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    // Copyright 2015-2016 Brian Smith.
    //
    // Permission to use, copy, modify, and/or distribute this software for any
    // purpose with or without fee is hereby granted, provided that the above
    // copyright notice and this permission notice appear in all copies.
    //
    // THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
    // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
    // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR
    // ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
    // ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    // OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

    //! untrusted.rs: Safe, fast, zero-panic, zero-crashing, zero-allocation
    //! parsing of untrusted inputs in Rust.
    //!
    //! <code>git clone https://github.com/briansmith/untrusted</code>
    //!
    //! untrusted.rs goes beyond Rust's normal safety guarantees by  also
    //! guaranteeing that parsing will be panic-free, as long as
    //! `untrusted::Input::as_slice_less_safe()` is not used. It avoids copying
    //! data and heap allocation and strives to prevent common pitfalls such as
    //! accidentally parsing input bytes multiple times. In order to meet these
    //! goals, untrusted.rs is limited in functionality such that it works best for
    //! input languages with a small fixed amount of lookahead such as ASN.1, TLS,
    //! TCP/IP, and many other networking, IPC, and related protocols. Languages
    //! that require more lookahead and/or backtracking require some significant
    //! contortions to parse using this framework. It would not be realistic to use
    //! it for parsing programming language code, for example.
    //!
    //! The overall pattern for using untrusted.rs is:
    //!
    //! 1. Write a recursive-descent-style parser for the input language, where the
    //!    input data is given as a `&mut untrusted::Reader` parameter to each
    //!    function. Each function should have a return type of `Result<V, E>` for
    //!    some value type `V` and some error type `E`, either or both of which may
    //!    be `()`. Functions for parsing the lowest-level language constructs
    //!    should be defined. Those lowest-level functions will parse their inputs
    //!    using `::read_byte()`, `Reader::peek()`, and similar functions.
    //!    Higher-level language constructs are then parsed by calling the
    //!    lower-level functions in sequence.
    //!
    //! 2. Wrap the top-most functions of your recursive-descent parser in
    //!    functions that take their input data as an `untrusted::Input`. The
    //!    wrapper functions should call the `Input`'s `read_all` (or a variant
    //!    thereof) method. The wrapper functions are the only ones that should be
    //!    exposed outside the parser's module.
    //!
    //! 3. After receiving the input data to parse, wrap it in an `untrusted::Input`
    //!    using `untrusted::Input::from()` as early as possible. Pass the
    //!    `untrusted::Input` to the wrapper functions when they need to be parsed.
    //!
    //! In general parsers built using `untrusted::Reader` do not need to explicitly
    //! check for end-of-input unless they are parsing optional constructs, because
    //! `Reader::read_byte()` will return `Err(EndOfInput)` on end-of-input.
    //! Similarly, parsers using `untrusted::Reader` generally don't need to check
    //! for extra junk at the end of the input as long as the parser's API uses the
    //! pattern described above, as `read_all` and its variants automatically check
    //! for trailing junk. `Reader::skip_to_end()` must be used when any remaining
    //! unread input should be ignored without triggering an error.
    //!
    //! untrusted.rs works best when all processing of the input data is done
    //! through the `untrusted::Input` and `untrusted::Reader` types. In
    //! particular, avoid trying to parse input data using functions that take
    //! byte slices. However, when you need to access a part of the input data as
    //! a slice to use a function that isn't written using untrusted.rs,
    //! `Input::as_slice_less_safe()` can be used.
    //!
    //! It is recommend to use `use untrusted;` and then `untrusted::Input`,
    //! `untrusted::Reader`, etc., instead of using `use untrusted::*`. Qualifying
    //! the names with `untrusted` helps remind the reader of the code that it is
    //! dealing with *untrusted* input.
    //!
    //! # Examples
    //!
    //! [*ring*](https://github.com/briansmith/ring)'s parser for the subset of
    //! ASN.1 DER it needs to understand,
    //! [`ring::der`](https://github.com/briansmith/ring/blob/master/src/der.rs),
    //! is built on top of untrusted.rs. *ring* also uses untrusted.rs to parse ECC
    //! public keys, RSA PKCS#1 1.5 padding, and for all other parsing it does.
    //!
    //! All of [webpki](https://github.com/briansmith/webpki)'s parsing of X.509
    //! certificates (also ASN.1 DER) is done using untrusted.rs.

    #![doc(html_root_url="https://briansmith.org/rustdoc/")]

    #![no_std]

    /// A wrapper around `&'a [u8]` that helps in writing panic-free code.
    ///
    /// No methods of `Input` will ever panic.
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub struct Input<'a> {
        value: no_panic::Slice<'a>
    }

    impl<'a> Input<'a> {
        /// Construct a new `Input` for the given input `bytes`.
        pub fn from(bytes: &'a [u8]) -> Input<'a> {
            // This limit is important for avoiding integer overflow. In particular,
            // `Reader` assumes that an `i + 1 > i` if `input.value.get(i)` does
            // not return `None`. According to the Rust language reference, the
            // maximum object size is `core::isize::MAX`, and in practice it is
            // impossible to create an object of size `core::usize::MAX` or larger.
            debug_assert!(bytes.len() < core::usize::MAX);
            Input { value: no_panic::Slice::new(bytes) }
        }

        /// Returns `true` if the input is empty and false otherwise.
        #[inline]
        pub fn is_empty(&self) -> bool { self.value.is_empty() }

        /// Returns an iterator over the input.
        #[inline]
        pub fn iter(&self) -> <&[u8] as IntoIterator>::IntoIter {
            self.value.into_iter()
        }

        /// Returns the length of the `Input`.
        #[inline]
        pub fn len(&self) -> usize { self.value.len() }

        /// Calls `read` with the given input as a `Reader`, ensuring that `read`
        /// consumed the entire input. If `read` does not consume the entire input,
        /// `incomplete_read` is returned.
        pub fn read_all<F, R, E>(&self, incomplete_read: E, read: F)
                                 -> Result<R, E>
                                 where F: FnOnce(&mut Reader<'a>) -> Result<R, E> {
            let mut input = Reader::new(*self);
            let result = try!(read(&mut input));
            if input.at_end() {
                Ok(result)
            } else {
                Err(incomplete_read)
            }
        }

        /// Like `read_all`, except taking an `FnMut`.
        pub fn read_all_mut<F, R, E>(&self, incomplete_read: E, mut read: F)
                                     -> Result<R, E>
                                     where F: FnMut(&mut Reader<'a>)
                                                    -> Result<R, E> {
            let mut input = Reader::new(*self);
            let result = try!(read(&mut input));
            if input.at_end() {
                Ok(result)
            } else {
                Err(incomplete_read)
            }
        }

        /// Access the input as a slice so it can be processed by functions that
        /// are not written using the Input/Reader framework.
        #[inline]
        pub fn as_slice_less_safe(&self) -> &'a [u8] {
            self.value.as_slice_less_safe()
        }
    }

    impl <'a, 'b> PartialEq<&'b [u8]> for Input<'a> {
        fn eq(&self, other: &&'b [u8]) -> bool {
            self.as_slice_less_safe() == *other
        }
    }

    /// Calls `read` with the given input as a `Reader`, ensuring that `read`
    /// consumed the entire input. When `input` is `None`, `read` will be
    /// called with `None`.
    pub fn read_all_optional<'a, F, R, E>(input: Option<Input<'a>>,
                                          incomplete_read: E, read: F)
                                          -> Result<R, E>
                                          where F: FnOnce(Option<&mut Reader>)
                                                          -> Result<R, E> {
        match input {
            Some(input) => {
                let mut input = Reader::new(input);
                let result = try!(read(Some(&mut input)));
                if input.at_end() {
                    Ok(result)
                } else {
                    Err(incomplete_read)
                }
            },
            None => read(None)
        }
    }

    /// A read-only, forward-only* cursor into the data in an `Input`.
    ///
    /// Using `Reader` to parse input helps to ensure that no byte of the input
    /// will be accidentally processed more than once. Using `Reader` in
    /// conjunction with `read_all`, `read_all_mut`, and `read_all_optional`
    /// helps ensure that no byte of the input is accidentally left unprocessed.
    /// The methods of `Reader` never panic, so `Reader` also assists the writing
    /// of panic-free code.
    ///
    /// \* `Reader` is not strictly forward-only because of the method
    /// `get_input_between_marks`, which is provided mainly to support calculating
    /// digests over parsed data.
    #[derive(Debug)]
    pub struct Reader<'a> {
        input: no_panic::Slice<'a>,
        i: usize
    }

    /// An index into the already-parsed input of a `Reader`.
    pub struct Mark {
        i: usize
    }

    impl<'a> Reader<'a> {
        /// Construct a new Reader for the given input. Use `read_all`,
        /// `read_all_mut`, or `read_all_optional` instead of `Reader::new`
        /// whenever possible.
        #[inline]
        pub fn new(input: Input<'a>) -> Reader<'a> {
            Reader { input: input.value, i: 0 }
        }

        /// Returns `true` if the reader is at the end of the input, and `false`
        /// otherwise.
        #[inline]
        pub fn at_end(&self) -> bool { self.i == self.input.len() }

        /// Returns an `Input` for already-parsed input that has had its boundaries
        /// marked using `mark`.
        #[inline]
        pub fn get_input_between_marks(&self, mark1: Mark, mark2: Mark)
                                       -> Result<Input<'a>, EndOfInput> {
            self.input.get_slice(mark1.i..mark2.i)
                      .map(|subslice| Input { value: subslice })
                      .ok_or(EndOfInput)
        }

        /// Return the current position of the `Reader` for future use in a call
        /// to `get_input_between_marks`.
        #[inline]
        pub fn mark(&self) -> Mark { Mark { i: self.i } }

        /// Returns `true` if there is at least one more byte in the input and that
        /// byte is equal to `b`, and false otherwise.
        pub fn peek(&self, b: u8) -> bool {
            match self.input.get(self.i) {
                Some(actual_b) => return b == *actual_b,
                None => false
            }
        }

        /// Reads the next input byte.
        ///
        /// Returns `Ok(b)` where `b` is the next input byte, or `Err(EndOfInput)`
        /// if the `Reader` is at the end of the input.
        pub fn read_byte(&mut self) -> Result<u8, EndOfInput> {
            match self.input.get(self.i) {
                Some(b) => {
                    self.i += 1; // safe from overflow; see Input::from().
                    Ok(*b)
                }
                None => Err(EndOfInput)
            }
        }

        /// Skips `num_bytes` of the input.
        ///
        /// Returns `Ok(())` if there are at least `num_bytes` of input remaining,
        /// and `Err(EndOfInput)` otherwise.
        pub fn skip(&mut self, num_bytes: usize) -> Result<(), EndOfInput> {
            self.skip_and_get_input(num_bytes).map(|_| ())
        }

        /// Skips `num_bytes` of the input, returning the skipped input as an `Input`.
        ///
        /// Returns `Ok(i)` where `i` is an `Input` if there are at least
        /// `num_bytes` of input remaining, and `Err(EndOfInput)` otherwise.
        pub fn skip_and_get_input(&mut self, num_bytes: usize)
                                  -> Result<Input<'a>, EndOfInput> {
            let new_i = try!(self.i.checked_add(num_bytes).ok_or(EndOfInput));
            let ret = self.input.get_slice(self.i..new_i)
                                .map(|subslice| Input { value: subslice })
                                .ok_or(EndOfInput);
            self.i = new_i;
            ret
        }

        /// Skips the reader to the end of the input, returning the skipped input
        /// as an `Input`.
        pub fn skip_to_end(&mut self) -> Input<'a> {
            let to_skip = self.input.len() - self.i;
            self.skip_and_get_input(to_skip).unwrap()
        }
    }

    /// The error type used to indicate the end of the input was reached before the
    /// operation could be completed.
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub struct EndOfInput;

    mod no_panic {
        use core;

        /// A wrapper around a slice that exposes no functions that can panic.
        #[derive(Clone, Copy, Debug, Eq, PartialEq)]
        pub struct Slice<'a> {
            bytes: &'a [u8]
        }

        impl<'a> Slice<'a> {
            #[inline]
            pub fn new(bytes: &'a [u8]) -> Slice<'a> {
                Slice { bytes }
            }

            #[inline]
            // TODO: https://github.com/rust-lang/rust/issues/35729#issuecomment-280872145
            //      pub fn get<I>(&self, i: I) -> Option<&I::Output>
            //          where I: core::slice::SliceIndex<u8>
            pub fn get(&self, i: usize) -> Option<&u8> { self.bytes.get(i) }

            // TODO: This will be replaced with `get()` once `get()` is made
            // generic over `SliceIndex`.
            #[inline]
            pub fn get_slice(&self, r: core::ops::Range<usize>)
                             -> Option<Slice<'a>> {
                self.bytes.get(r).map(|bytes| Slice { bytes })
            }

            #[inline]
            pub fn into_iter(&self) -> <&'a [u8] as IntoIterator>::IntoIter {
                self.bytes.into_iter()
            }

            #[inline]
            pub fn is_empty(&self) -> bool { self.bytes.is_empty() }

            #[inline]
            pub fn len(&self) -> usize { self.bytes.len() }

            #[inline]
            pub fn as_slice_less_safe(&self) -> &'a [u8] { self.bytes }
        }

    } // mod no_panic

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn test_input_from() {
            let _ = Input::from(b"foo");
        }

        #[test]
        fn test_input_is_empty() {
            let input = Input::from(b"");
            assert!(input.is_empty());
            let input = Input::from(b"foo");
            assert!(!input.is_empty());
        }

        #[test]
        fn test_input_len() {
            let input = Input::from(b"foo");
            assert_eq!(input.len(), 3);
        }

        #[test]
        fn test_input_read_all() {
            let input = Input::from(b"foo");
            let result = input.read_all(EndOfInput, |input| {
                assert_eq!(b'f', try!(input.read_byte()));
                assert_eq!(b'o', try!(input.read_byte()));
                assert_eq!(b'o', try!(input.read_byte()));
                assert!(input.at_end());
                Ok(())
            });
            assert_eq!(result, Ok(()));
        }

        #[test]
        fn test_input_read_all_unconsume() {
            let input = Input::from(b"foo");
            let result = input.read_all(EndOfInput, |input| {
                assert_eq!(b'f', try!(input.read_byte()));
                assert!(!input.at_end());
                Ok(())
            });
            assert_eq!(result, Err(EndOfInput));
        }

        #[test]
        fn test_input_as_slice_less_safe() {
            let slice = b"foo";
            let input = Input::from(slice);
            assert_eq!(input.as_slice_less_safe(), slice);
        }
    }

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