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//! Command line arguments by reference: `Iterator<Item = &'static OsStr>`
//!
//! The standard library's [`std::env::args_os`] iterator produces an owned
//! string (`OsString`) for each argument. In some use cases it can be more
//! convenient for the arguments to be produced by static reference (`&'static
//! OsStr`).
//!
//! [`std::env::args_os`]: https://doc.rust-lang.org/std/env/fn.args_os.html
//!
//! # Examples
//!
//! ```
//! fn main() {
//!     for arg in argv::iter() {
//!         // arg is a &'static OsStr.
//!         println!("{}", arg.to_string_lossy());
//!     }
//! }
//! ```
//!
//! # Portability
//!
//! This crate is intended to be used on Linux and macOS, on which command line
//! arguments naturally live for the duration of the program. This crate
//! implements the same API on other platforms as well, such as Windows, but
//! leaks memory on platforms other than Linux and macOS.

#![doc(html_root_url = "https://docs.rs/argv/0.1.0")]

use std::ffi::OsStr;

/// Returns an iterator over command line arguments.
pub fn iter() -> Iter {
    Iter {
        platform_specific: crate::r#impl::iter()
    }
}

/// Iterator over command line arguments.
pub struct Iter {
    platform_specific: crate::r#impl::Iter,
}

impl Iterator for Iter {
    type Item = &'static OsStr;

    fn next(&mut self) -> Option<Self::Item> {
        self.platform_specific.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.platform_specific.size_hint()
    }
}

impl ExactSizeIterator for Iter {
    fn len(&self) -> usize {
        self.platform_specific.len()
    }
}

#[cfg(any(target_os = "linux", target_os = "macos"))]
mod r#impl {
    use std::ffi::{CStr, OsStr};
    use std::mem;
    use std::os::raw::{c_char, c_int};
    use std::os::unix::ffi::OsStrExt;
    use std::ptr;

    static mut ARGC: c_int = 0;
    static mut ARGV: *const *const c_char = ptr::null();

    #[cfg(target_os = "linux")]
    #[link_section = ".init_array"]
    #[used]
    static CAPTURE: unsafe extern "C" fn(c_int, *const *const c_char) = capture;

    #[cfg_attr(target_os = "macos", link_section = "__DATA,__mod_init_func")]
    unsafe extern "C" fn capture(argc: c_int, argv: *const *const c_char) {
        ARGC = argc;
        ARGV = argv;
    }

    pub fn iter() -> Iter {
        // These are only mutated before main so they are safe to read once main
        // has begun.
        let argc = unsafe { ARGC };
        let argv = unsafe { ARGV };

        // We count on the OS to provide argv for which argv + argc does not
        // overflow.
        let end = unsafe { argv.offset(argc as isize) };

        Iter { next: argv, end }
    }

    pub struct Iter {
        next: *const *const c_char,
        end: *const *const c_char,
    }

    impl Iterator for Iter {
        type Item = &'static OsStr;

        fn next(&mut self) -> Option<Self::Item> {
            if self.next == self.end {
                None
            } else {
                let ptr = unsafe { *self.next };
                let c_str = unsafe { CStr::from_ptr(ptr) };
                self.next = unsafe { self.next.offset(1) };
                Some(OsStr::from_bytes(c_str.to_bytes()))
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let len = self.len();
            (len, Some(len))
        }
    }

    impl ExactSizeIterator for Iter {
        fn len(&self) -> usize {
            (self.end as usize - self.next as usize) / mem::size_of::<*const c_char>()
        }
    }
}

#[cfg(not(any(target_os = "linux", target_os = "macos")))]
mod r#impl {
    use std::env;
    use std::ffi::OsStr;

    pub fn iter() -> Iter {
        Iter {
            args: env::args_os(),
        }
    }

    pub struct Iter {
        args: env::ArgsOs,
    }

    impl Iterator for Iter {
        type Item = &'static OsStr;

        fn next(&mut self) -> Option<Self::Item> {
            let arg = self.args.next()?;
            Some(Box::leak(arg.into_boxed_os_str()))
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.args.size_hint()
        }
    }

    impl ExactSizeIterator for Iter {
        fn len(&self) -> usize {
            self.args.len()
        }
    }
}