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
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168

use std::slice::{from_raw_parts, from_raw_parts_mut};
use std::cmp::Ordering::{Less, Greater};
use std::{fmt, marker};
use crate::{exponential_search_by, offset_from};

macro_rules! exponential_group_by_key {
    (struct $name:ident, $elem:ty, $mkslice:ident) => {
        impl<'a, T: 'a, F> $name<'a, T, F> {
            #[inline]
            pub fn is_empty(&self) -> bool {
                self.ptr == self.end
            }

            #[inline]
            pub fn remainder_len(&self) -> usize {
                unsafe { offset_from(self.end, self.ptr) }
            }
        }

        impl<'a, T: 'a, F, K> std::iter::Iterator for $name<'a, T, F>
        where F: FnMut(&T) -> K,
              K: PartialEq
        {
            type Item = $elem;

            fn next(&mut self) -> Option<Self::Item> {
                if self.is_empty() { return None }

                let first = unsafe { &*self.ptr };

                let len = self.remainder_len();
                let tail = unsafe { $mkslice(self.ptr.add(1), len - 1) };

                let predicate = |x: &T| if (self.func)(first) == (self.func)(x) { Less } else { Greater };
                let index = exponential_search_by(tail, predicate).unwrap_err();

                let left = unsafe { $mkslice(self.ptr, index + 1) };
                self.ptr = unsafe { self.ptr.add(index + 1) };

                Some(left)
            }

            fn size_hint(&self) -> (usize, Option<usize>) {
                if self.is_empty() { return (0, Some(0)) }

                let len = self.remainder_len();
                (1, Some(len))
            }

            fn last(mut self) -> Option<Self::Item> {
                self.next_back()
            }
        }

        impl<'a, T: 'a, F, K> std::iter::DoubleEndedIterator for $name<'a, T, F>
        where F: FnMut(&T) -> K,
              K: PartialEq
        {
            fn next_back(&mut self) -> Option<Self::Item> {
                if self.is_empty() { return None }

                let last = unsafe { &*self.end.sub(1) };

                let len = self.remainder_len();
                let head = unsafe { $mkslice(self.ptr, len - 1) };

                let predicate = |x: &T| if (self.func)(last) == (self.func)(x) { Greater } else { Less };
                let index = exponential_search_by(head, predicate).unwrap_err();

                let right = unsafe { $mkslice(self.ptr.add(index), len - index) };
                self.end = unsafe { self.end.sub(len - index) };

                Some(right)
            }
        }

        impl<'a, T: 'a, F, K> std::iter::FusedIterator for $name<'a, T, F>
        where F: FnMut(&T) -> K,
              K: PartialEq
        { }
    }
}

/// An iterator that will reutrn non-overlapping groups in the slice using *exponential search*.
///
/// It will give an element to the given function, producing a key and comparing
/// the keys to determine groups.
pub struct ExponentialGroupByKey<'a, T, F> {
    ptr: *const T,
    end: *const T,
    func: F,
    _phantom: marker::PhantomData<&'a T>,
}

impl<'a, T: 'a, F> ExponentialGroupByKey<'a, T, F> {
    pub fn new(slice: &'a [T], func: F) -> Self {
        ExponentialGroupByKey {
            ptr: slice.as_ptr(),
            end: unsafe { slice.as_ptr().add(slice.len()) },
            func,
            _phantom: marker::PhantomData,
        }
    }
}

impl<'a, T: 'a, F> ExponentialGroupByKey<'a, T, F> {
    /// Returns the remainder of the original slice that is going to be
    /// returned by the iterator.
    pub fn remainder(&self) -> &[T] {
        let len = self.remainder_len();
        unsafe { from_raw_parts(self.ptr, len) }
    }
}

impl<'a, T: 'a + fmt::Debug, F> fmt::Debug for ExponentialGroupByKey<'a, T, F> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("ExponentialGroupByKey")
            .field("remainder", &self.remainder())
            .finish()
    }
}

exponential_group_by_key!{ struct ExponentialGroupByKey, &'a [T], from_raw_parts }

/// An iterator that will reutrn non-overlapping *mutable* groups
/// in the slice using *exponential search*.
///
/// It will give an element to the given function, producing a key and comparing
/// the keys to determine groups.
pub struct ExponentialGroupByKeyMut<'a, T, F> {
    ptr: *mut T,
    end: *mut T,
    func: F,
    _phantom: marker::PhantomData<&'a mut T>,
}

impl<'a, T: 'a, F> ExponentialGroupByKeyMut<'a, T, F> {
    pub fn new(slice: &'a mut [T], func: F) -> Self {
        ExponentialGroupByKeyMut {
            ptr: slice.as_mut_ptr(),
            end: unsafe { slice.as_mut_ptr().add(slice.len()) },
            func,
            _phantom: marker::PhantomData,
        }
    }
}

impl<'a, T: 'a, F> ExponentialGroupByKeyMut<'a, T, F> {
    /// Returns the remainder of the original slice that is going to be
    /// returned by the iterator.
    pub fn into_remainder(self) -> &'a mut [T] {
        let len = self.remainder_len();
        unsafe { from_raw_parts_mut(self.ptr, len) }
    }
}

impl<'a, T: 'a + fmt::Debug, F> fmt::Debug for ExponentialGroupByKeyMut<'a, T, F> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let len = self.remainder_len();
        let remainder = unsafe { from_raw_parts(self.ptr, len) };

        f.debug_struct("ExponentialGroupByKeyMut")
            .field("remaining", &remainder)
            .finish()
    }
}

exponential_group_by_key!{ struct ExponentialGroupByKeyMut, &'a mut [T], from_raw_parts_mut }