rucc-lower 0.10.43

//! An `asm` at file scope whose template is directives, read into the objects it defines.
//!
//! Design: `spec/11-asm-objects-debug.md` sections 11.1 and 11.2.
//!
//! # What this is for
//!
//! A template with no instructions in it is the case to build first, and 11.2 says so about the
//! `asm` inside a function. The same sentence is true of the one outside a function and for the
//! same reason: what a program writes there is nearly always a run of directives that puts bytes
//! and names in the output, and none of it needs the assembler 11.1 is waiting for. `.incbin` is
//! the whole of what the incbin header does, an alias table is `.globl` and a label, and a
//! version script's worth of `.set` is the same shape again.
//!
//! So a template made of directives is read here and becomes the globals it defines, and a
//! template with an instruction in it is refused by name until there is something that can turn
//! one into bytes. That is the same line drawn in the same place as for the `asm` in a function,
//! which refuses a template with anything in it today.
//!
//! # Why globals rather than text
//!
//! This compiler writes object files itself and does not print assembly for somebody else to
//! read, so there is no output for a template to be copied into. What the directives say is that
//! a section holds these bytes under these names with these alignments, and a global is exactly
//! that, so the reading is a translation and not an approximation. It also means `-S` and `-c`
//! cannot disagree about what the file contains, because both are written from the same globals.
//!
//! # Why the labels land next to each other
//!
//! A block usually names more than one thing and means them to be adjacent. The incbin header
//! writes a label, the file's bytes, a second label, and a size worked out as the distance
//! between them, and a program that reads the size and walks to the end symbol is reading a
//! promise about where the two sit. One global per label keeps that promise because the object
//! writer lays globals out in the order the module holds them, padding each to its own
//! alignment, which is the same walk an assembler makes over the same directives. What it takes
//! is for nothing to be placed between them, and that is why [`crate::unit`] reads the file
//! scope blocks before it walks the declarations: a block's globals are added together, so they
//! are a run, and a later declaration of one of the names finds the definition already there.
//!
//! The offsets this computes are counted from the start of the block rather than from the start
//! of the section, so the distance between two labels is right only if the block starts at an
//! offset every alignment inside it divides. That is arranged rather than hoped for: the first
//! global of a section is given the largest alignment anything in that section asked for.
//!
//! One difference from an assembler survives this and is deliberate. An alignment written after
//! the last byte of a section pads the section out in `gas` and does nothing here, because the
//! padding belongs to no label and a global has to have a name. Nothing can read those bytes: the
//! section still records the alignment it asked for, so the linker puts whatever follows it at
//! the same place either way, and every symbol in the file is at the same offset it would be.

use std::collections::HashMap;

use rucc_ir::{Linkage, Visibility};

/// What could not be done with a template, and which kind of answer that is.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) enum Failed {
    /// Something a reader of directives does not do, named so the message can say which.
    ///
    /// The string completes a sentence that ends in "is not supported yet", so it reads as a
    /// noun phrase and names the construct rather than describing the reader.
    Unsupported(String),
    /// A file `.incbin` named that could not be read, with the reason the operating system gave.
    Missing(String, String),
}

/// Which section the bytes of one global go in.
///
/// The four the directives name by themselves and the one they name with a string. They are kept
/// apart rather than turned into a section name here because what the object writer wants is what
/// kind of thing the global is: `.rodata` is the section a constant with no address in it lands
/// in anyway, and asking for it by name would produce a writable section with the same spelling.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) enum Section {
    /// `.text`. Nothing may be emitted into it here, since bytes in it would be instructions.
    Text,
    /// `.data`, which is what a section directive naming nothing else gets.
    Data,
    /// `.rodata`, and on a Mach-O target `.const_data`.
    ReadOnly,
    /// `.bss`, which carries no image, so only zeros may be put in it.
    Bss,
    /// The section the template named, for anything the four above do not cover.
    Named(String),
}

/// One piece of an image, kept as what it is rather than as bytes.
///
/// An integer is not bytes yet because which end of it comes first is the target's answer and
/// not this reader's, and the module already writes a scalar the way the target wants it. Zeros
/// are not bytes either, so that a section carrying no image can say how much of it there is
/// without the compiler holding that many bytes to say it with.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) enum Item {
    /// Literal bytes, in the order they go in the image.
    Bytes(Vec<u8>),
    /// One integer that many bytes wide.
    Int {
        /// How many bytes of image it is.
        width: u8,
        /// Its value, which is what the expression worked out to.
        value: i64,
    },
    /// That many zero bytes, and no image for them.
    Zero(u64),
}

impl Item {
    /// How many bytes of image it is.
    fn size(&self) -> u64 {
        match self {
            Item::Bytes(bytes) => bytes.len() as u64,
            Item::Int { width, .. } => u64::from(*width),
            Item::Zero(bytes) => *bytes,
        }
    }
}

/// One global a template defines, which is one label and everything written under it.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) struct Piece {
    /// The name the label gave it.
    pub name: String,
    /// Which section it goes in.
    pub section: Section,
    /// What it has to be aligned to, which is what the last alignment directive in front of the
    /// label asked for.
    pub align: u32,
    /// Its image, in order.
    pub items: Vec<Item>,
    /// How many bytes the image adds up to.
    pub size: u64,
    /// How the linker sees the name, which is internal unless a directive exported it.
    pub linkage: Linkage,
    /// How far outside a shared library the name reaches.
    pub visibility: Visibility,
}

/// Reads a template and gives back the globals it defines, in the order it defined them.
///
/// `read` is asked for the bytes of a file `.incbin` names. It is a function rather than a list
/// of directories so that the walk over a template stays something that can be tested without a
/// file system, and so that where an assembler looks for a file is decided in one place by the
/// caller.
///
/// # Errors
///
/// [`Failed::Unsupported`] for a template holding anything this does not read, which is an
/// instruction, a directive not in the set below, or a use of a name it cannot work out.
/// [`Failed::Missing`] for a file `.incbin` names that `read` could not give.
pub(crate) fn assemble(
    template: &str,
    read: &mut dyn FnMut(&str) -> Result<Vec<u8>, String>,
) -> Result<Vec<Piece>, Failed> {
    let mut asm = Assembler::new(read);
    for statement in statements(template)? {
        asm.statement(&statement)?;
    }
    asm.finish()
}

/// Where a section is up to, and which section that is.
#[derive(Debug)]
struct Where {
    /// Which section this is.
    section: Section,
    /// How many bytes of it the block has written, which is where the next label lands.
    at: u64,
}

/// The global being filled in, which is the last label and what has been written since.
#[derive(Debug)]
struct Open {
    /// The label's name.
    name: String,
    /// Which of [`Assembler::sections`] it is in.
    section: usize,
    /// What the label was aligned to.
    align: u32,
    /// Its image so far.
    items: Vec<Item>,
    /// How many bytes that is.
    size: u64,
}

/// What one of the directives that talk about a name says about it.
///
/// The five of them differ in the one word they put on the name and in nothing else, so they are
/// one function taking this rather than five that each walk a list of names.
#[derive(Debug, Clone, Copy)]
enum Said {
    /// How the linker sees the name. `.globl`, `.local` and `.weak`.
    Linkage(Linkage),
    /// How far outside a shared library it reaches. `.hidden` and `.protected`.
    Reach(Visibility),
}

/// What an expression worked out to, which is a number and the section it counts from.
///
/// A label is a number only once the linker has placed the section it is in, so what is known
/// here is the offset and which section it is an offset into. Two labels of one section subtract
/// to a number that does not depend on where the section went, which is the whole reason a
/// program writes `end - start`, and this is the record that makes the subtraction possible.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Value {
    /// Which of [`Assembler::sections`] the number counts from, and nothing for a plain number.
    section: Option<usize>,
    /// The number itself.
    offset: i64,
}

impl Value {
    /// A plain number, which counts from nothing.
    const fn number(offset: i64) -> Value {
        Value { section: None, offset }
    }
}

/// The walk over a template, and everything it has worked out so far.
struct Assembler<'a> {
    /// The sections the template has named, in the order it first named them.
    sections: Vec<Where>,
    /// Which of them is being written to.
    current: usize,
    /// The globals finished so far.
    pieces: Vec<Piece>,
    /// The one being filled in.
    open: Option<Open>,
    /// Where every label of this block is.
    labels: HashMap<String, Value>,
    /// The names a directive exported, and how.
    linkage: HashMap<String, Linkage>,
    /// The names a directive said how far they reach.
    visibility: HashMap<String, Visibility>,
    /// What `.size` claimed each name is, to be held against what was written under it.
    sizes: HashMap<String, i64>,
    /// What the last alignment directive asked for and nothing has been aligned to yet.
    pending: u32,
    /// Where the bytes of a file come from.
    read: &'a mut dyn FnMut(&str) -> Result<Vec<u8>, String>,
}

impl<'a> Assembler<'a> {
    /// A walk that has read nothing, standing in `.text` the way an assembler starts.
    fn new(read: &'a mut dyn FnMut(&str) -> Result<Vec<u8>, String>) -> Assembler<'a> {
        Assembler {
            sections: vec![Where { section: Section::Text, at: 0 }],
            current: 0,
            pieces: Vec::new(),
            open: None,
            labels: HashMap::new(),
            linkage: HashMap::new(),
            visibility: HashMap::new(),
            sizes: HashMap::new(),
            pending: 1,
            read,
        }
    }

    /// Reads one statement, which is a label, a directive, or the end of the line.
    fn statement(&mut self, statement: &str) -> Result<(), Failed> {
        let rest = statement.trim();
        if rest.is_empty() {
            return Ok(());
        }
        let word = word_of(rest);
        if let Some(after) = rest[word.len()..].strip_prefix(':') {
            if word.is_empty() {
                return Err(unsupported("a label with no name at file scope".to_owned()));
            }
            self.label(word)?;
            return self.statement(after);
        }
        let operands = rest[word.len()..].trim();
        if !word.starts_with('.') {
            return Err(unsupported(format!("the instruction '{word}'")));
        }
        self.directive(word, operands)
    }

    /// One label, which closes the global above it and opens the one under it.
    fn label(&mut self, name: &str) -> Result<(), Failed> {
        if self.labels.contains_key(name) {
            return Err(unsupported(format!("the label '{name}' written twice")));
        }
        self.close();
        let align = std::mem::replace(&mut self.pending, 1);
        let at = round_up(self.sections[self.current].at, u64::from(align));
        self.sections[self.current].at = at;
        self.labels.insert(
            name.to_owned(),
            Value { section: Some(self.current), offset: i64::try_from(at).unwrap_or(0) },
        );
        self.open = Some(Open {
            name: name.to_owned(),
            section: self.current,
            align,
            items: Vec::new(),
            size: 0,
        });
        Ok(())
    }

    /// One directive and the text after its name.
    fn directive(&mut self, name: &str, operands: &str) -> Result<(), Failed> {
        match name {
            ".text" => self.section(Section::Text),
            ".data" => self.section(Section::Data),
            ".bss" => self.section(Section::Bss),
            ".section" => {
                let named = split(operands).into_iter().next().unwrap_or_default();
                self.section(section_named(named.trim_matches('"')));
            }
            ".globl" | ".global" => self.says(operands, Said::Linkage(Linkage::External)),
            ".local" => self.says(operands, Said::Linkage(Linkage::Internal)),
            ".weak" => self.says(operands, Said::Linkage(Linkage::Weak)),
            ".hidden" | ".internal" => self.says(operands, Said::Reach(Visibility::Hidden)),
            ".protected" => self.says(operands, Said::Reach(Visibility::Protected)),
            // What a name is is recorded by the object writer from what the global holds, so
            // there is nothing here to keep. It is accepted because every exported symbol in
            // real assembly is written with one and refusing it would refuse the whole file.
            ".type" | ".ident" | ".file" | ".cfi_sections" => {}
            ".size" => self.size(operands)?,
            ".balign" | ".align" => self.align(operands, false)?,
            ".p2align" => self.align(operands, true)?,
            ".byte" => self.ints(operands, 1)?,
            ".short" | ".hword" | ".word" | ".value" => self.ints(operands, 2)?,
            ".long" | ".int" => self.ints(operands, 4)?,
            ".quad" => self.ints(operands, 8)?,
            ".ascii" => self.ascii(operands, false)?,
            ".asciz" | ".string" => self.ascii(operands, true)?,
            ".zero" | ".space" | ".skip" => self.space(operands)?,
            ".incbin" => self.incbin(operands)?,
            _ => return Err(unsupported(format!("the '{name}' directive at file scope"))),
        }
        Ok(())
    }

    /// Moves to a section, which closes whatever was being written to the last one.
    fn section(&mut self, section: Section) {
        self.close();
        self.pending = 1;
        self.current = match self.sections.iter().position(|held| held.section == section) {
            Some(index) => index,
            None => {
                self.sections.push(Where { section, at: 0 });
                self.sections.len() - 1
            }
        };
    }

    /// A directive that says one thing about each of the names it lists.
    fn says(&mut self, operands: &str, said: Said) {
        for name in split(operands) {
            let name = name.trim().to_owned();
            match said {
                Said::Linkage(linkage) => {
                    self.linkage.insert(name, linkage);
                }
                Said::Reach(visibility) => {
                    self.visibility.insert(name, visibility);
                }
            }
        }
    }

    /// `.size name, expression`, kept so it can be held against what was written.
    fn size(&mut self, operands: &str) -> Result<(), Failed> {
        let parts = split(operands);
        let [name, expression] = parts.as_slice() else {
            return Err(unsupported(
                "a '.size' directive that is not a name and a size".to_owned(),
            ));
        };
        let value = self.value(expression)?;
        if value.section.is_some() {
            return Err(unsupported(format!("a '.size' of '{name}' that is not a number")));
        }
        self.sizes.insert(name.trim().to_owned(), value.offset);
        Ok(())
    }

    /// An alignment directive, whose operand is a count of bytes or a power of two.
    fn align(&mut self, operands: &str, power: bool) -> Result<(), Failed> {
        let parts = split(operands);
        let first = parts.first().map_or("", |text| text.trim());
        if first.is_empty() {
            return Ok(());
        }
        let asked = self.value(first)?;
        if asked.section.is_some() {
            return Err(unsupported("an alignment that is not a number".to_owned()));
        }
        let Ok(asked) = u32::try_from(asked.offset) else {
            return Err(unsupported(format!("an alignment of {}", asked.offset)));
        };
        let bytes = if power {
            if asked >= 32 {
                return Err(unsupported(format!("an alignment of two to the {asked}")));
            }
            1u32 << asked
        } else {
            asked
        };
        if bytes == 0 || !bytes.is_power_of_two() {
            return Err(unsupported(format!("an alignment of {bytes}")));
        }
        self.pending = self.pending.max(bytes);
        Ok(())
    }

    /// A run of integers of one width, which is what the data directives are.
    fn ints(&mut self, operands: &str, width: u8) -> Result<(), Failed> {
        for operand in split(operands) {
            let value = self.value(&operand)?;
            if value.section.is_some() {
                return Err(unsupported(format!(
                    "the address of '{}' in an image",
                    operand.trim()
                )));
            }
            self.emit(Item::Int { width, value: value.offset })?;
        }
        Ok(())
    }

    /// A run of strings, with a terminator after each one under `.asciz`.
    fn ascii(&mut self, operands: &str, terminated: bool) -> Result<(), Failed> {
        for operand in split(operands) {
            let mut bytes = string_of(operand.trim())?;
            if terminated {
                bytes.push(0);
            }
            self.emit(Item::Bytes(bytes))?;
        }
        Ok(())
    }

    /// `.zero`, `.space` and `.skip`, which are that many bytes of the fill, and the fill is zero.
    fn space(&mut self, operands: &str) -> Result<(), Failed> {
        let parts = split(operands);
        let [count] = parts.as_slice() else {
            return Err(unsupported("a run of a fill that is not zero".to_owned()));
        };
        let value = self.value(count)?;
        let Ok(bytes) = u64::try_from(value.offset) else {
            return Err(unsupported(format!("a run of {} bytes", value.offset)));
        };
        self.zeros(bytes)
    }

    /// `.incbin`, which is the bytes of a file and is the whole reason this reader exists.
    fn incbin(&mut self, operands: &str) -> Result<(), Failed> {
        let parts = split(operands);
        let [name] = parts.as_slice() else {
            return Err(unsupported("an '.incbin' that skips or counts bytes".to_owned()));
        };
        let name = String::from_utf8(string_of(name.trim())?)
            .map_err(|_| unsupported("an '.incbin' of a name that is not text".to_owned()))?;
        match (self.read)(&name) {
            Ok(bytes) => self.emit(Item::Bytes(bytes)),
            Err(why) => Err(Failed::Missing(name, why)),
        }
    }

    /// That many zero bytes, said the way the section they go in can carry them.
    fn zeros(&mut self, bytes: u64) -> Result<(), Failed> {
        if bytes == 0 {
            return Ok(());
        }
        if self.sections[self.current].section == Section::Bss {
            return self.emit(Item::Zero(bytes));
        }
        self.emit(Item::Bytes(vec![0; usize::try_from(bytes).unwrap_or(usize::MAX)]))
    }

    /// Writes one piece of an image, after the padding any alignment directive in front of it
    /// asked for.
    fn emit(&mut self, item: Item) -> Result<(), Failed> {
        let pending = std::mem::replace(&mut self.pending, 1);
        if pending > 1 {
            let at = self.sections[self.current].at;
            let padding = round_up(at, u64::from(pending)) - at;
            if padding > 0 {
                self.put(Item::Bytes(vec![0; usize::try_from(padding).unwrap_or(usize::MAX)]))?;
            }
        }
        self.put(item)
    }

    /// Writes one piece of an image where the position already is.
    fn put(&mut self, item: Item) -> Result<(), Failed> {
        let section = &self.sections[self.current].section;
        if *section == Section::Text {
            return Err(unsupported("data in the text section at file scope".to_owned()));
        }
        if *section == Section::Bss && !matches!(item, Item::Zero(_)) {
            return Err(unsupported("data in a section that carries none".to_owned()));
        }
        let Some(open) = self.open.as_mut() else {
            return Err(unsupported("data at file scope under no label".to_owned()));
        };
        let size = item.size();
        open.size += size;
        open.items.push(item);
        self.sections[self.current].at += size;
        Ok(())
    }

    /// Finishes the global being written, if there is one.
    fn close(&mut self) {
        let Some(open) = self.open.take() else { return };
        self.pieces.push(Piece {
            name: open.name,
            section: self.sections[open.section].section.clone(),
            align: open.align,
            items: open.items,
            size: open.size,
            linkage: Linkage::Internal,
            visibility: Visibility::Default,
        });
    }

    /// The globals the template defined, with what the directives said about the names put on.
    fn finish(mut self) -> Result<Vec<Piece>, Failed> {
        self.close();
        // A name in the text section is a function, and what is under it here is nothing, since
        // anything that would have been is an instruction and was refused where it was written.
        if let Some(piece) = self.pieces.iter().find(|piece| piece.section == Section::Text) {
            return Err(unsupported(format!("the label '{}' in the text section", piece.name)));
        }
        for piece in &mut self.pieces {
            piece.linkage = self.linkage.get(&piece.name).copied().unwrap_or(Linkage::Internal);
            piece.visibility =
                self.visibility.get(&piece.name).copied().unwrap_or(Visibility::Default);
            if let Some(&said) = self.sizes.get(&piece.name) {
                if u64::try_from(said) != Ok(piece.size) {
                    return Err(unsupported(format!(
                        "a '.size' of '{}' that is not what was written under it",
                        piece.name
                    )));
                }
            }
        }
        // The offsets above are counted from the start of the block and the block starts wherever
        // the section had got to, so a label two hundred and fifty six bytes in is at a multiple
        // of two hundred and fifty six only if the block is. Giving the first global of a section
        // the largest alignment anything in that section asked for is what makes that true, and
        // it is the same thing an assembler does when it records a section's alignment.
        for index in 0..self.sections.len() {
            let section = self.sections[index].section.clone();
            let largest =
                self.pieces.iter().filter(|p| p.section == section).map(|p| p.align).max();
            let Some(largest) = largest else { continue };
            if let Some(first) = self.pieces.iter_mut().find(|p| p.section == section) {
                first.align = largest;
            }
        }
        Ok(self.pieces)
    }

    /// What one expression works out to.
    fn value(&self, text: &str) -> Result<Value, Failed> {
        let mut chars: Vec<char> = text.chars().collect();
        chars.retain(|c| !c.is_whitespace());
        let mut at = 0;
        let value = self.bitwise(&chars, &mut at)?;
        if at != chars.len() {
            return Err(unsupported(format!("the expression '{}'", text.trim())));
        }
        Ok(value)
    }

    /// The loosest binding operators, which are the bitwise ones.
    fn bitwise(&self, text: &[char], at: &mut usize) -> Result<Value, Failed> {
        let mut left = self.shift(text, at)?;
        while let Some(&c) = text.get(*at) {
            if !matches!(c, '|' | '&' | '^') {
                break;
            }
            *at += 1;
            let right = self.shift(text, at)?;
            let (a, b) = (absolute(left, text)?, absolute(right, text)?);
            left = Value::number(match c {
                '|' => a | b,
                '&' => a & b,
                _ => a ^ b,
            });
        }
        Ok(left)
    }

    /// The shifts, which bind tighter than the bitwise operators and looser than addition.
    fn shift(&self, text: &[char], at: &mut usize) -> Result<Value, Failed> {
        let mut left = self.sum(text, at)?;
        while let (Some(&c), Some(&next)) = (text.get(*at), text.get(*at + 1)) {
            if c != next || !matches!(c, '<' | '>') {
                break;
            }
            *at += 2;
            let right = self.sum(text, at)?;
            let (a, b) = (absolute(left, text)?, absolute(right, text)?);
            let by = u32::try_from(b).unwrap_or(64);
            left = Value::number(if c == '<' {
                a.checked_shl(by).unwrap_or(0)
            } else {
                a.checked_shr(by).unwrap_or(0)
            });
        }
        Ok(left)
    }

    /// Addition and subtraction, which is where a label may still be one side.
    fn sum(&self, text: &[char], at: &mut usize) -> Result<Value, Failed> {
        let mut left = self.product(text, at)?;
        while let Some(&c) = text.get(*at) {
            if !matches!(c, '+' | '-') {
                break;
            }
            *at += 1;
            let right = self.product(text, at)?;
            left = if c == '+' { add(left, right, text)? } else { subtract(left, right, text)? };
        }
        Ok(left)
    }

    /// Multiplication and its two relatives.
    fn product(&self, text: &[char], at: &mut usize) -> Result<Value, Failed> {
        let mut left = self.unary(text, at)?;
        while let Some(&c) = text.get(*at) {
            if !matches!(c, '*' | '/' | '%') {
                break;
            }
            *at += 1;
            let right = self.unary(text, at)?;
            let (a, b) = (absolute(left, text)?, absolute(right, text)?);
            if b == 0 && c != '*' {
                return Err(unsupported(format!("a division by zero in '{}'", written(text))));
            }
            left = Value::number(match c {
                '*' => a.wrapping_mul(b),
                '/' => a.wrapping_div(b),
                _ => a.wrapping_rem(b),
            });
        }
        Ok(left)
    }

    /// A sign or a complement in front of something, and the something on its own.
    fn unary(&self, text: &[char], at: &mut usize) -> Result<Value, Failed> {
        match text.get(*at) {
            Some('-') => {
                *at += 1;
                let value = self.unary(text, at)?;
                Ok(Value::number(absolute(value, text)?.wrapping_neg()))
            }
            Some('+') => {
                *at += 1;
                self.unary(text, at)
            }
            Some('~') => {
                *at += 1;
                let value = self.unary(text, at)?;
                Ok(Value::number(!absolute(value, text)?))
            }
            _ => self.atom(text, at),
        }
    }

    /// A number, a character, a label, the position, or a bracketed expression.
    fn atom(&self, text: &[char], at: &mut usize) -> Result<Value, Failed> {
        match text.get(*at) {
            Some('(') => {
                *at += 1;
                let value = self.bitwise(text, at)?;
                if text.get(*at) != Some(&')') {
                    return Err(unsupported(format!("the expression '{}'", written(text))));
                }
                *at += 1;
                Ok(value)
            }
            Some('\'') => {
                *at += 1;
                let Some(&c) = text.get(*at) else {
                    return Err(unsupported(format!("the expression '{}'", written(text))));
                };
                *at += 1;
                if text.get(*at) == Some(&'\'') {
                    *at += 1;
                }
                Ok(Value::number(i64::from(u32::from(c))))
            }
            Some(c) if c.is_ascii_digit() => number(text, at),
            Some(c) if is_name(*c) => {
                let start = *at;
                while text.get(*at).is_some_and(|&c| is_name(c)) {
                    *at += 1;
                }
                let name: String = text[start..*at].iter().collect();
                if name == "." {
                    let here = self.sections[self.current].at;
                    return Ok(Value {
                        section: Some(self.current),
                        offset: i64::try_from(here).unwrap_or(0),
                    });
                }
                self.labels
                    .get(&name)
                    .copied()
                    .ok_or_else(|| unsupported(format!("the name '{name}' in an expression")))
            }
            _ => Err(unsupported(format!("the expression '{}'", written(text)))),
        }
    }
}

/// Whether that character may be part of a name, which is more than C allows.
fn is_name(c: char) -> bool {
    c.is_ascii_alphanumeric() || matches!(c, '_' | '.' | '$')
}

/// A number in one of the four bases an assembler writes them in.
fn number(text: &[char], at: &mut usize) -> Result<Value, Failed> {
    let start = *at;
    let (radix, from) = match (text.get(*at), text.get(*at + 1)) {
        (Some('0'), Some('x' | 'X')) => (16, start + 2),
        (Some('0'), Some('b' | 'B')) => (2, start + 2),
        (Some('0'), Some(c)) if c.is_ascii_digit() => (8, start + 1),
        _ => (10, start),
    };
    *at = from;
    while text.get(*at).is_some_and(|c| c.is_digit(radix)) {
        *at += 1;
    }
    let digits: String = text[from..*at].iter().collect();
    if digits.is_empty() {
        // A lone zero, whose one digit was read as the marker of an octal number.
        *at = start + 1;
        return Ok(Value::number(0));
    }
    i64::from_str_radix(&digits, radix)
        .map(Value::number)
        .map_err(|_| unsupported(format!("the number '{digits}'")))
}

/// A value that has to be a plain number, and the message for one that is not.
fn absolute(value: Value, text: &[char]) -> Result<i64, Failed> {
    match value.section {
        None => Ok(value.offset),
        Some(_) => Err(unsupported(format!("an address in the expression '{}'", written(text)))),
    }
}

/// The sum of two values, where at most one of them may count from a section.
fn add(left: Value, right: Value, text: &[char]) -> Result<Value, Failed> {
    let section = match (left.section, right.section) {
        (None, other) | (other, None) => other,
        (Some(_), Some(_)) => {
            return Err(unsupported(format!("two addresses added in '{}'", written(text))));
        }
    };
    Ok(Value { section, offset: left.offset.wrapping_add(right.offset) })
}

/// The difference of two values, which is a plain number when both count from one section.
fn subtract(left: Value, right: Value, text: &[char]) -> Result<Value, Failed> {
    let section = match (left.section, right.section) {
        (held, None) => held,
        (Some(a), Some(b)) if a == b => None,
        _ => {
            return Err(unsupported(format!(
                "a difference of addresses in two sections in '{}'",
                written(text)
            )));
        }
    };
    Ok(Value { section, offset: left.offset.wrapping_sub(right.offset) })
}

/// The characters of an expression, for a message about it.
fn written(text: &[char]) -> String {
    text.iter().collect()
}

/// The first word of a statement, which is a label's name or a directive's.
fn word_of(text: &str) -> &str {
    let end = text.find(|c: char| c.is_whitespace() || c == ':' || c == ',').unwrap_or(text.len());
    &text[..end]
}

/// Which section a name stands for, with the four an object writer has answers of its own for
/// told apart from the rest.
fn section_named(name: &str) -> Section {
    match name {
        ".text" => Section::Text,
        ".data" => Section::Data,
        ".rodata" | ".const_data" | ".const" => Section::ReadOnly,
        ".bss" => Section::Bss,
        _ => Section::Named(name.to_owned()),
    }
}

/// The bytes of a quoted string, with the escapes an assembler reads.
fn string_of(text: &str) -> Result<Vec<u8>, Failed> {
    let inner = text
        .strip_prefix('"')
        .and_then(|rest| rest.strip_suffix('"'))
        .ok_or_else(|| unsupported(format!("the operand '{text}'")))?;
    let mut bytes = Vec::with_capacity(inner.len());
    let mut chars = inner.chars();
    while let Some(c) = chars.next() {
        if c != '\\' {
            let mut buffer = [0u8; 4];
            bytes.extend_from_slice(c.encode_utf8(&mut buffer).as_bytes());
            continue;
        }
        match chars.next() {
            Some('n') => bytes.push(b'\n'),
            Some('t') => bytes.push(b'\t'),
            Some('r') => bytes.push(b'\r'),
            Some('0') => bytes.push(0),
            Some('\\') => bytes.push(b'\\'),
            Some('"') => bytes.push(b'"'),
            Some(other) => {
                return Err(unsupported(format!("the escape '\\{other}' in a string")));
            }
            None => return Err(unsupported(format!("the operand '{text}'"))),
        }
    }
    Ok(bytes)
}

/// Splits a directive's operands on the commas between them, leaving the ones inside a string or
/// a bracket where they are.
fn split(operands: &str) -> Vec<String> {
    let mut parts = Vec::new();
    let mut held = String::new();
    let mut quoted = false;
    let mut depth = 0u32;
    for c in operands.chars() {
        match c {
            '"' => {
                quoted = !quoted;
                held.push(c);
            }
            '(' if !quoted => {
                depth += 1;
                held.push(c);
            }
            ')' if !quoted => {
                depth = depth.saturating_sub(1);
                held.push(c);
            }
            ',' if !quoted && depth == 0 => parts.push(std::mem::take(&mut held)),
            _ => held.push(c),
        }
    }
    if !held.trim().is_empty() || !parts.is_empty() {
        parts.push(held);
    }
    parts.into_iter().filter(|part| !part.trim().is_empty()).collect()
}

/// Cuts a template into the statements it is made of, with the comments taken out.
///
/// A newline and a semicolon both end one, which is what an assembler does on every target this
/// compiles for. A `#` runs to the end of the line and a `/*` runs to its closing pair, and
/// neither of them means anything inside a string, which is why this is a walk over characters
/// rather than a split.
fn statements(template: &str) -> Result<Vec<String>, Failed> {
    let mut out = Vec::new();
    let mut held = String::new();
    let mut chars = template.chars().peekable();
    let mut quoted = false;
    while let Some(c) = chars.next() {
        if quoted {
            held.push(c);
            if c == '\\' {
                if let Some(next) = chars.next() {
                    held.push(next);
                }
            } else if c == '"' {
                quoted = false;
            }
            continue;
        }
        match c {
            '"' => {
                quoted = true;
                held.push(c);
            }
            '#' => {
                while chars.peek().is_some_and(|&next| next != '\n') {
                    chars.next();
                }
            }
            '/' if chars.peek() == Some(&'*') => {
                chars.next();
                let mut closed = false;
                while let Some(next) = chars.next() {
                    if next == '*' && chars.peek() == Some(&'/') {
                        chars.next();
                        closed = true;
                        break;
                    }
                }
                if !closed {
                    return Err(unsupported("a comment with no end in it".to_owned()));
                }
            }
            '\n' | ';' => out.push(std::mem::take(&mut held)),
            _ => held.push(c),
        }
    }
    out.push(held);
    Ok(out)
}

/// The refusal for something this does not read.
fn unsupported(what: String) -> Failed {
    Failed::Unsupported(what)
}

/// That number rounded up to a multiple of the alignment.
fn round_up(at: u64, align: u64) -> u64 {
    match align {
        0 | 1 => at,
        _ => at.div_ceil(align) * align,
    }
}

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

    /// Reads a template, with no file for `.incbin` to find.
    fn read(template: &str) -> Result<Vec<Piece>, Failed> {
        assemble(template, &mut |name| Err(format!("no file '{name}'")))
    }

    /// Reads a template, with one file `.incbin` may name.
    fn with_file(template: &str, bytes: &[u8]) -> Result<Vec<Piece>, Failed> {
        let held = bytes.to_vec();
        assemble(template, &mut |_| Ok(held.clone()))
    }

    /// The bytes a piece's image adds up to, with a zero run written out.
    fn image(piece: &Piece) -> Vec<u8> {
        let mut bytes = Vec::new();
        for item in &piece.items {
            match item {
                Item::Bytes(held) => bytes.extend_from_slice(held),
                Item::Int { width, value } => {
                    bytes.extend_from_slice(&value.to_le_bytes()[..usize::from(*width)]);
                }
                Item::Zero(count) => {
                    let want = bytes.len() + usize::try_from(*count).expect("a run that fits");
                    bytes.resize(want, 0);
                }
            }
        }
        bytes
    }

    /// What the incbin header writes, which is the whole reason this reader exists.
    ///
    /// Three names, the middle one at the end of the first one's bytes and the third holding the
    /// distance between them. The test is that the three land where the header's own arithmetic
    /// says they do, since a program that reads the size and walks to the end symbol is reading
    /// that and nothing else.
    #[test]
    fn a_file_pulled_in_by_incbin_becomes_the_three_names_the_header_promises() {
        let template = "\
.section .rodata
.global gData
.type gData, @object
.balign 16
gData:
.incbin \"file.bin\"
.global gEnd
.type gEnd, @object
.balign 1
gEnd:
.byte 1
.global gSize
.type gSize, @object
.balign 16
gSize:
.int gEnd - gData
.balign 16
.text
";
        let pieces = with_file(template, &[7u8; 962]).expect("the template is read");
        let names: Vec<&str> = pieces.iter().map(|piece| piece.name.as_str()).collect();
        assert_eq!(names, ["gData", "gEnd", "gSize"]);
        assert!(pieces.iter().all(|piece| piece.section == Section::ReadOnly));
        assert!(pieces.iter().all(|piece| piece.linkage == Linkage::External));

        assert_eq!(pieces[0].size, 962);
        assert_eq!(pieces[0].align, 16, "the first of a section carries the largest alignment");
        assert_eq!(pieces[1].size, 1, "the end marker is the one byte written under it");
        assert_eq!(pieces[1].align, 1, "and is where the file's bytes stopped");
        assert_eq!(pieces[2].items, [Item::Int { width: 4, value: 962 }], "the size is a distance");
        assert_eq!(pieces[2].align, 16);
    }

    /// An alignment in front of a label is the label's, and one in front of data is padding.
    #[test]
    fn an_alignment_moves_the_next_label_and_pads_in_front_of_the_next_byte() {
        let pieces =
            read(".data\nx:\n.byte 1\n.balign 4\n.byte 2\n").expect("the template is read");
        assert_eq!(pieces.len(), 1);
        assert_eq!(image(&pieces[0]), [1, 0, 0, 0, 2]);

        let pieces = read(".data\nx:\n.byte 1\n.balign 4\ny:\n.byte 2\n").expect("read");
        assert_eq!(pieces.len(), 2);
        assert_eq!(image(&pieces[0]), [1]);
        assert_eq!(pieces[1].align, 4, "the label took the alignment rather than the bytes");
        assert_eq!(pieces[0].align, 4, "and the first of the section carries the largest");
    }

    /// A name is internal unless something exported it, which is what an assembler does.
    #[test]
    fn a_label_nothing_exported_is_one_the_linker_does_not_see() {
        let pieces = read(".data\nx:\n.byte 1\n.globl y\ny:\n.byte 2\n").expect("read");
        assert_eq!(pieces[0].linkage, Linkage::Internal);
        assert_eq!(pieces[1].linkage, Linkage::External);

        let pieces = read(".data\n.weak w\n.hidden w\nw:\n.byte 1\n").expect("read");
        assert_eq!(pieces[0].linkage, Linkage::Weak);
        assert_eq!(pieces[0].visibility, Visibility::Hidden);
    }

    /// The sections the four bare directives name, and one the template named itself.
    #[test]
    fn the_section_a_label_is_in_is_the_one_the_directive_before_it_named() {
        let template = ".section .rodata\na:\n.byte 1\n.data\nb:\n.byte 2\n\
                        .bss\nc:\n.zero 8\n.section .init_array\nd:\n.quad 0\n";
        let pieces = read(template).expect("the template is read");
        let sections: Vec<&Section> = pieces.iter().map(|piece| &piece.section).collect();
        assert_eq!(
            sections,
            [
                &Section::ReadOnly,
                &Section::Data,
                &Section::Bss,
                &Section::Named(".init_array".to_owned()),
            ]
        );
        assert_eq!(pieces[2].items, [Item::Zero(8)], "a section with no image says how much");
    }

    /// The arithmetic an assembler does, which is more than the one subtraction incbin needs.
    #[test]
    fn an_expression_is_worked_out_the_way_an_assembler_works_one_out() {
        let cases = [
            (".byte 1 + 2 * 3", vec![7u8]),
            (".byte (1 + 2) * 3", vec![9]),
            (".byte 0x10", vec![16]),
            (".byte 0b101", vec![5]),
            (".byte 010", vec![8]),
            (".byte 0", vec![0]),
            (".byte 'A'", vec![65]),
            (".byte ~0 & 0xff", vec![255]),
            (".byte 1 << 3", vec![8]),
            (".byte -1 & 3", vec![3]),
            (".short 0x1234", vec![0x34, 0x12]),
        ];
        for (directive, want) in cases {
            let pieces = read(&format!(".data\nx:\n{directive}\n")).expect(directive);
            assert_eq!(image(&pieces[0]), want, "{directive}");
        }
    }

    /// A string directive is bytes, and `.asciz` is the same bytes with a terminator.
    #[test]
    fn a_string_directive_is_the_bytes_of_the_string_it_was_given() {
        let pieces = read(".data\nx:\n.ascii \"hi\"\n.asciz \"yo\"\n").expect("read");
        assert_eq!(image(&pieces[0]), b"hiyo\0");

        let pieces = read(".data\nx:\n.string \"a\\nb\"\n").expect("read");
        assert_eq!(image(&pieces[0]), b"a\nb\0");
    }

    /// Comments and semicolons, since a template written in C is one long string either way.
    #[test]
    fn a_comment_and_a_semicolon_end_what_an_assembler_says_they_end() {
        let pieces =
            read(".data; x: ; .byte 1 # and the rest of the line\n.byte 2\n").expect("read");
        assert_eq!(image(&pieces[0]), [1, 2]);

        let pieces = read(".data\nx:\n/* nothing */ .byte 3\n").expect("read");
        assert_eq!(image(&pieces[0]), [3]);
    }

    /// An instruction is the thing this does not do, and the message says which one it was.
    #[test]
    fn a_template_with_an_instruction_in_it_is_refused_by_name() {
        let failed = read(".text\nf:\n movq %rsp, %rbp\n ret\n").expect_err("it is refused");
        assert_eq!(failed, Failed::Unsupported("the instruction 'movq'".to_owned()));
    }

    /// The rest of what is refused, each with a message that names what was written.
    #[test]
    fn what_a_reader_of_directives_does_not_do_is_refused_rather_than_guessed_at() {
        let cases = [
            (".data\nx:\n.byte 1\n.byte 1\nx:\n", "the label 'x' written twice"),
            (".data\n.byte 1\n", "data at file scope under no label"),
            (".text\nx:\n.byte 1\n", "data in the text section at file scope"),
            (".text\nx:\n", "the label 'x' in the text section"),
            (".bss\nx:\n.byte 1\n", "data in a section that carries none"),
            (".data\nx:\n.quad elsewhere\n", "the name 'elsewhere' in an expression"),
            (".data\nx:\n.cfi_startproc\n", "the '.cfi_startproc' directive at file scope"),
            (".data\n.balign 3\nx:\n.byte 1\n", "an alignment of 3"),
        ];
        for (template, want) in cases {
            let failed = read(template).expect_err(template);
            assert_eq!(failed, Failed::Unsupported(want.to_owned()), "{template}");
        }
    }

    /// A file that is not there is its own answer, because the program is right and the build
    /// is wrong, and the message a reader needs names the file rather than the directive.
    #[test]
    fn a_file_incbin_names_and_cannot_read_is_reported_as_the_file_it_is() {
        let failed = read(".data\nx:\n.incbin \"missing.bin\"\n").expect_err("it is refused");
        assert_eq!(
            failed,
            Failed::Missing("missing.bin".to_owned(), "no file 'missing.bin'".to_owned())
        );
    }

    /// `.size` is held against what was written under the name rather than believed.
    #[test]
    fn a_size_directive_that_disagrees_with_the_bytes_under_it_is_refused() {
        let pieces = read(".data\nx:\n.byte 1\n.byte 2\n.size x, .-x\n").expect("read");
        assert_eq!(pieces[0].size, 2);

        let failed = read(".data\nx:\n.byte 1\n.size x, 4\n").expect_err("it is refused");
        assert_eq!(
            failed,
            Failed::Unsupported(
                "a '.size' of 'x' that is not what was written under it".to_owned()
            )
        );
    }

    /// An empty template is no globals rather than a refusal, since a program writes one.
    #[test]
    fn a_template_with_nothing_in_it_defines_nothing() {
        for template in ["", "\n\t\n", ".text\n"] {
            assert!(read(template).expect("read").is_empty(), "{template:?}");
        }
    }
}