,
    spans::LightSpan,
    value::{Builtin, BuiltinArgument, NixAttrs, Thunk},
    vm::VM,
    SourceCode, Value,
};

#[builtins]
mod impure_builtins {
    use super::*;
    use crate::builtins::coerce_value_to_path;

    #[builtin("getEnv")]
    fn builtin_get_env(_: &mut VM, var: Value) -> Result<Value, ErrorKind> {
        Ok(env::var(var.to_str()?).unwrap_or_else(|_| "".into()).into())
    }

    #[builtin("pathExists")]
    fn builtin_path_exists(vm: &mut VM, s: Value) -> Result<Value, ErrorKind> {
        let path = coerce_value_to_path(&s, vm)?;
        vm.io().path_exists(path).map(Value::Bool)
    }

    #[builtin("readDir")]
    fn builtin_read_dir(vm: &mut VM, path: Value) -> Result<Value, ErrorKind> {
        let path = coerce_value_to_path(&path, vm)?;

        let res = vm.io().read_dir(path)?.into_iter().map(|(name, ftype)| {
            (
                name,
                Value::String(
                    SmolStr::new(match ftype {
                        FileType::Directory => "directory",
                        FileType::Regular => "regular",
                        FileType::Symlink => "symlink",
                        FileType::Unknown => "unknown",
                    })
                    .into(),
                ),
            )
        });

        Ok(Value::attrs(NixAttrs::from_iter(res)))
    }

    #[builtin("readFile")]
    fn builtin_read_file(vm: &mut VM, path: Value) -> Result<Value, ErrorKind> {
        let path = coerce_value_to_path(&path, vm)?;
        vm.io()
            .read_to_string(path)
            .map(|s| Value::String(s.into()))
    }
}

/// Return all impure builtins, that is all builtins which may perform I/O
/// outside of the VM and so cannot be used in all contexts (e.g. WASM).
pub(super) fn builtins() -> BTreeMap<&'static str, Value> {
    let mut map: BTreeMap<&'static str, Value> = impure_builtins::builtins()
        .into_iter()
        .map(|b| (b.name(), Value::Builtin(b)))
        .collect();

    map.insert(
        "storeDir",
        Value::Thunk(Thunk::new_suspended_native(Rc::new(Box::new(
            |vm: &mut VM| match vm.io().store_dir() {
                None => Ok(Value::Null),
                Some(dir) => Ok(Value::String(dir.into())),
            },
        )))),
    );

    // currentTime pins the time at which evaluation was started
    {
        let seconds = match SystemTime::now().duration_since(UNIX_EPOCH) {
            Ok(dur) => dur.as_secs() as i64,

            // This case is hit if the system time is *before* epoch.
            Err(err) => -(err.duration().as_secs() as i64),
        };

        map.insert("currentTime", Value::Integer(seconds));
    }

    map
}

/// Constructs and inserts the `import` builtin. This builtin is special in that
/// it needs to capture the [crate::SourceCode] structure to correctly track
/// source code locations while invoking a compiler.
// TODO: need to be able to pass through a CompilationObserver, too.
pub fn builtins_import(globals: &Weak<GlobalsMap>, source: SourceCode) -> Builtin {
    // This (very cheap, once-per-compiler-startup) clone exists
    // solely in order to keep the borrow checker happy.  It
    // resolves the tension between the requirements of
    // Rc::new_cyclic() and Builtin::new()
    let globals = globals.clone();

    Builtin::new(
        "import",
        &[BuiltinArgument {
            strict: true,
            name: "path",
        }],
        None,
        move |mut args: Vec<Value>, vm: &mut VM| {
            let mut path = super::coerce_value_to_path(&args.pop().unwrap(), vm)?;
            if path.is_dir() {
                path.push("default.nix");
            }

            let current_span = vm.current_span();

            if let Some(cached) = vm.import_cache.get(&path) {
                return Ok(cached.clone());
            }

            let contents = vm.io().read_to_string(path.clone())?;

            let parsed = rnix::ast::Root::parse(&contents);
            let errors = parsed.errors();

            let file = source.add_file(path.to_string_lossy().to_string(), contents);

            if !errors.is_empty() {
                return Err(ErrorKind::ImportParseError {
                    path,
                    file,
                    errors: errors.to_vec(),
                });
            }

            let result = crate::compiler::compile(
                &parsed.tree().expr().unwrap(),
                Some(path.clone()),
                file,
                // The VM must ensure that a strong reference to the
                // globals outlives any self-references (which are
                // weak) embedded within the globals.  If the
                // expect() below panics, it means that did not
                // happen.
                globals
                    .upgrade()
                    .expect("globals dropped while still in use"),
                &mut NoOpObserver::default(),
            )
            .map_err(|err| ErrorKind::ImportCompilerError {
                path: path.clone(),
                errors: vec![err],
            })?;

            if !result.errors.is_empty() {
                return Err(ErrorKind::ImportCompilerError {
                    path,
                    errors: result.errors,
                });
            }

            // Compilation succeeded, we can construct a thunk from whatever it spat
            // out and return that.
            let res = Value::Thunk(Thunk::new_suspended(
                result.lambda,
                LightSpan::new_actual(current_span),
            ));

            vm.import_cache.insert(path, res.clone());

            for warning in result.warnings {
                vm.push_warning(warning);
            }

            Ok(res)
        },
    )
}