22. 位置と大きさ — block は縦に積み、inline は横に並べ、右端で折り返す

// src/renderer/layout.rs の位置と大きさを決める部分
fn layout_block(b: &mut LayoutBox, x: f32, y: f32, container_width: f32) {
    b.x = x;
    b.y = y;
    // 幅: 指定があればそれ、無ければ親いっぱい(auto)
    b.width = b.style.width.unwrap_or(container_width);
    let width = b.width;
    let mut cur = Cursor {
        x,
        y,
        left: x,
        right: x + width,
        line_height: 0.0,
    };
    layout_flow(&mut b.children, &mut cur, width);
    if cur.line_height > 0.0 {
        cur.newline(); // 最後の行を確定する
    }
    // 高さ: 指定があればそれ、無ければ中身の分(auto)
    b.height = b.style.height.unwrap_or(cur.y - y);
}

fn layout_flow(children: &mut [LayoutBox], cur: &mut Cursor, container_width: f32) {
    for child in children.iter_mut() {
        match child.kind.clone() {
            BoxKind::Block => {
                if cur.line_height > 0.0 {
                    cur.newline();
                }
                layout_block(child, cur.left, cur.y, container_width);
                cur.y = child.y + child.height;
                cur.x = cur.left;
                cur.line_height = 0.0;
            }
            BoxKind::Inline => {
                child.x = cur.x;
                child.y = cur.y;
                let start_y = cur.y;
                layout_flow(&mut child.children, cur, container_width);
                // inline の箱の大きさは「子が占めた範囲」の近似
                child.width = if cur.y == start_y { cur.x - child.x } else { container_width };
                child.height = (cur.y + cur.line_height) - start_y;
            }
            BoxKind::Text(text) => {
                let font = child.style.font_size;
                let char_w = font * CHAR_WIDTH_RATIO;
                let line_h = font * LINE_HEIGHT_RATIO;
                child.x = cur.x;
                child.y = cur.y;
                let chars: Vec<char> = text.chars().collect();
                let mut rest: &[char] = &chars;
                let mut lines = Vec::new();
                while !rest.is_empty() {
                    let mut avail = ((cur.right - cur.x) / char_w).floor() as usize;
                    if avail == 0 {
                        if cur.x > cur.left {
                            cur.newline(); // 行末に来たので折り返す
                            continue;
                        }
                        avail = 1; // 行頭でも 1 文字入らないなら、はみ出してでも 1 文字置く
                    }
                    let take = avail.min(rest.len());
                    let (head, tail) = rest.split_at(take);
                    lines.push(TextLine {
                        text: head.iter().collect(),
                        x: cur.x,
                        y: cur.y,
                    });
                    cur.x += take as f32 * char_w;
                    cur.line_height = cur.line_height.max(line_h);
                    rest = tail;
                    if !rest.is_empty() {
                        cur.newline();
                    }
                }
                child.lines = lines;
                child.width = (text.chars().count() as f32 * char_w).min(container_width);
                child.height = (cur.y + cur.line_height) - child.y;
            }
        }
    }
}