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

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

//! Defines kernel to extract a substring of a \[Large\]StringArray

use crate::{
    array::*,
    datatypes::DataType,
    error::{ArrowError, Result},
};

fn utf8_substring<O: Offset>(array: &Utf8Array<O>, start: O, length: &Option<O>) -> Utf8Array<O> {
    let length = length.map(|v| v.to_usize());

    let iter = array.values_iter().map(|str_val| {
        // compute where we should start slicing this entry.
        let start = if start >= O::zero() {
            start.to_usize()
        } else {
            let start = (O::zero() - start).to_usize();
            str_val
                .char_indices()
                .rev()
                .nth(start)
                .map(|(idx, _)| idx + 1)
                .unwrap_or(0)
        };

        let mut iter_chars = str_val.char_indices();
        if let Some((start_idx, _)) = iter_chars.nth(start) {
            // length of the str
            let len_end = str_val.len() - start_idx;

            // length to slice
            let length = length.unwrap_or(len_end);

            if length == 0 {
                return "";
            }
            // compute
            let end_idx = iter_chars
                .nth(length.saturating_sub(1))
                .map(|(idx, _)| idx)
                .unwrap_or(str_val.len());

            &str_val[start_idx..end_idx]
        } else {
            ""
        }
    });

    let new = Utf8Array::<O>::from_trusted_len_values_iter(iter);
    new.with_validity(array.validity().cloned())
}

fn binary_substring<O: Offset>(
    array: &BinaryArray<O>,
    start: O,
    length: &Option<O>,
) -> BinaryArray<O> {
    let validity = array.validity();
    let offsets = array.offsets();
    let values = array.values();

    let mut new_offsets = Vec::<O>::with_capacity(array.len() + 1);
    let mut new_values = Vec::<u8>::new(); // we have no way to estimate how much this will be.

    let mut length_so_far = O::zero();
    new_offsets.push(length_so_far);

    offsets.windows(2).for_each(|windows| {
        let length_i: O = windows[1] - windows[0];

        // compute where we should start slicing this entry
        let start = windows[0]
            + if start >= O::zero() {
                start
            } else {
                length_i + start
            };
        let start = start.max(windows[0]).min(windows[1]);

        let length: O = length
            .unwrap_or(length_i)
            // .max(0) is not needed as it is guaranteed
            .min(windows[1] - start); // so we do not go beyond this entry
        length_so_far += length;
        new_offsets.push(length_so_far);

        // we need usize for ranges
        let start = start.to_usize();
        let length = length.to_usize();

        new_values.extend_from_slice(&values[start..start + length]);
    });

    BinaryArray::<O>::new(
        array.data_type().clone(),
        new_offsets.into(),
        new_values.into(),
        validity.cloned(),
    )
}

/// Returns an ArrayRef with a substring starting from `start` and with optional length `length` of each of the elements in `array`.
/// `start` can be negative, in which case the start counts from the end of the string.
/// this function errors when the passed array is not a \[Large\]String array.
pub fn substring(array: &dyn Array, start: i64, length: &Option<u64>) -> Result<Box<dyn Array>> {
    match array.data_type() {
        DataType::Binary => Ok(Box::new(binary_substring(
            array
                .as_any()
                .downcast_ref::<BinaryArray<i32>>()
                .expect("A binary is expected"),
            start as i32,
            &length.map(|e| e as i32),
        ))),
        DataType::LargeBinary => Ok(Box::new(binary_substring(
            array
                .as_any()
                .downcast_ref::<BinaryArray<i64>>()
                .expect("A large binary is expected"),
            start,
            &length.map(|e| e as i64),
        ))),
        DataType::LargeUtf8 => Ok(Box::new(utf8_substring(
            array
                .as_any()
                .downcast_ref::<Utf8Array<i64>>()
                .expect("A large string is expected"),
            start,
            &length.map(|e| e as i64),
        ))),
        DataType::Utf8 => Ok(Box::new(utf8_substring(
            array
                .as_any()
                .downcast_ref::<Utf8Array<i32>>()
                .expect("A string is expected"),
            start as i32,
            &length.map(|e| e as i32),
        ))),
        _ => Err(ArrowError::InvalidArgumentError(format!(
            "substring does not support type {:?}",
            array.data_type()
        ))),
    }
}

/// Checks if an array of type `datatype` can perform substring operation
///
/// # Examples
/// ```
/// use arrow2::compute::substring::can_substring;
/// use arrow2::datatypes::{DataType};
///
/// let data_type = DataType::Utf8;
/// assert_eq!(can_substring(&data_type), true);

/// let data_type = DataType::Null;
/// assert_eq!(can_substring(&data_type), false);
/// ```
pub fn can_substring(data_type: &DataType) -> bool {
    matches!(
        data_type,
        DataType::LargeUtf8 | DataType::Utf8 | DataType::LargeBinary | DataType::Binary
    )
}