//! The Windows command line is just a string
//! <https://docs.microsoft.com/en-us/archive/blogs/larryosterman/the-windows-command-line-is-just-a-string>
//!
//! This module implements the parsing necessary to turn that string into a list of arguments.

#[cfg(test)]
mod tests;

use crate::ffi::OsString;
use crate::fmt;
use crate::marker::PhantomData;
use crate::num::NonZeroU16;
use crate::os::windows::prelude::*;
use crate::path::PathBuf;
use crate::ptr::NonNull;
use crate::sys::c;
use crate::sys::windows::os::current_exe;
use crate::vec;

use core::iter;

pub fn args() -> Args {
    // SAFETY: `GetCommandLineW` returns a pointer to a null terminated UTF-16
    // string so it's safe for `WStrUnits` to use.
    unsafe {
        let lp_cmd_line = c::GetCommandLineW();
        let parsed_args_list = parse_lp_cmd_line(WStrUnits::new(lp_cmd_line), || {
            current_exe().map(PathBuf::into_os_string).unwrap_or_else(|_| OsString::new())
        });

        Args { parsed_args_list: parsed_args_list.into_iter() }
    }
}

/// Implements the Windows command-line argument parsing algorithm.
///
/// Microsoft's documentation for the Windows CLI argument format can be found at
/// <https://docs.microsoft.com/en-us/cpp/cpp/main-function-command-line-args?view=msvc-160#parsing-c-command-line-arguments>
///
/// A more in-depth explanation is here:
/// <https://daviddeley.com/autohotkey/parameters/parameters.htm#WIN>
///
/// Windows includes a function to do command line parsing in shell32.dll.
/// However, this is not used for two reasons:
///
/// 1. Linking with that DLL causes the process to be registered as a GUI application.
/// GUI applications add a bunch of overhead, even if no windows are drawn. See
/// <https://randomascii.wordpress.com/2018/12/03/a-not-called-function-can-cause-a-5x-slowdown/>.
///
/// 2. It does not follow the modern C/C++ argv rules outlined in the first two links above.
///
/// This function was tested for equivalence to the C/C++ parsing rules using an
/// extensive test suite available at
/// <https://github.com/ChrisDenton/winarg/tree/std>.
fn parse_lp_cmd_line<'a, F: Fn() -> OsString>(
    lp_cmd_line: Option<WStrUnits<'a>>,
    exe_name: F,
) -> Vec<OsString> {
    const BACKSLASH: NonZeroU16 = NonZeroU16::new(b'\\' as u16).unwrap();
    const QUOTE: NonZeroU16 = NonZeroU16::new(b'"' as u16).unwrap();
    const TAB: NonZeroU16 = NonZeroU16::new(b'\t' as u16).unwrap();
    const SPACE: NonZeroU16 = NonZeroU16::new(b' ' as u16).unwrap();

    let mut ret_val = Vec::new();
    // If the cmd line pointer is null or it points to an empty string then
    // return the name of the executable as argv[0].
    if lp_cmd_line.as_ref().and_then(|cmd| cmd.peek()).is_none() {
        ret_val.push(exe_name());
        return ret_val;
    }
    let mut code_units = lp_cmd_line.unwrap();

    // The executable name at the beginning is special.
    let mut in_quotes = false;
    let mut cur = Vec::new();
    for w in &mut code_units {
        match w {
            // A quote mark always toggles `in_quotes` no matter what because
            // there are no escape characters when parsing the executable name.
            QUOTE => in_quotes = !in_quotes,
            // If not `in_quotes` then whitespace ends argv[0].
            SPACE | TAB if !in_quotes => break,
            // In all other cases the code unit is taken literally.
            _ => cur.push(w.get()),
        }
    }
    // Skip whitespace.
    code_units.advance_while(|w| w == SPACE || w == TAB);
    ret_val.push(OsString::from_wide(&cur));

    // Parse the arguments according to these rules:
    // * All code units are taken literally except space, tab, quote and backslash.
    // * When not `in_quotes`, space and tab separate arguments. Consecutive spaces and tabs are
    // treated as a single separator.
    // * A space or tab `in_quotes` is taken literally.
    // * A quote toggles `in_quotes` mode unless it's escaped. An escaped quote is taken literally.
    // * A quote can be escaped if preceded by an odd number of backslashes.
    // * If any number of backslashes is immediately followed by a quote then the number of
    // backslashes is halved (rounding down).
    // * Backslashes not followed by a quote are all taken literally.
    // * If `in_quotes` then a quote can also be escaped using another quote
    // (i.e. two consecutive quotes become one literal quote).
    let mut cur = Vec::new();
    let mut in_quotes = false;
    while let Some(w) = code_units.next() {
        match w {
            // If not `in_quotes`, a space or tab ends the argument.
            SPACE | TAB if !in_quotes => {
                ret_val.push(OsString::from_wide(&cur[..]));
                cur.truncate(0);

                // Skip whitespace.
                code_units.advance_while(|w| w == SPACE || w == TAB);
            }
            // Backslashes can escape quotes or backslashes but only if consecutive backslashes are followed by a quote.
            BACKSLASH => {
                let backslash_count = code_units.advance_while(|w| w == BACKSLASH) + 1;
                if code_units.peek() == Some(QUOTE) {
                    cur.extend(iter::repeat(BACKSLASH.get()).take(backslash_count / 2));
                    // The quote is escaped if there are an odd number of backslashes.
                    if backslash_count % 2 == 1 {
                        code_units.next();
                        cur.push(QUOTE.get());
                    }
                } else {
                    // If there is no quote on the end then there is no escaping.
                    cur.extend(iter::repeat(BACKSLASH.get()).take(backslash_count));
                }
            }
            // If `in_quotes` and not backslash escaped (see above) then a quote either
            // unsets `in_quote` or is escaped by another quote.
            QUOTE if in_quotes => match code_units.peek() {
                // Two consecutive quotes when `in_quotes` produces one literal quote.
                Some(QUOTE) => {
                    cur.push(QUOTE.get());
                    code_units.next();
                }
                // Otherwise set `in_quotes`.
                Some(_) => in_quotes = false,
                // The end of the command line.
                // Push `cur` even if empty, which we do by breaking while `in_quotes` is still set.
                None => break,
            },
            // If not `in_quotes` and not BACKSLASH escaped (see above) then a quote sets `in_quote`.
            QUOTE => in_quotes = true,
            // Everything else is always taken literally.
            _ => cur.push(w.get()),
        }
    }
    // Push the final argument, if any.
    if !cur.is_empty() || in_quotes {
        ret_val.push(OsString::from_wide(&cur[..]));
    }
    ret_val
}

pub struct Args {
    parsed_args_list: vec::IntoIter<OsString>,
}

impl fmt::Debug for Args {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.parsed_args_list.as_slice().fmt(f)
    }
}

impl Iterator for Args {
    type Item = OsString;
    fn next(&mut self) -> Option<OsString> {
        self.parsed_args_list.next()
    }
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.parsed_args_list.size_hint()
    }
}

impl DoubleEndedIterator for Args {
    fn next_back(&mut self) -> Option<OsString> {
        self.parsed_args_list.next_back()
    }
}

impl ExactSizeIterator for Args {
    fn len(&self) -> usize {
        self.parsed_args_list.len()
    }
}

/// A safe iterator over a LPWSTR
/// (aka a pointer to a series of UTF-16 code units terminated by a NULL).
struct WStrUnits<'a> {
    // The pointer must never be null...
    lpwstr: NonNull<u16>,
    // ...and the memory it points to must be valid for this lifetime.
    lifetime: PhantomData<&'a [u16]>,
}
impl WStrUnits<'_> {
    /// Create the iterator. Returns `None` if `lpwstr` is null.
    ///
    /// SAFETY: `lpwstr` must point to a null-terminated wide string that lives
    /// at least as long as the lifetime of this struct.
    unsafe fn new(lpwstr: *const u16) -> Option<Self> {
        Some(Self { lpwstr: NonNull::new(lpwstr as _)?, lifetime: PhantomData })
    }
    fn peek(&self) -> Option<NonZeroU16> {
        // SAFETY: It's always safe to read the current item because we don't
        // ever move out of the array's bounds.
        unsafe { NonZeroU16::new(*self.lpwstr.as_ptr()) }
    }
    /// Advance the iterator while `predicate` returns true.
    /// Returns the number of items it advanced by.
    fn advance_while<P: FnMut(NonZeroU16) -> bool>(&mut self, mut predicate: P) -> usize {
        let mut counter = 0;
        while let Some(w) = self.peek() {
            if !predicate(w) {
                break;
            }
            counter += 1;
            self.next();
        }
        counter
    }
}
impl Iterator for WStrUnits<'_> {
    // This can never return zero as that marks the end of the string.
    type Item = NonZeroU16;
    fn next(&mut self) -> Option<NonZeroU16> {
        // SAFETY: If NULL is reached we immediately return.
        // Therefore it's safe to advance the pointer after that.
        unsafe {
            let next = self.peek()?;
            self.lpwstr = NonNull::new_unchecked(self.lpwstr.as_ptr().add(1));
            Some(next)
        }
    }
}