DarkRoom/core/dr-export/src/inscribed.rs

//! TRACES: FR-MRG-4
//! The largest rectangle inside a coverage mask, found a row at a time.
//!
//! A merged panorama has ragged edges: the frames' footprints under a
//! cylinder or a sphere are not rectangles, and the composite carries a
//! black border where none of them reached. FR-MRG-4 asks for an auto-crop
//! to the largest inscribed rectangle. This finds it as the bands are
//! produced, so the composite is never held to be measured (FR-MRG-11):
//! each row extends a running histogram of consecutive covered rows above
//! it, and the largest rectangle ending on that row is the largest
//! rectangle under the histogram — a stack pass, linear in the width.
//!
//! The crop is written as the DNG's `DefaultCropOrigin`/`DefaultCropSize`,
//! which every reader honours and which discards nothing: the pixels
//! outside it are still in the file for a photographer who wants them.

/// The rectangle so far, in pixels from the top left.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub struct Rect {
    pub x: u32,
    pub y: u32,
    pub width: u32,
    pub height: u32,
}

impl Rect {
    pub fn area(&self) -> u64 {
        u64::from(self.width) * u64::from(self.height)
    }
}

/// Feed rows top to bottom; ask for the best at any point.
#[derive(Debug, Clone)]
pub struct Inscribed {
    width: usize,
    /// How many consecutive covered rows end at the last row fed, per column.
    heights: Vec<u32>,
    rows: u32,
    best: Rect,
}

impl Inscribed {
    pub fn new(width: u32) -> Self {
        Inscribed {
            width: width as usize,
            heights: vec![0; width as usize],
            rows: 0,
            best: Rect::default(),
        }
    }

    /// One more row of coverage, `width` long.
    pub fn push_row(&mut self, covered: &[bool]) {
        debug_assert_eq!(covered.len(), self.width);
        for (h, &c) in self.heights.iter_mut().zip(covered) {
            *h = if c { *h + 1 } else { 0 };
        }
        self.rows += 1;
        // Largest rectangle under the histogram, with a sentinel column of
        // height 0 at the end so every bar is popped.
        let mut stack: Vec<usize> = Vec::new();
        for i in 0..=self.width {
            let h = if i < self.width { self.heights[i] } else { 0 };
            while let Some(&top) = stack.last() {
                if self.heights[top] <= h {
                    break;
                }
                stack.pop();
                let height = self.heights[top];
                let left = stack.last().map_or(0, |&l| l + 1);
                let width = (i - left) as u32;
                let area = u64::from(width) * u64::from(height);
                if area > self.best.area() {
                    self.best = Rect {
                        x: left as u32,
                        y: self.rows - height,
                        width,
                        height,
                    };
                }
            }
            stack.push(i);
        }
    }

    /// Several rows at once, as a band hands them over.
    pub fn push_rows(&mut self, covered: &[bool], rows: u32) {
        for r in 0..rows as usize {
            self.push_row(&covered[r * self.width..(r + 1) * self.width]);
        }
    }

    pub fn best(&self) -> Rect {
        self.best
    }
}

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

    fn from_art(art: &[&str]) -> Rect {
        let mut ins = Inscribed::new(art[0].len() as u32);
        for row in art {
            let covered: Vec<bool> = row.chars().map(|c| c == '#').collect();
            ins.push_row(&covered);
        }
        ins.best()
    }

    #[test]
    fn a_full_mask_is_its_own_rectangle() {
        let r = from_art(&["####", "####", "####"]);
        assert_eq!(
            r,
            Rect {
                x: 0,
                y: 0,
                width: 4,
                height: 3
            }
        );
    }

    #[test]
    fn ragged_edges_are_cut_off() {
        // A cylinder's footprint: narrower at top and bottom.
        let r = from_art(&[
            "..####..", ".######.", "########", "########", ".######.", "..####..",
        ]);
        // 6 wide × 4 tall = 24 beats 8 × 2 = 16 and 4 × 6 = 24 ties; the
        // first found wins a tie, which is the wider one here.
        assert_eq!(r.area(), 24);
        assert!(r.width == 6 && r.height == 4 || r.width == 4 && r.height == 6);
    }

    #[test]
    fn a_hole_is_avoided() {
        let r = from_art(&["#####", "##.##", "#####", "#####"]);
        // Left of the hole: 2 × 4 = 8; right: 2 × 4 = 8; below: 5 × 2 = 10.
        assert_eq!(
            r,
            Rect {
                x: 0,
                y: 2,
                width: 5,
                height: 2
            }
        );
    }

    #[test]
    fn nothing_covered_is_nothing() {
        assert_eq!(from_art(&["....", "...."]).area(), 0);
    }
}