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
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
use std::cmp;

use chained_hash_table::{ChainedHashTable, WINDOW_SIZE};
use huffman_table;

const MAX_MATCH: usize = huffman_table::MAX_MATCH as usize;
const MIN_MATCH: usize = huffman_table::MIN_MATCH as usize;

/// Get the length of the checked match
/// The function returns number of bytes at and including `current_pos` that are the same as the
/// ones at `pos_to_check`
pub fn get_match_length(data: &[u8], current_pos: usize, pos_to_check: usize) -> usize {
    // Unsafe version for comparison
    // This doesn't actually make it much faster

    // use std::mem::transmute_copy;

    // let mut counter = 0;
    // let max = cmp::min(data.len() - current_pos, MAX_MATCH);

    // unsafe {
    //     let mut cur = data.as_ptr().offset(current_pos as isize);
    //     let mut tc = data.as_ptr().offset(pos_to_check as isize);
    //     while (counter < max) &&
    //           (transmute_copy::<u8, u32>(&*cur) == transmute_copy::<u8, u32>(&*tc)) {
    //         counter += 4;
    //         cur = cur.offset(4);
    //         tc = tc.offset(4);
    //     }
    //     if counter > 3 {
    //         cur = cur.offset(-4);
    //         tc = tc.offset(-4);
    //         counter -= 4;
    //     }
    //     while counter < max && *cur == *tc {
    //         counter += 1;
    //         cur = cur.offset(1);
    //         tc = tc.offset(1);
    //     }
    // }

    //    counter
    data[current_pos..]
        .iter()
        .zip(data[pos_to_check..].iter())
        .take(MAX_MATCH)
        .take_while(|&(&a, &b)| a == b)
        .count()
}

/// Try finding the position and length of the longest match in the input data.
/// # Returns
/// (length, distance from position)
/// If no match is found that was better than `prev_length` or at all, or we are at the start,
/// the length value returned will be 2.
///
/// # Arguments:
/// `data`: The data to search in.
/// `hash_table`: Hash table to use for searching.
/// `position`: The position in the data to match against.
/// `prev_length`: The length of the previous `longest_match` check to compare against.
/// `max_hash_checks`: The maximum number of matching hash chain positions to check.
pub fn longest_match(
    data: &[u8],
    hash_table: &ChainedHashTable,
    position: usize,
    prev_length: usize,
    max_hash_checks: u16,
) -> (usize, usize) {

    // debug_assert_eq!(position, hash_table.current_head() as usize);

    // If we already have a match at the maximum length,
    // or we can't grow further, we stop here.
    if prev_length >= MAX_MATCH || position + prev_length >= data.len() {
        return (0, 0);
    }

    let limit = if position > WINDOW_SIZE {
        position - WINDOW_SIZE
    } else {
        0
    };

    // Make sure the length is at least one to simplify the matching code, as
    // otherwise the matching code might underflow.
    let prev_length = cmp::max(prev_length, MIN_MATCH - 1);

    let max_length = cmp::min((data.len() - position), MAX_MATCH);

    // The position in the hash chain we are currently checking.
    let mut current_head = position;

    // The best match length we've found so far, and it's distance.
    let mut best_length = prev_length;
    let mut best_distance = 0;

    // The position of the previous value in the hash chain.
    let mut prev_head;

    for _ in 0..max_hash_checks {
        prev_head = current_head;
        current_head = hash_table.get_prev(current_head) as usize;
        if current_head >= prev_head || current_head < limit {
            // If the current hash chain value refers to itself, or is referring to
            // a value that's higher (we only move backwars through the chain),
            // we are at the end and can stop.
            break;
        }

        // We only check further if the match length can actually increase
        // Checking if the end byte and the potential next byte matches is generally
        // more likely to give a quick answer rather than checking from the start first, given
        // that the hashes match.
        // If there is no previous match, best_length will be 1 and the two first bytes will
        // be checked instead.
        // Since we've made sure best_length is always at least 1, this shouldn't underflow.
        if data[position + best_length - 1..position + best_length + 1] ==
            data[current_head + best_length - 1..current_head + best_length + 1]
        {
            // Actually check how many bytes match.
            // At the moment this will check the two bytes we just checked again,
            // though adding code for skipping these bytes may not result in any speed
            // gain due to the added complexity.
            let length = get_match_length(data, position, current_head);
            if length > best_length {
                best_length = length;
                best_distance = position - current_head;
                if length == max_length {
                    // We are at the max length, so there is no point
                    // searching any longer
                    break;
                }
            }
        }
    }

    /*let r = if best_length > prev_length {
        best_length
    } else {
        0
    };*/
    if best_length > prev_length {
        (best_length, best_distance)
    } else {
        (0, 0)
    }
}

// Get the longest match from the current position of the hash table.
#[inline]
#[cfg(test)]
pub fn longest_match_current(data: &[u8], hash_table: &ChainedHashTable) -> (usize, usize) {
    use compression_options::MAX_HASH_CHECKS;
    longest_match(
        data,
        hash_table,
        hash_table.current_head() as usize,
        MIN_MATCH as usize - 1,
        MAX_HASH_CHECKS,
    )
}

#[cfg(test)]
mod test {
    use chained_hash_table::{filled_hash_table, HASH_BYTES, ChainedHashTable};
    use super::{get_match_length, longest_match};

    /// Test that match lengths are calculated correctly
    #[test]
    fn match_length() {
        let test_arr = [5u8, 5, 5, 5, 5, 9, 9, 2, 3, 5, 5, 5, 5, 5];
        let l = get_match_length(&test_arr, 9, 0);
        assert_eq!(l, 5);
        let l2 = get_match_length(&test_arr, 9, 7);
        assert_eq!(l2, 0);
        let l3 = get_match_length(&test_arr, 10, 0);
        assert_eq!(l3, 4);
    }

    /// Test that we get the longest of the matches
    #[test]
    fn get_longest_match() {
        let test_data = b"xTest data, Test_data,zTest data";
        let hash_table = filled_hash_table(&test_data[..23 + 1 + HASH_BYTES - 1]);

        let (length, distance) = super::longest_match_current(test_data, &hash_table);

        // We check that we get the longest match, rather than the shorter, but closer one.
        assert_eq!(distance, 22);
        assert_eq!(length, 9);
        let test_arr2 = [
            10u8,
            10,
            10,
            10,
            10,
            10,
            10,
            10,
            2,
            3,
            5,
            10,
            10,
            10,
            10,
            10,
        ];
        let hash_table = filled_hash_table(&test_arr2[..HASH_BYTES + 1 + 1 + 2]);
        let (length, distance) = super::longest_match_current(&test_arr2, &hash_table);

        assert_eq!(distance, 1);
        assert_eq!(length, 4);
    }

    /// Make sure we can get a match at index zero
    #[test]
    fn match_index_zero() {
        let test_data = b"AAAAAAA";

        let mut hash_table = ChainedHashTable::from_starting_values(test_data[0], test_data[1]);
        for (n, &b) in test_data[2..5].iter().enumerate() {
            hash_table.add_hash_value(n, b);
        }

        let (match_length, match_dist) = longest_match(test_data, &hash_table, 2, 0, 4096);

        assert_eq!(match_dist, 1);
        assert!(match_length > 2);
    }
}