uucore 0.4.0

uutils ~ 'core' uutils code library (cross-platform)
Documentation 
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

// This file is part of the uutils coreutils package.
//
// For the full copyright and license information, please view the LICENSE
// file that was distributed with this source code.

/// Fast increment function, operating on ASCII strings.
///
/// Add inc to the string val[start..end]. This operates on ASCII digits, assuming
/// val and inc are well formed.
///
/// Updates `start` if we have a carry, or if inc > start.
///
/// We also assume that there is enough space in val to expand if start needs
/// to be updated.
/// ```
/// use uucore::fast_inc::fast_inc;
///
/// // Start with a buffer containing "0", with one byte of head space
/// let mut val = Vec::from(".0".as_bytes());
/// let mut start = val.len()-1;
/// let end = val.len();
/// let inc = "6".as_bytes();
/// assert_eq!(&val[start..end], "0".as_bytes());
/// fast_inc(val.as_mut(), &mut start, end, inc);
/// assert_eq!(&val[start..end], "6".as_bytes());
/// fast_inc(val.as_mut(), &mut start, end, inc);
/// assert_eq!(&val[start..end], "12".as_bytes());
/// ```
#[inline]
pub fn fast_inc(val: &mut [u8], start: &mut usize, end: usize, inc: &[u8]) {
    // To avoid a lot of casts to signed integers, we make sure to decrement pos
    // as late as possible, so that it does not ever go negative.
    let mut pos = end;
    let mut carry = 0u8;

    // First loop, add all digits of inc into val.
    for inc_pos in (0..inc.len()).rev() {
        // The decrement operation would also panic in debug mode, print a message for developer convenience.
        debug_assert!(
            pos > 0,
            "Buffer overflowed, make sure you allocate val with enough headroom."
        );
        pos -= 1;

        let mut new_val = inc[inc_pos] + carry;
        // Be careful here, only add existing digit of val.
        if pos >= *start {
            new_val += val[pos] - b'0';
        }
        if new_val > b'9' {
            carry = 1;
            new_val -= 10;
        } else {
            carry = 0;
        }
        val[pos] = new_val;
    }

    // Done, now, if we have a carry, add that to the upper digits of val.
    if carry == 0 {
        *start = (*start).min(pos);
        return;
    }

    fast_inc_one(val, start, pos);
}

/// Fast increment by one function, operating on ASCII strings.
///
/// Add 1 to the string val[start..end]. This operates on ASCII digits, assuming
/// val is well formed.
///
/// Updates `start` if we have a carry, or if inc > start.
///
/// We also assume that there is enough space in val to expand if start needs
/// to be updated.
/// ```
/// use uucore::fast_inc::fast_inc_one;
///
/// // Start with a buffer containing "8", with one byte of head space
/// let mut val = Vec::from(".8".as_bytes());
/// let mut start = val.len()-1;
/// let end = val.len();
/// assert_eq!(&val[start..end], "8".as_bytes());
/// fast_inc_one(val.as_mut(), &mut start, end);
/// assert_eq!(&val[start..end], "9".as_bytes());
/// fast_inc_one(val.as_mut(), &mut start, end);
/// assert_eq!(&val[start..end], "10".as_bytes());
/// ```
#[inline]
pub fn fast_inc_one(val: &mut [u8], start: &mut usize, end: usize) {
    let mut pos = end;

    while pos > *start {
        pos -= 1;

        if val[pos] == b'9' {
            // 9+1 = 10. Carry propagating, keep going.
            val[pos] = b'0';
        } else {
            // Carry stopped propagating, return unchanged start.
            val[pos] += 1;
            return;
        }
    }

    // The following decrement operation would also panic in debug mode, print a message for developer convenience.
    debug_assert!(
        *start > 0,
        "Buffer overflowed, make sure you allocate val with enough headroom."
    );
    // The carry propagated so far that a new digit was added.
    val[*start - 1] = b'1';
    *start -= 1;
}

#[cfg(test)]
mod tests {
    use crate::fast_inc::fast_inc;
    use crate::fast_inc::fast_inc_one;

    #[test]
    fn test_fast_inc_simple() {
        let mut val = Vec::from("...0_".as_bytes());
        let mut start: usize = 3;
        let inc = "4".as_bytes();
        fast_inc(val.as_mut(), &mut start, 4, inc);
        assert_eq!(start, 3);
        assert_eq!(val, "...4_".as_bytes());
        fast_inc(val.as_mut(), &mut start, 4, inc);
        assert_eq!(start, 3);
        assert_eq!(val, "...8_".as_bytes());
        fast_inc(val.as_mut(), &mut start, 4, inc);
        assert_eq!(start, 2); // carried 1 more digit
        assert_eq!(val, "..12_".as_bytes());

        let mut val = Vec::from("0_".as_bytes());
        let mut start: usize = 0;
        let inc = "2".as_bytes();
        fast_inc(val.as_mut(), &mut start, 1, inc);
        assert_eq!(start, 0);
        assert_eq!(val, "2_".as_bytes());
        fast_inc(val.as_mut(), &mut start, 1, inc);
        assert_eq!(start, 0);
        assert_eq!(val, "4_".as_bytes());
        fast_inc(val.as_mut(), &mut start, 1, inc);
        assert_eq!(start, 0);
        assert_eq!(val, "6_".as_bytes());
    }

    // Check that we handle increment > val correctly.
    #[test]
    fn test_fast_inc_large_inc() {
        let mut val = Vec::from("...7_".as_bytes());
        let mut start: usize = 3;
        let inc = "543".as_bytes();
        fast_inc(val.as_mut(), &mut start, 4, inc);
        assert_eq!(start, 1); // carried 2 more digits
        assert_eq!(val, ".550_".as_bytes());
        fast_inc(val.as_mut(), &mut start, 4, inc);
        assert_eq!(start, 0); // carried 1 more digit
        assert_eq!(val, "1093_".as_bytes());
    }

    // Check that we handle longer carries
    #[test]
    fn test_fast_inc_carry() {
        let mut val = Vec::from(".999_".as_bytes());
        let mut start: usize = 1;
        let inc = "1".as_bytes();
        fast_inc(val.as_mut(), &mut start, 4, inc);
        assert_eq!(start, 0);
        assert_eq!(val, "1000_".as_bytes());

        let mut val = Vec::from(".999_".as_bytes());
        let mut start: usize = 1;
        let inc = "11".as_bytes();
        fast_inc(val.as_mut(), &mut start, 4, inc);
        assert_eq!(start, 0);
        assert_eq!(val, "1010_".as_bytes());
    }

    #[test]
    fn test_fast_inc_one_simple() {
        let mut val = Vec::from("...8_".as_bytes());
        let mut start: usize = 3;
        fast_inc_one(val.as_mut(), &mut start, 4);
        assert_eq!(start, 3);
        assert_eq!(val, "...9_".as_bytes());
        fast_inc_one(val.as_mut(), &mut start, 4);
        assert_eq!(start, 2); // carried 1 more digit
        assert_eq!(val, "..10_".as_bytes());
        fast_inc_one(val.as_mut(), &mut start, 4);
        assert_eq!(start, 2);
        assert_eq!(val, "..11_".as_bytes());

        let mut val = Vec::from("0_".as_bytes());
        let mut start: usize = 0;
        fast_inc_one(val.as_mut(), &mut start, 1);
        assert_eq!(start, 0);
        assert_eq!(val, "1_".as_bytes());
        fast_inc_one(val.as_mut(), &mut start, 1);
        assert_eq!(start, 0);
        assert_eq!(val, "2_".as_bytes());
        fast_inc_one(val.as_mut(), &mut start, 1);
        assert_eq!(start, 0);
        assert_eq!(val, "3_".as_bytes());
    }
}