use std::path::PathBuf;

use fix_bytecode::Continuation;
use fix_error::Error;
use fix_lang::StringId;
use fix_runtime::{
    AttrSet, Break, BytecodeReader, CallFrame, Machine, MachineExt, Path, PendingLoad,
    PendingScope, Step, StrictValue, Value, VmRuntimeCtx, VmRuntimeCtxExt, canon_path_str,
};
use gc_arena::{Gc, Mutation};
use hashbrown::HashSet;

pub fn import<'gc, M: Machine<'gc>>(
    m: &mut M,
    ctx: &mut impl VmRuntimeCtx,
    reader: &mut BytecodeReader<'_>,
    mc: &Mutation<'gc>,
) -> Step {
    // stack: [path]
    let path_val = m.force_and_retry::<StrictValue>(reader, mc)?;
    let path_str = match ctx.get_string_or_path(path_val) {
        Some(s) => s.to_owned(),
        None => {
            return m.finish_err(Error::eval_error(format!(
                "expected a path or string, got {}",
                path_val.ty()
            )));
        }
    };
    let abs = match resolve_import_target(&path_str) {
        Ok(p) => p,
        Err(e) => return m.finish_err(e),
    };

    if let Some(cached) = m.import_cache_get(&abs) {
        return m.return_from_primop(cached, reader);
    }

    // Stash the resolved path on the stack as a string-id so the
    // finalizer can use it as the cache key. The slot we pop here was
    // freed by `force_and_retry`, so we simply push.
    let path_sid = ctx.intern_string(abs.to_string_lossy());
    m.push(Value::new(path_sid));
    let env = m.env();
    m.push_call_frame(CallFrame {
        pc: Continuation::PImportFinalize.ip() as usize,
        thunk: None,
        env,
    });

    m.set_pending_load(PendingLoad {
        path: abs,
        scope: None,
    });
    Step::Break(Break::LoadFile)
}

pub fn import_finalize<'gc, M: Machine<'gc>>(
    m: &mut M,
    ctx: &mut impl VmRuntimeCtx,
    reader: &mut BytecodeReader<'_>,
) -> Step {
    // stack: [path_sid, return_value]
    let val = m.pop();
    #[allow(clippy::unwrap_used)]
    let path_sid = m.pop().downcast::<StringId>().unwrap();
    // The cache key is keyed by the absolute path string we interned in
    // `import`. Resolve it back to the host PathBuf.
    let path_str = ctx.resolve_string(path_sid).to_owned();
    m.import_cache_insert(PathBuf::from(path_str), val);
    m.push(val);
    let Some(CallFrame {
        pc: ret_pc,
        thunk: _,
        env,
    }) = m.pop_call_frame()
    else {
        unreachable!()
    };
    reader.set_pc(ret_pc);
    // FIXME:
    // m.dec_call_depth();
    m.set_env(env);
    Step::Continue(())
}

pub fn scoped_import<'gc, M: Machine<'gc>>(
    m: &mut M,
    ctx: &mut impl VmRuntimeCtx,
    reader: &mut BytecodeReader<'_>,
    mc: &Mutation<'gc>,
) -> Step {
    // stack: [scope, path]
    let (scope_attrs, path_val) = m.force_and_retry::<(Gc<AttrSet>, StrictValue)>(reader, mc)?;
    let path_str = match ctx.get_string_or_path(path_val) {
        Some(s) => s.to_owned(),
        None => {
            return m.finish_err(Error::eval_error(format!(
                "expected a path or string, got {}",
                path_val.ty()
            )));
        }
    };
    let abs = match resolve_import_target(&path_str) {
        Ok(p) => p,
        Err(e) => return m.finish_err(e),
    };

    let keys: HashSet<StringId> = scope_attrs.entries.iter().map(|&(k, _)| k).collect();
    let slot_id = m.scope_slots_push(Value::new(scope_attrs));

    let env = m.env();
    m.push_call_frame(CallFrame {
        pc: Continuation::PScopedImportFinalize.ip() as usize,
        thunk: None,
        env,
    });

    m.set_pending_load(PendingLoad {
        path: abs,
        scope: Some(PendingScope { keys, slot_id }),
    });
    Step::Break(Break::LoadFile)
}

pub fn scoped_import_finalize<'gc, M: Machine<'gc>>(
    m: &mut M,
    _ctx: &mut impl VmRuntimeCtx,
    reader: &mut BytecodeReader<'_>,
    _mc: &Mutation<'gc>,
) -> Step {
    // stack: [return_value]
    // We intentionally do NOT pop the slot from `scope_slots` so that
    // closures or thunks created inside the imported file can still
    // resolve their scope after `scopedImport` returns.
    let val = m.pop();
    m.return_from_primop(val, reader)
}

pub fn path_exists<'gc, M: Machine<'gc>>(
    m: &mut M,
    ctx: &mut impl VmRuntimeCtx,
    reader: &mut BytecodeReader<'_>,
    mc: &Mutation<'gc>,
) -> Step {
    let path_val = m.force_and_retry::<StrictValue>(reader, mc)?;
    // pathExists requires an absolute path. A `Path` value is
    // always absolute; a string is accepted only if it starts with `/`.
    let (path, is_path_value) = if let Some(p) = path_val.downcast::<Path>() {
        (ctx.resolve_string(p.0).to_owned(), true)
    } else if let Some(s) = ctx.get_string(path_val) {
        (s.to_owned(), false)
    } else {
        return m.finish_err(Error::eval_error(format!(
            "expected a path or string, got {}",
            path_val.ty()
        )));
    };
    if !is_path_value && !path.starts_with('/') {
        return m.finish_err(Error::eval_error(format!(
            "string '{path}' doesn't represent an absolute path"
        )));
    }
    // CppNix collapses consecutive slashes and resolves `.` / `..` lexically
    // before checking. Trailing-slash / trailing-dot mean "must be a directory".
    let must_be_dir = path.ends_with('/') || path.ends_with("/.");
    let canon = canon_path_str(&path);
    let p = std::path::Path::new(&canon);
    let exists = if must_be_dir {
        std::fs::metadata(p).map(|m| m.is_dir()).unwrap_or(false)
    } else {
        std::fs::symlink_metadata(p).is_ok()
    };
    m.return_from_primop(Value::new(exists), reader)
}

/// Convert the user-supplied path string into an absolute, dotted-segment
/// resolved `PathBuf` and append `default.nix` if the target is a directory.
fn resolve_import_target(path: &str) -> Result<PathBuf, Box<Error>> {
    let mut abs = PathBuf::from(path);
    if !abs.is_absolute() {
        return Err(Error::eval_error(format!(
            "import: expected an absolute path, got '{path}'"
        )));
    }
    if abs.is_dir() {
        abs.push("default.nix");
    }
    Ok(abs)
}