#
rustfix/
replace.rs

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    //! A small module giving you a simple container that allows easy and cheap
    //! replacement of parts of its content, with the ability to prevent changing
    //! the same parts multiple times.

    use std::rc::Rc;

    use crate::error::Error;

    /// Indicates the change state of a [`Span`].
    #[derive(Debug, Clone, PartialEq, Eq)]
    enum State {
        /// The initial state. No change applied.
        Initial,
        /// Has been replaced.
        Replaced(Rc<[u8]>),
        /// Has been inserted.
        Inserted(Rc<[u8]>),
    }

    impl State {
        fn is_inserted(&self) -> bool {
            matches!(*self, State::Inserted(..))
        }
    }

    /// Span with a change [`State`].
    #[derive(Debug, Clone, PartialEq, Eq)]
    struct Span {
        /// Start of this span in parent data
        start: usize,
        /// up to end excluding
        end: usize,
        /// Whether the span is inserted, replaced or still fresh.
        data: State,
    }

    /// A container that allows easily replacing chunks of its data
    #[derive(Debug, Clone, Default)]
    pub struct Data {
        /// Original data.
        original: Vec<u8>,
        /// [`Span`]s covering the full range of the original data.
        parts: Vec<Span>,
    }

    impl Data {
        /// Create a new data container from a slice of bytes
        pub fn new(data: &[u8]) -> Self {
            Data {
                original: data.into(),
                parts: vec![Span {
                    data: State::Initial,
                    start: 0,
                    end: data.len(),
                }],
            }
        }

        /// Render this data as a vector of bytes
        pub fn to_vec(&self) -> Vec<u8> {
            if self.original.is_empty() {
                return Vec::new();
            }

            self.parts.iter().fold(Vec::new(), |mut acc, d| {
                match d.data {
                    State::Initial => acc.extend_from_slice(&self.original[d.start..d.end]),
                    State::Replaced(ref d) | State::Inserted(ref d) => acc.extend_from_slice(d),
                };
                acc
            })
        }

        /// Replace a chunk of data with the given slice, erroring when this part
        /// was already changed previously.
        pub fn replace_range(
            &mut self,
            range: std::ops::Range<usize>,
            data: &[u8],
        ) -> Result<(), Error> {
            if range.start > range.end {
                return Err(Error::InvalidRange(range));
            }

            if range.end > self.original.len() {
                return Err(Error::DataLengthExceeded(range, self.original.len()));
            }

            let insert_only = range.start == range.end;

            // Since we error out when replacing an already replaced chunk of data,
            // we can take some shortcuts here. For example, there can be no
            // overlapping replacements -- we _always_ split a chunk of 'initial'
            // data into three[^empty] parts, and there can't ever be two 'initial'
            // parts touching.
            //
            // [^empty]: Leading and trailing ones might be empty if we replace
            // the whole chunk. As an optimization and without loss of generality we
            // don't add empty parts.
            let new_parts = {
                let Some(index_of_part_to_split) = self.parts.iter().position(|p| {
                    !p.data.is_inserted() && p.start <= range.start && p.end >= range.end
                }) else {
                    if tracing::enabled!(tracing::Level::DEBUG) {
                        let slices = self
                            .parts
                            .iter()
                            .map(|p| {
                                (
                                    p.start,
                                    p.end,
                                    match p.data {
                                        State::Initial => "initial",
                                        State::Replaced(..) => "replaced",
                                        State::Inserted(..) => "inserted",
                                    },
                                )
                            })
                            .collect::<Vec<_>>();
                        tracing::debug!(
                            "no single slice covering {}..{}, current slices: {:?}",
                            range.start,
                            range.end,
                            slices,
                        );
                    }

                    return Err(Error::MaybeAlreadyReplaced(range));
                };

                let part_to_split = &self.parts[index_of_part_to_split];

                // If this replacement matches exactly the part that we would
                // otherwise split then we ignore this for now. This means that you
                // can replace the exact same range with the exact same content
                // multiple times and we'll process and allow it.
                //
                // This is currently done to alleviate issues like
                // rust-lang/rust#51211 although this clause likely wants to be
                // removed if that's fixed deeper in the compiler.
                if part_to_split.start == range.start && part_to_split.end == range.end {
                    if let State::Replaced(ref replacement) = part_to_split.data {
                        if &**replacement == data {
                            return Ok(());
                        }
                    }
                }

                if part_to_split.data != State::Initial {
                    return Err(Error::AlreadyReplaced);
                }

                let mut new_parts = Vec::with_capacity(self.parts.len() + 2);

                // Previous parts
                if let Some(ps) = self.parts.get(..index_of_part_to_split) {
                    new_parts.extend_from_slice(ps);
                }

                // Keep initial data on left side of part
                if range.start > part_to_split.start {
                    new_parts.push(Span {
                        start: part_to_split.start,
                        end: range.start,
                        data: State::Initial,
                    });
                }

                // New part
                new_parts.push(Span {
                    start: range.start,
                    end: range.end,
                    data: if insert_only {
                        State::Inserted(data.into())
                    } else {
                        State::Replaced(data.into())
                    },
                });

                // Keep initial data on right side of part
                if range.end < part_to_split.end {
                    new_parts.push(Span {
                        start: range.end,
                        end: part_to_split.end,
                        data: State::Initial,
                    });
                }

                // Following parts
                if let Some(ps) = self.parts.get(index_of_part_to_split + 1..) {
                    new_parts.extend_from_slice(ps);
                }

                new_parts
            };

            self.parts = new_parts;

            Ok(())
        }
    }

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

        fn str(i: &[u8]) -> &str {
            ::std::str::from_utf8(i).unwrap()
        }

        #[test]
        fn insert_at_beginning() {
            let mut d = Data::new(b"foo bar baz");
            d.replace_range(0..0, b"oh no ").unwrap();
            assert_eq!("oh no foo bar baz", str(&d.to_vec()));
        }

        #[test]
        fn insert_at_end() {
            let mut d = Data::new(b"foo bar baz");
            d.replace_range(11..11, b" oh no").unwrap();
            assert_eq!("foo bar baz oh no", str(&d.to_vec()));
        }

        #[test]
        fn replace_some_stuff() {
            let mut d = Data::new(b"foo bar baz");
            d.replace_range(4..7, b"lol").unwrap();
            assert_eq!("foo lol baz", str(&d.to_vec()));
        }

        #[test]
        fn replace_a_single_char() {
            let mut d = Data::new(b"let y = true;");
            d.replace_range(4..5, b"mut y").unwrap();
            assert_eq!("let mut y = true;", str(&d.to_vec()));
        }

        #[test]
        fn replace_multiple_lines() {
            let mut d = Data::new(b"lorem\nipsum\ndolor");

            d.replace_range(6..11, b"lol").unwrap();
            assert_eq!("lorem\nlol\ndolor", str(&d.to_vec()));

            d.replace_range(12..17, b"lol").unwrap();
            assert_eq!("lorem\nlol\nlol", str(&d.to_vec()));
        }

        #[test]
        fn replace_multiple_lines_with_insert_only() {
            let mut d = Data::new(b"foo!");

            d.replace_range(3..3, b"bar").unwrap();
            assert_eq!("foobar!", str(&d.to_vec()));

            d.replace_range(0..3, b"baz").unwrap();
            assert_eq!("bazbar!", str(&d.to_vec()));

            d.replace_range(3..4, b"?").unwrap();
            assert_eq!("bazbar?", str(&d.to_vec()));
        }

        #[test]
        fn replace_invalid_range() {
            let mut d = Data::new(b"foo!");

            assert!(d.replace_range(2..1, b"bar").is_err());
            assert!(d.replace_range(0..3, b"bar").is_ok());
        }

        #[test]
        fn empty_to_vec_roundtrip() {
            let s = "";
            assert_eq!(s.as_bytes(), Data::new(s.as_bytes()).to_vec().as_slice());
        }

        #[test]
        fn replace_overlapping_stuff_errs() {
            let mut d = Data::new(b"foo bar baz");

            d.replace_range(4..7, b"lol").unwrap();
            assert_eq!("foo lol baz", str(&d.to_vec()));

            assert!(matches!(
                d.replace_range(4..7, b"lol2").unwrap_err(),
                Error::AlreadyReplaced,
            ));
        }

        #[test]
        fn broken_replacements() {
            let mut d = Data::new(b"foo");
            assert!(matches!(
                d.replace_range(4..8, b"lol").unwrap_err(),
                Error::DataLengthExceeded(std::ops::Range { start: 4, end: 8 }, 3),
            ));
        }

        #[test]
        fn replace_same_twice() {
            let mut d = Data::new(b"foo");
            d.replace_range(0..1, b"b").unwrap();
            d.replace_range(0..1, b"b").unwrap();
            assert_eq!("boo", str(&d.to_vec()));
        }

        proptest! {
            #[test]
            fn new_to_vec_roundtrip(ref s in "\\PC*") {
                assert_eq!(s.as_bytes(), Data::new(s.as_bytes()).to_vec().as_slice());
            }

            #[test]
            fn replace_random_chunks(
                ref data in "\\PC*",
                ref replacements in prop::collection::vec(
                    (any::<::std::ops::Range<usize>>(), any::<Vec<u8>>()),
                    1..100,
                )
            ) {
                let mut d = Data::new(data.as_bytes());
                for &(ref range, ref bytes) in replacements {
                    let _ = d.replace_range(range.clone(), bytes);
                }
            }
        }
    }