//! When calling function with dynamic library,
//! function return value indicates whether function has been successfully executed.
//! When function return `false`, programmer may want to know which error occurs.
//! 
//! This module provide **thread independent** error message string storage,
//! which is more like Win32 `GetLastError()` but return error message instead of error code.
//! These module provided functions will be called when executing main module functions.

use std::cell::RefCell;
use std::ffi::{CString, c_char};

struct LastError {
    msg: CString,
}

impl LastError {
    fn new() -> Self {
        Self {
            msg: CString::new("").expect("empty string must be valid for CString"),
        }
    }

    pub fn set_msg(&mut self, msg: &str) {
        self.msg = CString::new(msg).expect("unexpected blank in error message output");
    }

    pub fn get_msg(&self) -> *const c_char {
        self.msg.as_ptr()
    }

    pub fn clear_msg(&mut self) {
        self.msg = CString::new("").expect("empty string must be valid for CString");
    }
}

thread_local! {
    static LAST_ERROR: RefCell<LastError> = RefCell::new(LastError::new());
}

/// Set thread local error message.
pub fn set_last_error(msg: &str) {
    LAST_ERROR.with(|e| {
        e.borrow_mut().set_msg(msg);
    });
}

/// Get const pointer to thread local error message string.
/// If there is no error, return pointer will point to empty string.
pub fn get_last_error() -> *const c_char {
    LAST_ERROR.with(|e| e.borrow().get_msg())
}

/// Clear thread local error message (reset to empty string).
pub fn clear_last_error() {
    LAST_ERROR.with(|e| {
        e.borrow_mut().clear_msg();
    });
}