jixel 0.2.3

Tiny JPEG XL encoder
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

/*
 * // Copyright (c) Radzivon Bartoshyk 5/2026. All rights reserved.
 * //
 * // Redistribution and use in source and binary forms, with or without modification,
 * // are permitted provided that the following conditions are met:
 * //
 * // 1.  Redistributions of source code must retain the above copyright notice, this
 * // list of conditions and the following disclaimer.
 * //
 * // 2.  Redistributions in binary form must reproduce the above copyright notice,
 * // this list of conditions and the following disclaimer in the documentation
 * // and/or other materials provided with the distribution.
 * //
 * // 3.  Neither the name of the copyright holder nor the names of its
 * // contributors may be used to endorse or promote products derived from
 * // this software without specific prior written permission.
 * //
 * // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * // AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * // DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
 * // FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * // DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * // SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * // CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * // OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

struct HuffmanNode {
    total_count: u32,
    index_left: i16,
    index_right_or_value: i16, // for leaves: the symbol; for internal: right child index
}

const SENTINEL: HuffmanNode = HuffmanNode {
    total_count: u32::MAX,
    index_left: -1,
    index_right_or_value: -1,
};

fn set_depth(idx: usize, pool: &[HuffmanNode], depth: &mut [u8], level: u8) {
    let node = &pool[idx];
    if node.index_left >= 0 {
        let new_level = level + 1;
        set_depth(node.index_left as usize, pool, depth, new_level);
        set_depth(node.index_right_or_value as usize, pool, depth, new_level);
    } else {
        depth[node.index_right_or_value as usize] = level;
    }
}

pub(crate) fn create_huffman_tree(data: &[u32], tree_limit: u8, depth: &mut [u8]) {
    let length = data.len();
    debug_assert!(depth.len() >= length);
    for d in depth[..length].iter_mut() {
        *d = 0;
    }

    let mut count_limit: u32 = 1;
    loop {
        let mut tree: Vec<HuffmanNode> = Vec::with_capacity(2 * length + 1);

        // Gather nonzero-count leaves in reverse order; stable sort keeps
        // tie-breaking deterministic and matching libjxl.
        for (i, &data) in data[..length].iter().enumerate().rev() {
            if data != 0 {
                let count = data.max(count_limit.saturating_sub(1));
                tree.push(HuffmanNode {
                    total_count: count,
                    index_left: -1,
                    index_right_or_value: i as i16,
                });
            }
        }

        let n = tree.len();
        if n == 1 {
            // Single symbol: placeholder depth. The simple-Huffman-tree fast
            // path in the writer handles the actual emission.
            depth[tree[0].index_right_or_value as usize] = 1;
            return;
        }

        // Stable sort by ascending count.
        tree.sort_by_key(|a| a.total_count);

        tree.push(SENTINEL);
        tree.push(SENTINEL);

        let mut i = 0usize; // next leaf
        let mut j = n + 1; // next internal node

        // Build n - 1 internal nodes.
        for _ in 0..(n - 1) {
            let left;
            if tree[i].total_count <= tree[j].total_count {
                left = i;
                i += 1;
            } else {
                left = j;
                j += 1;
            }
            let right;
            if tree[i].total_count <= tree[j].total_count {
                right = i;
                i += 1;
            } else {
                right = j;
                j += 1;
            }

            // Overwrite the trailing sentinel as the new parent.
            let parent = tree.len() - 1;
            tree[parent].total_count = tree[left].total_count + tree[right].total_count;
            tree[parent].index_left = left as i16;
            tree[parent].index_right_or_value = right as i16;

            // Replace the trailing sentinel for next iteration.
            tree.push(SENTINEL);
        }
        debug_assert_eq!(tree.len(), 2 * n + 1);

        // Root is at 2n - 1.
        set_depth(2 * n - 1, &tree, depth, 0);

        let max_depth = depth[..length].iter().copied().max().unwrap_or(0);
        if max_depth <= tree_limit {
            return;
        }

        count_limit = count_limit
            .checked_mul(2)
            .expect("huffman depth limit unreachable");
    }
}