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use std::ptr::null_mut;

use winapi::um::oleauto::{SysAllocStringLen, SysFreeString, SysStringLen};
use widestring::U16String;

use super::errors::BStringError;
use super::ptr::Ptr;

// pub type wchar_t = u16;
// pub type WCHAR = wchar_t;
// pub type OLECHAR = WCHAR;
// pub type BSTR = *mut OLECHAR;

//This is how C/Rust look at it, but the memory returned by SysX methods is a bit different
type BSTR = *mut u16; 

/// This trait is implemented on String to enable the convenient and safe conversion of
/// It utilizes the Sys* functions to manage the allocated memory. 
/// Generally you will want to use `.allocate_managed_bstr()` because it provides a
/// type that will automatically free the BSTR when dropped. 
/// 
/// For FFI, you **cannot** use a straight up *mut u16 when an interface calls for a 
/// BSTR. The reason being is that at the four bytes before where the BSTR pointer points to, 
/// there is a length prefix. In addition, the memory will be freed by the same allocator used in 
/// `SysAllocString`, which can cause UB if you didn't allocate the memory that way. **Any** other
/// allocation method will cause UB and crashes. 
/// 
pub trait BStringExt {
    /// Allocates a Ptr<u16> (aka a *mut u16 aka a BSTR)
    fn allocate_bstr(&mut self) -> Result<Ptr<u16>, BStringError>;
    /// Allocates a DroppableBString container - automatically frees the memory properly if dropped.
    fn allocate_managed_bstr(&mut self) -> Result<DroppableBString, BStringError>;
    /// Manually and correct free the memory allocated via Sys* methods
    fn deallocate_bstr(bstr: Ptr<u16>);
    /// Convenience method for conversion to a good intermediary type
    fn from_bstr(bstr: *mut u16) -> U16String;
    /// Convenience method for conversion to a good intermediary type
    fn from_pbstr(bstr: Ptr<u16>) -> U16String;
    /// Convenience method for conversion to a good intermediary type
    fn from_boxed_bstr(bstr: Box<u16>) -> U16String;
}

impl BStringExt for U16String {
    fn allocate_bstr(&mut self) -> Result<Ptr<u16>, BStringError> {
        let sz = self.len();
        let cln = self.clone();
        let rw = cln.as_ptr();
        let bstr: BSTR = unsafe {SysAllocStringLen(rw, sz as u32)};
        match Ptr::with_checked(bstr) {
            Some(pbstr) => Ok(pbstr), 
            None => Err(BStringError::AllocateFailed{len: sz})
        }
    }

    fn allocate_managed_bstr(&mut self) -> Result<DroppableBString, BStringError> {
        Ok(DroppableBString{ inner: Some(self.allocate_bstr()?) })
    }

    fn deallocate_bstr(bstr: Ptr<u16>) {
        let bstr: BSTR = bstr.as_ptr();
        unsafe { SysFreeString(bstr) }
    }

    fn from_bstr(bstr: *mut u16) -> U16String {
        assert!(!bstr.is_null());
        let sz = unsafe {SysStringLen(bstr)};
        unsafe {U16String::from_ptr(bstr, sz as usize)}
    }

    fn from_pbstr(bstr: Ptr<u16>) -> U16String {
        U16String::from_bstr(bstr.as_ptr())
    }

    fn from_boxed_bstr(bstr: Box<u16>) -> U16String {
        U16String::from_bstr(Box::into_raw(bstr))
    }
}

/// Struct that holds pointer to Sys* allocated memory. 
/// It will automatically free the memory via the Sys* 
/// functions unless it has been consumed. 
pub struct DroppableBString {
    inner: Option<Ptr<u16>>
}

impl DroppableBString {
    /// `consume()` -> `*mut u16` returns the contained data
    /// while also setting a flag that the data has been
    /// consumed. It is your responsibility to manage the 
    /// memory yourself. Most uses of BSTR in FFI will
    /// free the memory for you. 
    #[allow(dead_code)]
    pub fn consume(&mut self) -> *mut u16 {
        let ret = match self.inner {
            Some(ptr) => ptr.as_ptr(), 
            None => null_mut()
        };
        self.inner = None;
        ret
    }
}

impl Drop for DroppableBString {
    fn drop(&mut self) {
        match self.inner {
            Some(ptr) => {
                unsafe { SysFreeString(ptr.as_ptr())}
            }, 
            None => {}
        }
    }
}