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
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
use std::hash::Hash;

mod private {
    use std::collections::HashMap;
    use std::hash::Hash;
    use std::fmt;

    #[derive(Clone)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    pub struct DuplicatesBy<I: Iterator, Key, F> {
        pub(crate) iter: I,
        pub(crate) meta: Meta<Key, F>,
    }

    impl<I, V, F> fmt::Debug for DuplicatesBy<I, V, F>
    where
        I: Iterator + fmt::Debug,
        V: fmt::Debug + Hash + Eq,
    {
        debug_fmt_fields!(DuplicatesBy, iter, meta.used);
    }

    impl<I: Iterator, Key: Eq + Hash, F> DuplicatesBy<I, Key, F> {
        pub(crate) fn new(iter: I, key_method: F) -> Self {
            DuplicatesBy {
                iter,
                meta: Meta {
                    used: HashMap::new(),
                    pending: 0,
                    key_method,
                },
            }
        }
    }

    #[derive(Clone)]
    pub struct Meta<Key, F> {
        used: HashMap<Key, bool>,
        pending: usize,
        key_method: F,
    }

    impl<Key, F> Meta<Key, F>
    where
        Key: Eq + Hash,
    {
        /// Takes an item and returns it back to the caller if it's the second time we see it.
        /// Otherwise the item is consumed and None is returned
        #[inline(always)]
        fn filter<I>(&mut self, item: I) -> Option<I>
        where
            F: KeyMethod<Key, I>,
        {
            let kv = self.key_method.make(item);
            match self.used.get_mut(kv.key_ref()) {
                None => {
                    self.used.insert(kv.key(), false);
                    self.pending += 1;
                    None
                }
                Some(true) => None,
                Some(produced) => {
                    *produced = true;
                    self.pending -= 1;
                    Some(kv.value())
                }
            }
        }
    }

    impl<I, Key, F> Iterator for DuplicatesBy<I, Key, F>
    where
        I: Iterator,
        Key: Eq + Hash,
        F: KeyMethod<Key, I::Item>,
    {
        type Item = I::Item;

        fn next(&mut self) -> Option<Self::Item> {
            let DuplicatesBy { iter, meta } = self;
            iter.find_map(|v| meta.filter(v))
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, hi) = self.iter.size_hint();
            let hi = hi.map(|hi| {
                if hi <= self.meta.pending {
                    // fewer or equally many iter-remaining elements than pending elements
                    // => at most, each iter-remaining element is matched
                    hi
                } else {
                    // fewer pending elements than iter-remaining elements
                    // => at most:
                    //    * each pending element is matched
                    //    * the other iter-remaining elements come in pairs
                    self.meta.pending + (hi - self.meta.pending) / 2
                }
            });
            // The lower bound is always 0 since we might only get unique items from now on
            (0, hi)
        }
    }

    impl<I, Key, F> DoubleEndedIterator for DuplicatesBy<I, Key, F>
    where
        I: DoubleEndedIterator,
        Key: Eq + Hash,
        F: KeyMethod<Key, I::Item>,
    {
        fn next_back(&mut self) -> Option<Self::Item> {
            let DuplicatesBy { iter, meta } = self;
            iter.rev().find_map(|v| meta.filter(v))
        }
    }

    /// A keying method for use with `DuplicatesBy`
    pub trait KeyMethod<K, V> {
        type Container: KeyXorValue<K, V>;

        fn make(&mut self, value: V) -> Self::Container;
    }

    /// Apply the identity function to elements before checking them for equality.
    #[derive(Debug)]
    pub struct ById;
    impl<V> KeyMethod<V, V> for ById {
        type Container = JustValue<V>;

        fn make(&mut self, v: V) -> Self::Container {
            JustValue(v)
        }
    }

    /// Apply a user-supplied function to elements before checking them for equality.
    pub struct ByFn<F>(pub(crate) F);
    impl<F> fmt::Debug for ByFn<F> {
        debug_fmt_fields!(ByFn,);
    }
    impl<K, V, F> KeyMethod<K, V> for ByFn<F>
    where
        F: FnMut(&V) -> K,
    {
        type Container = KeyValue<K, V>;

        fn make(&mut self, v: V) -> Self::Container {
            KeyValue((self.0)(&v), v)
        }
    }

    // Implementors of this trait can hold onto a key and a value but only give access to one of them
    // at a time. This allows the key and the value to be the same value internally
    pub trait KeyXorValue<K, V> {
        fn key_ref(&self) -> &K;
        fn key(self) -> K;
        fn value(self) -> V;
    }

    #[derive(Debug)]
    pub struct KeyValue<K, V>(K, V);
    impl<K, V> KeyXorValue<K, V> for KeyValue<K, V> {
        fn key_ref(&self) -> &K {
            &self.0
        }
        fn key(self) -> K {
            self.0
        }
        fn value(self) -> V {
            self.1
        }
    }

    #[derive(Debug)]
    pub struct JustValue<V>(V);
    impl<V> KeyXorValue<V, V> for JustValue<V> {
        fn key_ref(&self) -> &V {
            &self.0
        }
        fn key(self) -> V {
            self.0
        }
        fn value(self) -> V {
            self.0
        }
    }
}

/// An iterator adapter to filter for duplicate elements.
///
/// See [`.duplicates_by()`](crate::Itertools::duplicates_by) for more information.
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub type DuplicatesBy<I, V, F> = private::DuplicatesBy<I, V, private::ByFn<F>>;

/// Create a new `DuplicatesBy` iterator.
pub fn duplicates_by<I, Key, F>(iter: I, f: F) -> DuplicatesBy<I, Key, F>
where
    Key: Eq + Hash,
    F: FnMut(&I::Item) -> Key,
    I: Iterator,
{
    DuplicatesBy::new(iter, private::ByFn(f))
}

/// An iterator adapter to filter out duplicate elements.
///
/// See [`.duplicates()`](crate::Itertools::duplicates) for more information.
pub type Duplicates<I> = private::DuplicatesBy<I, <I as Iterator>::Item, private::ById>;

/// Create a new `Duplicates` iterator.
pub fn duplicates<I>(iter: I) -> Duplicates<I>
where
    I: Iterator,
    I::Item: Eq + Hash,
{
    Duplicates::new(iter, private::ById)
}