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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;
pub fn get_match_length(data: &[u8], current_pos: usize, pos_to_check: usize) -> usize {
data[current_pos..]
.iter()
.zip(data[pos_to_check..].iter())
.take(MAX_MATCH)
.take_while(|&(&a, &b)| a == b)
.count()
}
pub fn longest_match(
data: &[u8],
hash_table: &ChainedHashTable,
position: usize,
prev_length: usize,
max_hash_checks: u16,
) -> (usize, usize) {
if prev_length >= MAX_MATCH || position + prev_length >= data.len() {
return (0, 0);
}
let limit = if position > WINDOW_SIZE {
position - WINDOW_SIZE
} else {
0
};
let prev_length = cmp::max(prev_length, MIN_MATCH - 1);
let max_length = cmp::min((data.len() - position), MAX_MATCH);
let mut current_head = position;
let mut best_length = prev_length;
let mut best_distance = 0;
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 {
break;
}
if data[position + best_length - 1..position + best_length + 1] ==
data[current_head + best_length - 1..current_head + best_length + 1]
{
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 {
break;
}
}
}
}
if best_length > prev_length {
(best_length, best_distance)
} else {
(0, 0)
}
}
#[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]
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]
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);
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);
}
#[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);
}
}