image/codecs/jpeg/
entropy.rs

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1/// Given an array containing the number of codes of each code length,
2/// this function generates the huffman codes lengths and their respective
3/// code lengths as specified by the JPEG spec.
4const fn derive_codes_and_sizes(bits: &[u8; 16]) -> ([u8; 256], [u16; 256]) {
5    let mut huffsize = [0u8; 256];
6    let mut huffcode = [0u16; 256];
7
8    let mut k = 0;
9
10    // Annex C.2
11    // Figure C.1
12    // Generate table of individual code lengths
13    let mut i = 0;
14    while i < 16 {
15        let mut j = 0;
16        while j < bits[i as usize] {
17            huffsize[k] = i + 1;
18            k += 1;
19            j += 1;
20        }
21        i += 1;
22    }
23
24    huffsize[k] = 0;
25
26    // Annex C.2
27    // Figure C.2
28    // Generate table of huffman codes
29    k = 0;
30    let mut code = 0u16;
31    let mut size = huffsize[0];
32
33    while huffsize[k] != 0 {
34        huffcode[k] = code;
35        code += 1;
36        k += 1;
37
38        if huffsize[k] == size {
39            continue;
40        }
41
42        // FIXME there is something wrong with this code
43        let diff = huffsize[k].wrapping_sub(size);
44        code = if diff < 16 { code << diff as usize } else { 0 };
45
46        size = size.wrapping_add(diff);
47    }
48
49    (huffsize, huffcode)
50}
51
52pub(crate) const fn build_huff_lut_const(bits: &[u8; 16], huffval: &[u8]) -> [(u8, u16); 256] {
53    let mut lut = [(17u8, 0u16); 256];
54    let (huffsize, huffcode) = derive_codes_and_sizes(bits);
55
56    let mut i = 0;
57    while i < huffval.len() {
58        lut[huffval[i] as usize] = (huffsize[i], huffcode[i]);
59        i += 1;
60    }
61
62    lut
63}
image/codecs/jpeg/entropy.rs

image/codecs/jpeg/
entropy.rs
