agent-image-diff 0.2.4

/// Connected component labeling using two-pass algorithm with union-find.
///
/// Labels each true pixel in the mask with a component ID (1-indexed).
/// Background (false) pixels get label 0.

struct UnionFind {
    parent: Vec<u32>,
    rank: Vec<u8>,
}

impl UnionFind {
    fn new(size: usize) -> Self {
        Self {
            parent: (0..size as u32).collect(),
            rank: vec![0; size],
        }
    }

    fn find(&mut self, mut x: u32) -> u32 {
        while self.parent[x as usize] != x {
            self.parent[x as usize] = self.parent[self.parent[x as usize] as usize];
            x = self.parent[x as usize];
        }
        x
    }

    fn union(&mut self, a: u32, b: u32) {
        let ra = self.find(a);
        let rb = self.find(b);
        if ra == rb {
            return;
        }
        match self.rank[ra as usize].cmp(&self.rank[rb as usize]) {
            std::cmp::Ordering::Less => self.parent[ra as usize] = rb,
            std::cmp::Ordering::Greater => self.parent[rb as usize] = ra,
            std::cmp::Ordering::Equal => {
                self.parent[rb as usize] = ra;
                self.rank[ra as usize] += 1;
            }
        }
    }
}

/// Label connected components in a boolean diff mask.
///
/// Returns a Vec<u32> of the same length as mask, where 0 = background
/// and 1+ = component label.
pub fn label_components(mask: &[bool], width: u32, height: u32, connectivity: u8) -> Vec<u32> {
    let len = (width * height) as usize;
    assert_eq!(mask.len(), len);

    let mut labels = vec![0u32; len];
    let mut uf = UnionFind::new(len + 1); // +1 since labels are 1-indexed
    let mut next_label = 0u32;

    // Pass 1: assign provisional labels
    for y in 0..height {
        for x in 0..width {
            let idx = (y * width + x) as usize;
            if !mask[idx] {
                continue;
            }

            let mut neighbor_labels: Vec<u32> = Vec::with_capacity(4);

            // Check already-visited neighbors based on connectivity
            // For 4-connectivity: up, left
            // For 8-connectivity: up-left, up, up-right, left
            if y > 0 {
                // Up
                let up = ((y - 1) * width + x) as usize;
                if labels[up] > 0 {
                    neighbor_labels.push(labels[up]);
                }

                if connectivity == 8 {
                    // Up-left
                    if x > 0 {
                        let ul = ((y - 1) * width + (x - 1)) as usize;
                        if labels[ul] > 0 {
                            neighbor_labels.push(labels[ul]);
                        }
                    }
                    // Up-right
                    if x + 1 < width {
                        let ur = ((y - 1) * width + (x + 1)) as usize;
                        if labels[ur] > 0 {
                            neighbor_labels.push(labels[ur]);
                        }
                    }
                }
            }
            // Left
            if x > 0 {
                let left = (y * width + (x - 1)) as usize;
                if labels[left] > 0 {
                    neighbor_labels.push(labels[left]);
                }
            }

            if neighbor_labels.is_empty() {
                next_label += 1;
                labels[idx] = next_label;
            } else {
                let min_label = *neighbor_labels.iter().min().unwrap();
                labels[idx] = min_label;
                for &nl in &neighbor_labels {
                    if nl != min_label {
                        uf.union(min_label, nl);
                    }
                }
            }
        }
    }

    // Pass 2: replace each label with its root, renumber contiguously
    let mut remap = std::collections::HashMap::new();
    let mut next_id = 0u32;

    for label in labels.iter_mut() {
        if *label == 0 {
            continue;
        }
        let root = uf.find(*label);
        let id = *remap.entry(root).or_insert_with(|| {
            next_id += 1;
            next_id
        });
        *label = id;
    }

    labels
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn single_pixel() {
        let mask = vec![
            false, false, false,
            false, true, false,
            false, false, false,
        ];
        let labels = label_components(&mask, 3, 3, 8);
        assert_eq!(labels[4], 1);
        assert_eq!(labels.iter().filter(|&&l| l > 0).count(), 1);
    }

    #[test]
    fn two_disconnected_regions() {
        let mask = vec![
            true, true, false, false, false,
            true, true, false, false, false,
            false, false, false, false, false,
            false, false, false, true, true,
            false, false, false, true, true,
        ];
        let labels = label_components(&mask, 5, 5, 8);
        let unique: std::collections::HashSet<u32> = labels.iter().filter(|&&l| l > 0).copied().collect();
        assert_eq!(unique.len(), 2);
    }

    #[test]
    fn diagonal_connected_with_8_connectivity() {
        let mask = vec![
            true, false, false,
            false, true, false,
            false, false, true,
        ];
        let labels = label_components(&mask, 3, 3, 8);
        // All three should be the same component with 8-connectivity
        let unique: std::collections::HashSet<u32> = labels.iter().filter(|&&l| l > 0).copied().collect();
        assert_eq!(unique.len(), 1);
    }

    #[test]
    fn diagonal_disconnected_with_4_connectivity() {
        let mask = vec![
            true, false, false,
            false, true, false,
            false, false, true,
        ];
        let labels = label_components(&mask, 3, 3, 4);
        // Each pixel should be its own component with 4-connectivity
        let unique: std::collections::HashSet<u32> = labels.iter().filter(|&&l| l > 0).copied().collect();
        assert_eq!(unique.len(), 3);
    }

    #[test]
    fn l_shape_is_single_region() {
        let mask = vec![
            true, false,
            true, false,
            true, true,
        ];
        let labels = label_components(&mask, 2, 3, 4);
        let unique: std::collections::HashSet<u32> = labels.iter().filter(|&&l| l > 0).copied().collect();
        assert_eq!(unique.len(), 1);
    }
}