use std::ffi::{CStr, CString};
use std::os::raw::{c_char, c_int, c_void};

#[link(name = "guile-3.0")]
extern "C" {
    fn scm_init_guile();
    fn scm_c_eval_string(expr: *const c_char) -> *mut c_void;
    fn scm_to_locale_string(scm_obj: *mut c_void) -> *const c_char;
    fn scm_is_string(scm_obj: *mut c_void) -> c_int;
    fn scm_object_to_string(scm_obj: *mut c_void, printer: *mut c_void) -> *mut c_void;
}

#[link(name = "guiledefs")]
extern "C" {
    pub static scm_undefined: *mut c_void;
    pub static scm_unspecified: *mut c_void;
    fn defs_scm_is_eq(x: *mut c_void, y: *mut c_void) -> c_int;
    fn defs_scm_define_string(name: *const c_char, value: *const c_char);
}

/// Convert a scm object into a string using Guile-builtins
fn string_from_scm(scm_obj: *mut c_void) -> Result<String, ()> {
    unsafe {
        if scm_obj.is_null() {
            return Err(());
        }
        if defs_scm_is_eq(scm_obj, scm_unspecified) != 0 {
            return Ok(String::new());
        } else if scm_is_string(scm_obj) != 0 {
            let res_str = CStr::from_ptr(scm_to_locale_string(scm_obj))
                .to_string_lossy()
                .into_owned();
            return Ok(res_str);
        } else {
            let res = scm_object_to_string(scm_obj, scm_undefined);
            if res.is_null() {
                return Err(());
            }
            let res_str = CStr::from_ptr(scm_to_locale_string(res))
                .to_string_lossy()
                .into_owned();
            return Ok(res_str);
        }
    }
}

#[derive(Clone)]
pub struct Guile {}

impl Guile {
    pub fn new() -> Self {
        unsafe {
            scm_init_guile();
        }
        Guile {}
    }

    pub fn evaluate_expression(&self, expr: &str) -> Result<String, ()> {
        unsafe {
            let c_expr = CString::new(expr).map_err(|_| ())?;
            let result = scm_c_eval_string(c_expr.as_ptr());
            string_from_scm(result)
        }
    }

    pub fn define_string(&self, name: &str, value: &str) {
        let c_name = CString::new(name).expect("CString::new failed");
        let c_value = CString::new(value).expect("CString::new failed");
        unsafe {
            defs_scm_define_string(c_name.as_ptr(), c_value.as_ptr());
        }
    }
}

impl Drop for Guile {
    fn drop(&mut self) {
        if let Err(e) = self.evaluate_expression("(exit)") {
            panic!("Error while exiting {:#?}", e);
        }
    }
}