wip porting to rust

Cargo.lock

@@ -222,9 +222,11 @@ checksum = "42f5e15c9953c5e4ccceeb2e7382a716482c34515315f7b03532b8b4e8393d2d"
 name = "parse_powerisa_pdf"
 version = "0.1.0"
 dependencies = [
+ "indexmap",
  "libm",
  "mupdf",
  "quick-xml",
+ "serde",
 ]
 
 [[package]]
@@ -258,6 +260,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b66c2058c55a409d601666cffe35f04333cf1013010882cec174a7467cd4e21c"
 dependencies = [
  "memchr",
+ "serde",
 ]
 
 [[package]]
@@ -310,6 +313,16 @@ version = "1.0.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b39cdef0fa800fc44525c84ccb54a029961a8215f9619753635a9c0d2538d46d"
 
+[[package]]
+name = "serde"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9a8e94ea7f378bd32cbbd37198a4a91436180c5bb472411e48b5ec2e2124ae9e"
+dependencies = [
+ "serde_core",
+ "serde_derive",
+]
+
 [[package]]
 name = "serde_core"
 version = "1.0.228"

Cargo.toml

@@ -11,6 +11,8 @@ categories = []
 rust-version = "1.89.0"
 
 [dependencies]
+indexmap = "2.12.1"
 libm = "0.2.15"
 mupdf = { version = "0.5.0", default-features = false }
-quick-xml = "0.38.4"
+quick-xml = { version = "0.38.4", features = ["serialize"] }
+serde = { version = "1.0.228", features = ["derive"] }

src/main.rs

@@ -2,11 +2,13 @@
 // See Notices.txt for copyright information
 
 use crate::quad_tree::QuadTree;
+use indexmap::IndexSet;
 use non_nan_float::NonNaNF32;
 use std::{
-    borrow::Borrow,
+    borrow::{Borrow, Cow},
     cell::RefCell,
     collections::{BTreeMap, BTreeSet, HashMap, HashSet},
+    error::Error,
     fmt,
     num::NonZero,
     rc::Rc,
@@ -560,7 +562,7 @@ impl Char {
     fn height(&self) -> f32 {
         self.max_y.get() - self.min_y.get()
     }
-    fn top_down_left_to_right_sort_key(&self) -> impl Ord {
+    fn top_down_left_to_right_sort_key(&self) -> impl Ord + use<> {
         (-self.min_y, self.min_x)
     }
 }
@@ -1463,8 +1465,87 @@ enum LineOrRect {
 struct Page {
     page_num: u32,
     qt: BTreeMap<TextSection, QuadTree<PageItem>>,
-    unprocessed_chars: BTreeMap<TextSection, BTreeMap<Font, BTreeSet<Char>>>,
-    unprocessed_non_text: BTreeSet<LineOrRect>,
+    unprocessed_chars:
+        Rc<RefCell<BTreeMap<TextSection, Rc<RefCell<BTreeMap<Font, IndexSet<Char>>>>>>>,
+    unprocessed_non_text: Rc<RefCell<IndexSet<LineOrRect>>>,
+}
+
+struct Pages {
+    pages_gen: Option<Box<dyn Iterator<Item = Result<Page, Box<dyn Error>>>>>,
+    pages: BTreeMap<u32, Rc<Page>>,
+    max_page_num: u32,
+}
+
+impl fmt::Debug for Pages {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self {
+            pages_gen,
+            pages,
+            max_page_num,
+        } = self;
+        f.debug_struct("Pages")
+            .field(
+                "pages_gen",
+                &pages_gen.is_some().then_some(format_args!("...")),
+            )
+            .field("pages", pages)
+            .field("max_page_num", max_page_num)
+            .finish()
+    }
+}
+
+impl Pages {
+    fn new(pages_gen: Option<Box<dyn Iterator<Item = Result<Page, Box<dyn Error>>>>>) -> Self {
+        Self {
+            pages_gen,
+            pages: BTreeMap::new(),
+            max_page_num: 0,
+        }
+    }
+    fn close(&mut self) {
+        self.pages_gen = None;
+    }
+    fn is_past_end(&mut self, page_num: u32) -> Result<bool, Box<dyn Error>> {
+        while self.pages_gen.is_some() && page_num > self.max_page_num {
+            self.fill_page()?;
+        }
+        Ok(page_num > self.max_page_num)
+    }
+    fn fill_page(&mut self) -> Result<bool, Box<dyn Error>> {
+        let Some(pages_gen) = &mut self.pages_gen else {
+            return Ok(false);
+        };
+        let page = pages_gen.next();
+        let Some(page) = page else {
+            self.close();
+            return Ok(false);
+        };
+        let page = page?;
+        let page_num = page.page_num;
+        assert!(
+            page_num > self.max_page_num,
+            "page numbers must be a strictly-increasing positive integer sequence:\n\
+            got {page_num} which isn't more than {}",
+            self.max_page_num
+        );
+        self.pages.insert(page_num, Rc::new(page));
+        self.max_page_num = page_num;
+        Ok(true)
+    }
+    fn get(&mut self, page_num: u32) -> Result<Option<Rc<Page>>, Box<dyn Error>> {
+        loop {
+            if let Some(page) = self.pages.get(&page_num) {
+                return Ok(Some(page.clone()));
+            }
+            if self.pages_gen.is_none() {
+                return Ok(None);
+            }
+            if page_num < self.max_page_num {
+                return Ok(None);
+            }
+            self.fill_page()?;
+        }
+    }
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -1720,53 +1801,179 @@ impl TextSection {
             .expect("page_num out of range")
             .get_or_init(|| Self::page_sections_helper(page_num))
     }
+    fn for_position(page_num: u32, x: f32, y: f32) -> Option<Self> {
+        for &i in Self::page_sections(page_num) {
+            if i.min_x.get() <= x && x <= i.max_x.get() && i.min_y.get() <= y && y <= i.max_y.get()
+            {
+                return Some(i);
+            }
+        }
+        None
+    }
+}
+
+#[derive(Debug, Clone)]
+struct InsnHeader {
+    header_lines: Vec<ParsedTextLine>,
+    mnemonic_lines: Vec<ParsedTextLine>,
+    bit_fields: InsnBitFields,
+}
+
+impl InsnHeader {
+    fn min_y(&self) -> f32 {
+        self.bit_fields.box_min_y
+    }
+    fn write_xml(&self, parent: &mut xml_tree::Element) {
+        let header = parent.sub_element("header".into(), []);
+        header.text = "\n".into();
+        header.tail = "\n".into();
+        let title = header.sub_element("title".into(), []);
+        title.tail = "\n".into();
+        ParsedTextLine::write_xml_lines(&self.header_lines, title, false, false);
+        let mnemonics = header.sub_element("mnemonics".into(), []);
+        mnemonics.tail = "\n".into();
+        ParsedTextLine::write_xml_lines(&self.mnemonic_lines, mnemonics, false, false);
+        self.bit_fields.write_xml(header);
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Insn {
+    headers: Vec<InsnHeader>,
+    code_lines: Vec<ParsedTextLine>,
+    desc_lines: Vec<ParsedTextLine>,
+    sp_regs_altered: Option<InsnSpRegsAltered>,
+}
+
+impl Insn {
+    fn write_xml(&self, parent: &mut xml_tree::Element) {
+        let insn = parent.sub_element("instruction".into(), []);
+        insn.text = "\n".into();
+        insn.tail = "\n".into();
+        for header in &self.headers {
+            header.write_xml(insn);
+        }
+        if !self.code_lines.is_empty() {
+            let code = insn.sub_element("code".into(), []);
+            code.tail = "\n".into();
+            ParsedTextLine::write_xml_lines(&self.code_lines, code, false, false);
+        }
+        if !self.desc_lines.is_empty() {
+            let desc = insn.sub_element("description".into(), []);
+            desc.tail = "\n".into();
+            ParsedTextLine::write_xml_lines(&self.desc_lines, desc, false, false);
+        }
+        if let Some(sp_regs_altered) = &self.sp_regs_altered {
+            sp_regs_altered.write_xml(insn);
+        }
+    }
+}
+
+#[derive(Debug)]
+struct Parser {
+    pages: Pages,
+    text_section: TextSection,
+    insns: Vec<Insn>,
+}
+
+impl Parser {
+    fn new() -> Self {
+        Self {
+            pages: Pages::new(None),
+            text_section: TextSection::first(),
+            insns: Vec::new(),
+        }
+    }
+    fn page(&mut self) -> Result<Rc<Page>, Box<dyn Error>> {
+        Ok(self
+            .pages
+            .get(self.text_section.page_num)?
+            .ok_or("page_num is out of range")?)
+    }
+    fn unprocessed_chars(
+        &mut self,
+    ) -> Result<Rc<RefCell<BTreeMap<Font, IndexSet<Char>>>>, Box<dyn Error>> {
+        Ok(self
+            .page()?
+            .unprocessed_chars
+            .borrow_mut()
+            .entry(self.text_section)
+            .or_default()
+            .clone())
+    }
+    fn pages_gen(
+        file: &str,
+        page_numbers: Option<Vec<NonZero<u32>>>,
+    ) -> Result<Box<dyn Iterator<Item = Result<Page, Box<dyn Error>>>>, Box<dyn Error>> {
+        let page_numbers = page_numbers.map(|page_numbers| {
+            let mut retval = Vec::from_iter(page_numbers.into_iter().map(|v| v.get() - 1));
+            retval.sort();
+            retval
+        });
+        let document = mupdf::Document::open(file)?;
+        let pages: Vec<mupdf::Page> = document.pages().and_then(|pages| pages.collect())?;
+        Ok(Box::new(pages.into_iter().enumerate().map(
+            move |(i, page)| {
+                let page_num = match &page_numbers {
+                    Some(page_numbers) => page_numbers[i] + 1,
+                    None => i as u32 + 1,
+                };
+                println!("page {page_num}");
+                Ok(Page::from_mupdf_page(page_num, page)
+                    .map_err(|e| format!("error reading pdf page {page_num}: {e}"))?)
+            },
+        )))
+    }
+    fn parse_pdf<I: Iterator<Item = NonZero<u32>>>(
+        &mut self,
+        file: &str,
+        page_numbers: Option<I>,
+    ) -> Result<(), Box<dyn Error>> {
+        self.pages = Pages::new(Some(Self::pages_gen(
+            file,
+            page_numbers.map(|v| v.into_iter().collect()),
+        )?));
+        self.text_section = TextSection::first();
+        loop {
+            self.text_section = self.text_section.next();
+            if self.pages.is_past_end(self.text_section.page_num)? {
+                return Ok(());
+            }
+            if self.pages.get(self.text_section.page_num)?.is_some() {
+                println!("section {:?}", self.text_section);
+                self.note_text_section(Self::parse_text_section)?;
+            }
+        }
+    }
+    fn note_text_section(
+        &mut self,
+        f: impl FnOnce(&mut Self) -> Result<(), Box<dyn Error>>,
+    ) -> Result<(), Box<dyn Error>> {
+        let start_text_section = self.text_section;
+        match f(self) {
+            Ok(()) => Ok(()),
+            Err(e) => {
+                let note = if self.text_section == start_text_section {
+                    format!("text_section={:?}", self.text_section)
+                } else {
+                    format!(
+                        "start_text_section={start_text_section:?}\ntext_section={:?}",
+                        self.text_section
+                    )
+                };
+                Err(format!("{e}\nnote: {note}").into())
+            }
+        }
+    }
+    fn parse_text_section(&mut self) -> Result<(), Box<dyn Error>> {
+        todo!()
+    }
 }
 
 #[derive(Clone, Debug, Default)]
 struct MyDevice {
-    chars: Rc<RefCell<Vec<Char>>>,
-}
-
-impl MyDevice {
-    fn text(&mut self, text: &mupdf::Text, cmt: mupdf::Matrix) {
-        for span in text.spans() {
-            let span_font = span.font();
-            let font_name = span_font.name();
-            const ROUND_FACTOR: f32 = 1000.0;
-            let Some(size) =
-                NonNaNF32::new((span.trm().expansion() * ROUND_FACTOR).round() / ROUND_FACTOR)
-            else {
-                continue;
-            };
-            let font = Font::new(font_name, size);
-            for item in span.items() {
-                let Some(ch) = u32::try_from(item.ucs())
-                    .ok()
-                    .and_then(|v| char::try_from(v).ok())
-                else {
-                    continue;
-                };
-                let mut m = span.trm();
-                m.e = item.x();
-                m.f = item.y();
-                m.concat(cmt);
-                let (min_x, min_y, max_x, max_y) = match span.wmode() {
-                    mupdf::WriteMode::Horizontal => {
-                        todo!();
-                    }
-                    mupdf::WriteMode::Vertical => todo!(),
-                };
-                self.chars.borrow_mut().push(Char {
-                    font,
-                    text: String::from(ch),
-                    min_x,
-                    min_y,
-                    max_x,
-                    max_y,
-                });
-            }
-        }
-    }
+    qt: Rc<RefCell<BTreeMap<TextSection, QuadTree<PageItem>>>>,
+    unprocessed_non_text: Rc<RefCell<IndexSet<LineOrRect>>>,
 }
 
 impl mupdf::NativeDevice for MyDevice {
@@ -1815,48 +2022,190 @@ impl mupdf::NativeDevice for MyDevice {
     ) {
         // TODO
     }
+}
 
-    fn fill_text(
-        &mut self,
-        text: &mupdf::Text,
-        cmt: mupdf::Matrix,
-        _color_space: &mupdf::Colorspace,
-        _color: &[f32],
-        _alpha: f32,
-        _cp: mupdf::ColorParams,
-    ) {
-        self.text(text, cmt);
-    }
+#[derive(serde::Deserialize, Debug)]
+enum MuPdfXml<'a> {
+    #[serde(rename = "page")]
+    Page(MuPdfXmlPage<'a>),
+}
 
-    fn stroke_text(
-        &mut self,
-        text: &mupdf::Text,
-        _stroke_state: &mupdf::StrokeState,
-        cmt: mupdf::Matrix,
-        _color_space: &mupdf::Colorspace,
-        _color: &[f32],
-        _alpha: f32,
-        _cp: mupdf::ColorParams,
-    ) {
-        self.text(text, cmt);
-    }
+#[derive(serde::Deserialize, Debug)]
+struct MuPdfXmlPage<'a> {
+    #[serde(rename = "@id")]
+    id: Cow<'a, str>,
+    #[serde(rename = "@width")]
+    width: f32,
+    #[serde(rename = "@height")]
+    height: f32,
+    block: Vec<MuPdfXmlBlock<'a>>,
+}
 
-    fn clip_text(&mut self, text: &mupdf::Text, cmt: mupdf::Matrix, _scissor: mupdf::Rect) {
-        self.text(text, cmt);
-    }
+#[derive(serde::Deserialize, Debug)]
+struct MuPdfXmlBlock<'a> {
+    #[serde(rename = "@bbox")]
+    bbox: [f32; 4],
+    #[serde(rename = "@justify")]
+    justify: Cow<'a, str>,
+    line: Vec<MuPdfXmlLine<'a>>,
+}
 
-    fn clip_stroke_text(
-        &mut self,
-        text: &mupdf::Text,
-        _stroke_state: &mupdf::StrokeState,
-        cmt: mupdf::Matrix,
-        _scissor: mupdf::Rect,
-    ) {
-        self.text(text, cmt);
-    }
+#[derive(serde::Deserialize, Debug)]
+struct MuPdfXmlLine<'a> {
+    #[serde(rename = "@bbox")]
+    bbox: [f32; 4],
+    #[serde(rename = "@wmode")]
+    wmode: u8,
+    #[serde(rename = "@dir")]
+    dir: [f32; 2],
+    #[serde(rename = "@text")]
+    text: Cow<'a, str>,
+    font: Vec<MuPdfXmlFont<'a>>,
+}
 
-    fn ignore_text(&mut self, text: &mupdf::Text, cmt: mupdf::Matrix) {
-        self.text(text, cmt);
+#[derive(serde::Deserialize, Debug)]
+struct MuPdfXmlFont<'a> {
+    #[serde(rename = "@name")]
+    name: Cow<'a, str>,
+    #[serde(rename = "@size")]
+    size: f32,
+    char: Vec<MuPdfXmlChar<'a>>,
+}
+
+#[derive(serde::Deserialize, Debug)]
+struct MuPdfXmlChar<'a> {
+    #[serde(rename = "@quad")]
+    quad: [f32; 8],
+    #[serde(rename = "@x")]
+    x: f32,
+    #[serde(rename = "@y")]
+    y: f32,
+    #[serde(rename = "@bidi")]
+    bidi: u16,
+    #[serde(rename = "@color")]
+    color: Cow<'a, str>,
+    #[serde(rename = "@alpha")]
+    alpha: Cow<'a, str>,
+    #[serde(rename = "@flags")]
+    flags: u32,
+    #[serde(rename = "@c")]
+    c: Cow<'a, str>,
+}
+
+impl Page {
+    fn from_mupdf_page(
+        page_num: u32,
+        page: mupdf::Page,
+    ) -> Result<Self, Box<dyn std::error::Error>> {
+        let device = MyDevice::default();
+        page.run(
+            &mupdf::Device::from_native(device.clone())?,
+            &mupdf::Matrix::IDENTITY,
+        )?;
+        let MyDevice {
+            qt,
+            unprocessed_non_text,
+        } = device;
+        let mut qt = Rc::try_unwrap(qt)
+            .ok()
+            .expect("already dropped all other references")
+            .into_inner();
+        let unprocessed_chars: Rc<
+            RefCell<BTreeMap<TextSection, Rc<RefCell<BTreeMap<Font, IndexSet<Char>>>>>>,
+        > = Rc::default();
+        // we convert to xml and parse that becuase the mupdf rust crate doesn't include all the API surface we need.
+        let xml = page.to_xml()?;
+        let MuPdfXml::Page(xml_page) = quick_xml::de::from_str(&xml)?;
+        for xml_block in xml_page.block {
+            for xml_line in xml_block.line {
+                for xml_font in xml_line.font {
+                    const ROUND_FACTOR: f32 = 1000.0;
+                    let font_size = (xml_font.size * ROUND_FACTOR).round() / ROUND_FACTOR;
+                    let font_size = NonNaNF32::new(font_size).ok_or("font size must not be NaN")?;
+                    let font = Font::new(&xml_font.name, font_size);
+                    for xml_char in xml_font.char {
+                        let [x0, y0, x1, y1, x2, y2, x3, y3] = xml_char.quad;
+                        let min_x = x0.min(x1).min(x2).min(x3);
+                        let max_x = x0.max(x1).max(x2).max(x3);
+                        let min_y = y0.min(y1).min(y2).min(y3);
+                        let max_y = y0.max(y1).max(y2).max(y3);
+                        let Some(text_section) = TextSection::for_position(
+                            page_num,
+                            (min_x + max_x) * 0.5,
+                            (min_y + max_y) * 0.5,
+                        ) else {
+                            if PAGE_BODY_MIN_Y <= min_y && min_y <= PAGE_BODY_MAX_Y {
+                                if page_num != 1072 {
+                                    // page 1072 has characters in the margins
+                                    return Err(
+                                        format!("char not in text section: {xml_char:?}\npage_num={page_num}").into(),
+                                    );
+                                }
+                            }
+                            continue;
+                        };
+                        let char = Char {
+                            font: font.clone(),
+                            text: xml_char.c.into_owned(),
+                            min_x: NonNaNF32::new(min_x).ok_or("char position shouldn't be NaN")?,
+                            min_y: NonNaNF32::new(min_y).ok_or("char position shouldn't be NaN")?,
+                            max_x: NonNaNF32::new(max_x).ok_or("char position shouldn't be NaN")?,
+                            max_y: NonNaNF32::new(max_y).ok_or("char position shouldn't be NaN")?,
+                        };
+                        qt.entry(text_section).or_default().insert(
+                            min_x,
+                            min_y,
+                            PageItem::Char(char.clone()),
+                        );
+                        unprocessed_chars
+                            .borrow_mut()
+                            .entry(text_section)
+                            .or_default()
+                            .borrow_mut()
+                            .entry(char.font.clone())
+                            .or_default()
+                            .insert(char);
+                    }
+                }
+            }
+        }
+        for i in unprocessed_chars.borrow_mut().values_mut() {
+            for j in i.borrow_mut().values_mut() {
+                j.sort_by_key(Char::top_down_left_to_right_sort_key);
+            }
+        }
+        let mut unknown_fonts = Vec::new();
+        let mut unknown_font_errors = Vec::new();
+        for i in RefCell::borrow(&unprocessed_chars).values() {
+            for (font, chars) in RefCell::borrow(i).iter() {
+                if font.known_font_group().is_none() {
+                    let mut text = String::new();
+                    for char in chars {
+                        text += &char.text;
+                    }
+                    unknown_fonts.push(format!("{font:?},"));
+                    unknown_font_errors.push(format!(
+                        "unknown font {font:?}\nlast char: {:?}\ntext: {text:?}",
+                        chars.last()
+                    ));
+                }
+            }
+        }
+        unknown_fonts.sort();
+        if !unknown_fonts.is_empty() {
+            return Err(format!(
+                "\nunknown fonts:\n{}\n\n{}",
+                unknown_fonts.join("\n"),
+                unknown_font_errors.join("\n")
+            )
+            .into());
+        }
+        Ok(Self {
+            page_num,
+            qt,
+            unprocessed_chars,
+            unprocessed_non_text,
+        })
     }
 }
 
@@ -1880,20 +2229,25 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     } else {
         None
     };
-    let document = mupdf::Document::open(&args[1])?;
-    let pages: Vec<_> = document.pages()?.collect::<Result<_, _>>()?;
-    let page_numbers = page_numbers.unwrap_or_else(|| {
-        Box::new(
-            (0..pages.len()).map(|i| NonZero::new((i + 1) as u32).expect("known to be non-zero")),
-        )
-    });
-    for page_num in page_numbers {
-        let device = MyDevice::default();
-        pages[page_num.get() as usize - 1].run(
-            &mupdf::Device::from_native(device.clone())?,
-            &mupdf::Matrix::IDENTITY,
-        )?;
-        println!("{device:?}");
+    let mut parser = Parser::new();
+    let is_subset = page_numbers.is_some();
+    let file_name = &args[1];
+    parser.parse_pdf(file_name, page_numbers)?;
+    let mut insns = xml_tree::Element::new(
+        "instructions".into(),
+        [("is-subset".into(), is_subset.to_string())],
+    );
+    insns.text = "\n".into();
+    insns.tail = "\n".into();
+    let mut comment =
+        xml_tree::Element::comment(format!(" Automatically generated from {file_name} "));
+    comment.tail = "\n".into();
+    insns.children.push(comment);
+    for insn in parser.insns {
+        insn.write_xml(&mut insns);
     }
+    let mut output = Vec::new();
+    insns.write(&mut output, true)?;
+    std::fs::write("powerisa-instructions.xml", output)?;
     Ok(())
 }

src/xml_tree.rs

@@ -3,7 +3,7 @@
 
 use quick_xml::{
     Writer,
-    events::{BytesText, Event},
+    events::{BytesDecl, BytesText, Event},
 };
 use std::fmt;
 
@@ -110,47 +110,7 @@ pub(crate) struct Element {
 impl fmt::Display for Element {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut writer = Writer::new(FmtToIoAdaptor::new(f));
-        fn helper(
-            element: &Element,
-            writer: &mut Writer<impl std::io::Write>,
-        ) -> std::io::Result<()> {
-            let Element {
-                tag,
-                attrib,
-                text,
-                children,
-                tail,
-            } = element;
-            match tag {
-                ElementTag::Comment => {
-                    writer.write_event(Event::Comment(BytesText::new(text)))?;
-                }
-                ElementTag::Normal(tag) if tag.is_empty() => {
-                    writer.write_event(Event::Text(BytesText::new(text)))?;
-                }
-                ElementTag::Normal(tag) => {
-                    let mut element_writer = writer.create_element(tag);
-                    for (name, value) in attrib {
-                        element_writer =
-                            element_writer.with_attribute((name.as_str(), value.as_str()));
-                    }
-                    if text.is_empty() && children.is_empty() {
-                        element_writer.write_empty()?;
-                    } else {
-                        element_writer.write_inner_content(|writer| {
-                            writer.write_event(Event::Text(BytesText::new(text)))?;
-                            for child in children {
-                                helper(child, writer)?;
-                            }
-                            Ok(())
-                        })?;
-                    }
-                }
-            }
-            writer.write_event(Event::Text(BytesText::new(tail)))?;
-            Ok(())
-        }
-        helper(self, &mut writer).map_err(|_| fmt::Error)?;
+        self.write_to(&mut writer).map_err(|_| fmt::Error)?;
         writer.into_inner().finish()?;
         Ok(())
     }
@@ -166,6 +126,16 @@ impl Element {
             tail: String::new(),
         }
     }
+    /// equivalent of python's `xml.etree.ElementTree.Comment()`
+    pub(crate) fn comment(text: String) -> Self {
+        Self {
+            tag: ElementTag::Comment,
+            attrib: Vec::new(),
+            text,
+            children: Vec::new(),
+            tail: String::new(),
+        }
+    }
     /// equivalent to python `"".join(self.itertext())`
     pub(crate) fn inner_text(&self) -> String {
         let mut retval = String::new();
@@ -198,4 +168,53 @@ impl Element {
         self.children.push(Self::new(tag, attrib));
         self.children.last_mut().expect("just pushed")
     }
+    pub(crate) fn write_to(&self, writer: &mut Writer<impl std::io::Write>) -> std::io::Result<()> {
+        let Element {
+            tag,
+            attrib,
+            text,
+            children,
+            tail,
+        } = self;
+        match tag {
+            ElementTag::Comment => {
+                writer.write_event(Event::Comment(BytesText::new(text)))?;
+            }
+            ElementTag::Normal(tag) if tag.is_empty() => {
+                writer.write_event(Event::Text(BytesText::new(text)))?;
+            }
+            ElementTag::Normal(tag) => {
+                let mut element_writer = writer.create_element(tag);
+                for (name, value) in attrib {
+                    element_writer = element_writer.with_attribute((name.as_str(), value.as_str()));
+                }
+                if text.is_empty() && children.is_empty() {
+                    element_writer.write_empty()?;
+                } else {
+                    element_writer.write_inner_content(|writer| {
+                        writer.write_event(Event::Text(BytesText::new(text)))?;
+                        for child in children {
+                            child.write_to(writer)?;
+                        }
+                        Ok(())
+                    })?;
+                }
+            }
+        }
+        writer.write_event(Event::Text(BytesText::new(tail)))?;
+        Ok(())
+    }
+    /// equivalent of python's `xml.etree.ElementTree(self).write(writer, encoding='utf-8', xml_declaration=xml_declaration)`
+    pub(crate) fn write(
+        &self,
+        writer: impl std::io::Write,
+        xml_declaration: bool,
+    ) -> std::io::Result<()> {
+        let mut writer = Writer::new(writer);
+        if xml_declaration {
+            writer.write_event(Event::Decl(BytesDecl::new("1.0", Some("utf-8"), None)))?;
+            writer.write_event(Event::Text(BytesText::new("\n")))?;
+        }
+        self.write_to(&mut writer)
+    }
 }