1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
use smallvec::{Array, SmallVec};
use std::ptr;

pub trait MapInPlace<T>: Sized {
    fn map_in_place<F>(&mut self, mut f: F)
    where
        F: FnMut(T) -> T,
    {
        self.flat_map_in_place(|e| Some(f(e)))
    }

    fn flat_map_in_place<F, I>(&mut self, f: F)
    where
        F: FnMut(T) -> I,
        I: IntoIterator<Item = T>;
}

impl<T> MapInPlace<T> for Vec<T> {
    fn flat_map_in_place<F, I>(&mut self, mut f: F)
    where
        F: FnMut(T) -> I,
        I: IntoIterator<Item = T>,
    {
        let mut read_i = 0;
        let mut write_i = 0;
        unsafe {
            let mut old_len = self.len();
            self.set_len(0); // make sure we just leak elements in case of panic

            while read_i < old_len {
                // move the read_i'th item out of the vector and map it
                // to an iterator
                let e = ptr::read(self.get_unchecked(read_i));
                let iter = f(e).into_iter();
                read_i += 1;

                for e in iter {
                    if write_i < read_i {
                        ptr::write(self.get_unchecked_mut(write_i), e);
                        write_i += 1;
                    } else {
                        // If this is reached we ran out of space
                        // in the middle of the vector.
                        // However, the vector is in a valid state here,
                        // so we just do a somewhat inefficient insert.
                        self.set_len(old_len);
                        self.insert(write_i, e);

                        old_len = self.len();
                        self.set_len(0);

                        read_i += 1;
                        write_i += 1;
                    }
                }
            }

            // write_i tracks the number of actually written new items.
            self.set_len(write_i);
        }
    }
}

impl<T, A: Array<Item = T>> MapInPlace<T> for SmallVec<A> {
    fn flat_map_in_place<F, I>(&mut self, mut f: F)
    where
        F: FnMut(T) -> I,
        I: IntoIterator<Item = T>,
    {
        let mut read_i = 0;
        let mut write_i = 0;
        unsafe {
            let mut old_len = self.len();
            self.set_len(0); // make sure we just leak elements in case of panic

            while read_i < old_len {
                // move the read_i'th item out of the vector and map it
                // to an iterator
                let e = ptr::read(self.get_unchecked(read_i));
                let iter = f(e).into_iter();
                read_i += 1;

                for e in iter {
                    if write_i < read_i {
                        ptr::write(self.get_unchecked_mut(write_i), e);
                        write_i += 1;
                    } else {
                        // If this is reached we ran out of space
                        // in the middle of the vector.
                        // However, the vector is in a valid state here,
                        // so we just do a somewhat inefficient insert.
                        self.set_len(old_len);
                        self.insert(write_i, e);

                        old_len = self.len();
                        self.set_len(0);

                        read_i += 1;
                        write_i += 1;
                    }
                }
            }

            // write_i tracks the number of actually written new items.
            self.set_len(write_i);
        }
    }
}