rigless 0.2.1

// Copyright (c) 2026 Rigless Labs.
// SPDX-License-Identifier: MIT OR Apache-2.0
//! GDB's agent expressions, "Debugging with GDB" for GDB 18.1, appendix F: the bytecode a
//! breakpoint's conditions and commands come in, run on a stack of 64-bit integers. A register is
//! read as `g` reads it and memory as a debugger peeks it, so an expression changes nothing in the
//! run, and `printf` writes to what it's given. The trace and trace-variable bytecodes and floating
//! point aren't here, since the server keeps no trace, and an expression that uses one fails.
// ceiling: a stack of 64 and 10_000 bytecodes an evaluation; upgrade when a condition needs more

use super::packet::PACKET_SIZE;
use crate::emulator::DataModel;
use crate::soc::Width;

/// The most values the stack holds.
const STACK: usize = 64;

/// The most bytecodes one evaluation runs, so a loop ends.
const STEPS: u32 = 10_000;

/// The longest string `printf`'s `%s` reads before its NUL.
const LONGEST_STRING: u64 = 256;

/// The widest a conversion's width or precision is: more than one packet holds is no use to GDB.
const WIDEST: usize = PACKET_SIZE;

/// What an expression reads: a register by GDB's number, and memory.
pub(super) trait Inspect {
    /// The register GDB numbers `number`, or `None` for one there isn't.
    fn register(&mut self, number: u16) -> Option<u64>;
    /// The value of `width` at `address`, or `None` where nothing answers.
    fn read(&mut self, address: u64, width: Width) -> Option<u64>;
    /// How the family lays out its data, which says how wide a `long` is.
    fn data_model(&mut self) -> DataModel;
}

/// Why an expression couldn't be evaluated, which appendix F calls terminating with an error.
#[derive(Clone, Debug, PartialEq, Eq)]
pub(super) enum Failed {
    /// A bytecode this agent doesn't run, or none appendix F gives.
    Bytecode(u8),
    /// The expression ran off its end, or a bytecode's operand did.
    Truncated,
    /// The stack ran out, or past [`STACK`].
    Stack,
    /// A division or remainder by zero.
    DivideByZero,
    /// Nothing answers at this address.
    Memory(u64),
    /// No register with this number.
    Register(u16),
    /// Past [`STEPS`] bytecodes.
    TooLong,
    /// A `printf` format this agent can't follow.
    Format,
}

impl std::fmt::Display for Failed {
    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Bytecode(code) => write!(formatter, "bytecode {code:#04x} isn't one it runs"),
            Self::Truncated => write!(formatter, "it ends partway through a bytecode"),
            Self::Stack => write!(formatter, "its stack ran out or over {STACK}"),
            Self::DivideByZero => write!(formatter, "it divides by zero"),
            Self::Memory(address) => write!(formatter, "nothing answers at {address:#x}"),
            Self::Register(number) => write!(formatter, "there's no register {number}"),
            Self::TooLong => write!(formatter, "it ran past {STEPS} bytecodes"),
            Self::Format => write!(formatter, "its printf format isn't one it follows"),
        }
    }
}

/// Each bytecode this agent runs, by its number in "Bytecode Descriptions".
mod code {
    pub(super) const ADD: u8 = 0x02;
    pub(super) const SUB: u8 = 0x03;
    pub(super) const MUL: u8 = 0x04;
    pub(super) const DIV_SIGNED: u8 = 0x05;
    pub(super) const DIV_UNSIGNED: u8 = 0x06;
    pub(super) const REM_SIGNED: u8 = 0x07;
    pub(super) const REM_UNSIGNED: u8 = 0x08;
    pub(super) const LSH: u8 = 0x09;
    pub(super) const RSH_SIGNED: u8 = 0x0A;
    pub(super) const RSH_UNSIGNED: u8 = 0x0B;
    pub(super) const LOG_NOT: u8 = 0x0E;
    pub(super) const BIT_AND: u8 = 0x0F;
    pub(super) const BIT_OR: u8 = 0x10;
    pub(super) const BIT_XOR: u8 = 0x11;
    pub(super) const BIT_NOT: u8 = 0x12;
    pub(super) const EQUAL: u8 = 0x13;
    pub(super) const LESS_SIGNED: u8 = 0x14;
    pub(super) const LESS_UNSIGNED: u8 = 0x15;
    pub(super) const EXT: u8 = 0x16;
    pub(super) const REF8: u8 = 0x17;
    pub(super) const REF16: u8 = 0x18;
    pub(super) const REF32: u8 = 0x19;
    pub(super) const REF64: u8 = 0x1A;
    pub(super) const IF_GOTO: u8 = 0x20;
    pub(super) const GOTO: u8 = 0x21;
    pub(super) const CONST8: u8 = 0x22;
    pub(super) const CONST16: u8 = 0x23;
    pub(super) const CONST32: u8 = 0x24;
    pub(super) const CONST64: u8 = 0x25;
    pub(super) const REG: u8 = 0x26;
    pub(super) const END: u8 = 0x27;
    pub(super) const DUP: u8 = 0x28;
    pub(super) const POP: u8 = 0x29;
    pub(super) const ZERO_EXT: u8 = 0x2A;
    pub(super) const SWAP: u8 = 0x2B;
    pub(super) const PICK: u8 = 0x32;
    pub(super) const ROT: u8 = 0x33;
    pub(super) const PRINTF: u8 = 0x34;
}

/// The stack and where the expression has got to.
struct Machine<'a> {
    code: &'a [u8],
    at: usize,
    stack: Vec<u64>,
}

impl Machine<'_> {
    fn pop(&mut self) -> Result<u64, Failed> {
        self.stack.pop().ok_or(Failed::Stack)
    }

    fn push(&mut self, value: u64) -> Result<(), Failed> {
        if self.stack.len() == STACK {
            return Err(Failed::Stack);
        }
        self.stack.push(value);
        Ok(())
    }

    /// The next `bytes` of the expression as an operand, most significant byte first.
    fn operand(&mut self, bytes: usize) -> Result<u64, Failed> {
        let end = self.at.checked_add(bytes).ok_or(Failed::Truncated)?;
        let field = self.code.get(self.at..end).ok_or(Failed::Truncated)?;
        self.at = end;
        Ok(field
            .iter()
            .fold(0, |value, &byte| value << 8 | u64::from(byte)))
    }

    /// Pops `b`, then `a`, and pushes what `with` makes of them.
    fn binary(&mut self, with: Binary) -> Result<(), Failed> {
        let b = self.pop()?;
        let a = self.pop()?;
        self.push(with(a, b)?)
    }
}

/// Evaluates `expression` reading through `inspect`, with what its `printf`s print appended to
/// `printed`, and answers the value on top of the stack at its `end`, or 0 for an empty stack.
pub(super) fn evaluate(
    expression: &[u8],
    inspect: &mut dyn Inspect,
    printed: &mut Vec<u8>,
) -> Result<u64, Failed> {
    let mut machine = Machine {
        code: expression,
        at: 0,
        stack: Vec::new(),
    };
    for _ in 0..STEPS {
        let &bytecode = machine.code.get(machine.at).ok_or(Failed::Truncated)?;
        machine.at += 1;
        if let Some(operation) = binary(bytecode) {
            machine.binary(operation)?;
            continue;
        }
        match bytecode {
            code::END => return Ok(machine.stack.last().copied().unwrap_or(0)),
            code::LOG_NOT => {
                let a = machine.pop()?;
                machine.push(u64::from(a == 0))?;
            }
            code::BIT_NOT => {
                let a = machine.pop()?;
                machine.push(!a)?;
            }
            code::EXT => {
                let bits = machine.operand(1)?;
                let a = machine.pop()?;
                machine.push(sign_extend(a, bits))?;
            }
            code::ZERO_EXT => {
                let bits = machine.operand(1)?;
                let a = machine.pop()?;
                machine.push(if bits >= 64 { a } else { a & ((1 << bits) - 1) })?;
            }
            code::REF8 | code::REF16 | code::REF32 | code::REF64 => {
                let width = match bytecode {
                    code::REF8 => Width::W8,
                    code::REF16 => Width::W16,
                    code::REF32 => Width::W32,
                    _ => Width::W64,
                };
                let address = machine.pop()?;
                let value = inspect
                    .read(address, width)
                    .ok_or(Failed::Memory(address))?;
                machine.push(value)?;
            }
            code::IF_GOTO | code::GOTO => {
                let to =
                    usize::try_from(machine.operand(2)?).map_err(|_overflow| Failed::Truncated)?;
                if bytecode == code::GOTO || machine.pop()? != 0 {
                    machine.at = to;
                }
            }
            code::CONST8 | code::CONST16 | code::CONST32 | code::CONST64 => {
                let bytes = 1 << (bytecode - code::CONST8);
                let value = machine.operand(bytes)?;
                machine.push(value)?;
            }
            code::REG => {
                let number = u16::try_from(machine.operand(2)?).expect("two bytes");
                let value = inspect.register(number).ok_or(Failed::Register(number))?;
                machine.push(value)?;
            }
            code::DUP => {
                let a = *machine.stack.last().ok_or(Failed::Stack)?;
                machine.push(a)?;
            }
            code::POP => {
                machine.pop()?;
            }
            code::SWAP => {
                let b = machine.pop()?;
                let a = machine.pop()?;
                machine.push(b)?;
                machine.push(a)?;
            }
            code::PICK => {
                let depth = usize::try_from(machine.operand(1)?).expect("a byte");
                let index = machine
                    .stack
                    .len()
                    .checked_sub(depth + 1)
                    .ok_or(Failed::Stack)?;
                let value = machine.stack[index];
                machine.push(value)?;
            }
            code::ROT => {
                let c = machine.pop()?;
                let b = machine.pop()?;
                let a = machine.pop()?;
                for value in [c, a, b] {
                    machine.push(value)?;
                }
            }
            code::PRINTF => printf(&mut machine, inspect, printed)?,
            other => return Err(Failed::Bytecode(other)),
        }
    }
    Err(Failed::TooLong)
}

/// What a bytecode that pops two values and pushes one does, given `a`, pushed first, and `b`.
type Binary = fn(u64, u64) -> Result<u64, Failed>;

/// The bytecode's [`Binary`], if it's one.
fn binary(bytecode: u8) -> Option<Binary> {
    Some(match bytecode {
        code::ADD => |a, b| Ok(a.wrapping_add(b)),
        code::SUB => |a, b| Ok(a.wrapping_sub(b)),
        code::MUL => |a, b| Ok(a.wrapping_mul(b)),
        // The least `i64` over -1 wraps, as the target's own division does.
        code::DIV_SIGNED => |a: u64, b: u64| {
            let (a, b) = (a.cast_signed(), b.cast_signed());
            (b != 0)
                .then(|| a.wrapping_div(b).cast_unsigned())
                .ok_or(Failed::DivideByZero)
        },
        code::DIV_UNSIGNED => |a: u64, b| a.checked_div(b).ok_or(Failed::DivideByZero),
        code::REM_SIGNED => |a: u64, b: u64| {
            let (a, b) = (a.cast_signed(), b.cast_signed());
            (b != 0)
                .then(|| a.wrapping_rem(b).cast_unsigned())
                .ok_or(Failed::DivideByZero)
        },
        code::REM_UNSIGNED => |a: u64, b| a.checked_rem(b).ok_or(Failed::DivideByZero),
        code::LSH => |a: u64, b| Ok(a.checked_shl(shift(b)).unwrap_or(0)),
        code::RSH_SIGNED => |a: u64, b| Ok((a.cast_signed() >> shift(b).min(63)).cast_unsigned()),
        code::RSH_UNSIGNED => |a: u64, b| Ok(a.checked_shr(shift(b)).unwrap_or(0)),
        code::BIT_AND => |a, b| Ok(a & b),
        code::BIT_OR => |a, b| Ok(a | b),
        code::BIT_XOR => |a, b| Ok(a ^ b),
        code::EQUAL => |a, b| Ok(u64::from(a == b)),
        code::LESS_SIGNED => |a: u64, b: u64| Ok(u64::from(a.cast_signed() < b.cast_signed())),
        code::LESS_UNSIGNED => |a, b| Ok(u64::from(a < b)),
        _ => return None,
    })
}

/// A shift's count, past 64 shifting everything out.
fn shift(count: u64) -> u32 {
    u32::try_from(count).unwrap_or(u32::MAX)
}

/// `value` sign-extended from its low `bits`, which at 64 or more leaves it as it is.
fn sign_extend(value: u64, bits: u64) -> u64 {
    if bits == 0 || bits >= 64 {
        return value;
    }
    let unused = 64 - bits;
    ((value << unused).cast_signed() >> unused).cast_unsigned()
}

/// `printf`: the count of arguments and the format, two bytes of its length then its bytes and a
/// NUL, which pops the function and the channel GDB pushed last, then each argument, the first on
/// top, and appends what the format makes of them.
fn printf(
    machine: &mut Machine<'_>,
    inspect: &mut dyn Inspect,
    printed: &mut Vec<u8>,
) -> Result<(), Failed> {
    let count = usize::try_from(machine.operand(1)?).expect("a byte");
    let length = usize::try_from(machine.operand(2)?).expect("two bytes");
    let end = machine.at.checked_add(length).ok_or(Failed::Truncated)?;
    let format = machine.code.get(machine.at..end).ok_or(Failed::Truncated)?;
    machine.at = end;
    let format = format.strip_suffix(&[0]).ok_or(Failed::Format)?;
    // The function and the channel, which say where a target's own printf would print.
    machine.pop()?;
    machine.pop()?;
    let arguments = (0..count)
        .map(|_| machine.pop())
        .collect::<Result<Vec<u64>, Failed>>()?;
    let format = unescape(format).ok_or(Failed::Format)?;
    format_into(&format, &arguments, inspect, printed)
}

/// `text` with the escapes C source writes in a string, which GDB sends as they're written, as the
/// bytes they stand for.
fn unescape(text: &[u8]) -> Option<Vec<u8>> {
    let mut bytes = Vec::with_capacity(text.len());
    let mut rest = text.iter().copied().peekable();
    while let Some(byte) = rest.next() {
        if byte != b'\\' {
            bytes.push(byte);
            continue;
        }
        let escaped = rest.next()?;
        bytes.push(match escaped {
            b'n' => b'\n',
            b't' => b'\t',
            b'r' => b'\r',
            b'a' => 0x07,
            b'b' => 0x08,
            b'f' => 0x0C,
            b'v' => 0x0B,
            b'e' => 0x1B,
            b'0'..=b'7' => {
                let mut value = u32::from(escaped - b'0');
                for _ in 0..2 {
                    match rest.peek() {
                        Some(&digit @ b'0'..=b'7') => {
                            value = value * 8 + u32::from(digit - b'0');
                            rest.next();
                        }
                        _ => break,
                    }
                }
                u8::try_from(value).ok()?
            }
            b'x' => {
                let mut value = 0u32;
                let mut digits = 0;
                while let Some(digit) = rest.peek().and_then(|&byte| char::from(byte).to_digit(16))
                {
                    value = value.checked_mul(16)? + digit;
                    digits += 1;
                    rest.next();
                }
                if digits == 0 {
                    return None;
                }
                u8::try_from(value).ok()?
            }
            other => other,
        });
    }
    Some(bytes)
}

/// A conversion's flags, width and precision, as C's printf reads them.
#[derive(Default)]
struct Spec {
    left: bool,
    zero: bool,
    /// `+` or a space before a signed number that isn't negative, or nothing.
    sign: &'static [u8],
    alternate: bool,
    width: usize,
    precision: Option<usize>,
}

impl Spec {
    /// `digits`, a number's digits, after `sign` and `prefix`, padded as the spec says.
    fn pad(&self, digits: &[u8], sign: &[u8], prefix: &[u8]) -> Vec<u8> {
        let mut body: Vec<u8> = match self.precision {
            // C prints nothing for zero at a precision of zero.
            Some(0) if digits == b"0" => Vec::new(),
            Some(precision) => {
                let mut body = vec![b'0'; precision.saturating_sub(digits.len())];
                body.extend_from_slice(digits);
                body
            }
            None => digits.to_vec(),
        };
        let fill = self
            .width
            .saturating_sub(sign.len() + prefix.len() + body.len());
        let zeros = self.zero && !self.left && self.precision.is_none();
        let mut padded = Vec::with_capacity(fill + sign.len() + prefix.len() + body.len());
        if !self.left && !zeros {
            padded.resize(fill, b' ');
        }
        padded.extend_from_slice(sign);
        padded.extend_from_slice(prefix);
        if zeros {
            padded.resize(padded.len() + fill, b'0');
        }
        padded.append(&mut body);
        if self.left {
            padded.resize(padded.len() + fill, b' ');
        }
        padded
    }

    /// `text` padded to the width.
    fn pad_text(&self, text: &[u8]) -> Vec<u8> {
        let fill = self.width.saturating_sub(text.len());
        let mut padded = Vec::with_capacity(fill + text.len());
        if !self.left {
            padded.resize(fill, b' ');
        }
        padded.extend_from_slice(text);
        if self.left {
            padded.resize(padded.len() + fill, b' ');
        }
        padded
    }
}

/// Formats `arguments` into `printed` as C's printf does with `format`: `%d`, `%i`, `%u`, `%x`,
/// `%X`, `%o`, `%c`, `%s` from memory, `%p` and `%%`, with flags, a width, a precision and a
/// length, a `long` as wide as the family's data model has it. A width or a precision past
/// [`WIDEST`] fails.
fn format_into(
    format: &[u8],
    arguments: &[u64],
    inspect: &mut dyn Inspect,
    printed: &mut Vec<u8>,
) -> Result<(), Failed> {
    let mut arguments = arguments.iter().copied();
    let mut rest = format.iter().copied().peekable();
    while let Some(byte) = rest.next() {
        if byte != b'%' {
            printed.push(byte);
            continue;
        }
        let mut spec = Spec::default();
        while let Some(&flag) = rest.peek() {
            match flag {
                b'-' => spec.left = true,
                b'0' => spec.zero = true,
                b'+' => spec.sign = b"+",
                // C ignores a space when there's a `+` too.
                b' ' if spec.sign.is_empty() => spec.sign = b" ",
                b' ' => {}
                b'#' => spec.alternate = true,
                _ => break,
            }
            rest.next();
        }
        let number = |rest: &mut std::iter::Peekable<_>| {
            let mut value = 0usize;
            while let Some(digit) = rest
                .peek()
                .and_then(|&byte: &u8| char::from(byte).to_digit(10))
            {
                value = value.saturating_mul(10).saturating_add(digit as usize);
                rest.next();
            }
            (value <= WIDEST).then_some(value).ok_or(Failed::Format)
        };
        spec.width = number(&mut rest)?;
        if rest.peek() == Some(&b'.') {
            rest.next();
            spec.precision = Some(number(&mut rest)?);
        }
        let (mut halves, mut longs, mut widest) = (0, 0, false);
        while let Some(&length) = rest.peek() {
            match length {
                b'h' => halves += 1,
                b'l' => longs += 1,
                b'j' | b'q' => widest = true,
                b'z' | b't' => {}
                _ => break,
            }
            rest.next();
        }
        let bits = match (halves, longs) {
            _ if widest || longs >= 2 => 64,
            (0, 1) => u64::from(inspect.data_model().long().bits()),
            (0, _) => 32,
            (1, _) => 16,
            _ => 8,
        };
        let conversion = rest.next().ok_or(Failed::Format)?;
        if conversion == b'%' {
            printed.push(b'%');
            continue;
        }
        let value = arguments.next().ok_or(Failed::Format)?;
        let unsigned = if bits == 64 {
            value
        } else {
            value & ((1 << bits) - 1)
        };
        let text = match conversion {
            b'd' | b'i' => {
                let signed = sign_extend(value, bits).cast_signed();
                let sign = if signed < 0 { b"-" } else { spec.sign };
                spec.pad(signed.unsigned_abs().to_string().as_bytes(), sign, b"")
            }
            b'u' => spec.pad(unsigned.to_string().as_bytes(), b"", b""),
            b'x' | b'X' => {
                let digits = if conversion == b'X' {
                    format!("{unsigned:X}")
                } else {
                    format!("{unsigned:x}")
                };
                let prefix: &[u8] = match (spec.alternate && unsigned != 0, conversion) {
                    (false, _) => b"",
                    (true, b'X') => b"0X",
                    (true, _) => b"0x",
                };
                spec.pad(digits.as_bytes(), b"", prefix)
            }
            b'o' => {
                let digits = format!("{unsigned:o}");
                // `#` makes the first digit a zero, raising the precision only if it has to.
                if spec.alternate && !digits.starts_with('0') {
                    spec.precision = Some(spec.precision.unwrap_or(0).max(digits.len() + 1));
                } else if spec.alternate {
                    spec.precision = Some(spec.precision.unwrap_or(1).max(1));
                }
                spec.pad(digits.as_bytes(), b"", b"")
            }
            b'p' => spec.pad(format!("{value:x}").as_bytes(), b"", b"0x"),
            b'c' => spec.pad_text(&[u8::try_from(value & 0xFF).expect("a byte")]),
            b's' => {
                let longest = spec
                    .precision
                    .map_or(LONGEST_STRING, |precision| precision as u64);
                let mut bytes = Vec::new();
                for offset in 0..longest {
                    let address = value.checked_add(offset).ok_or(Failed::Memory(value))?;
                    match inspect
                        .read(address, Width::W8)
                        .ok_or(Failed::Memory(address))?
                    {
                        0 => break,
                        byte => bytes.push(u8::try_from(byte).expect("a byte")),
                    }
                }
                spec.pad_text(&bytes)
            }
            _ => return Err(Failed::Format),
        };
        printed.extend_from_slice(&text);
    }
    Ok(())
}

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

    /// Registers that read as their number times ten, and memory that holds `bytes` from 0x100.
    struct Fake {
        bytes: Vec<u8>,
        model: DataModel,
    }

    impl Inspect for Fake {
        fn register(&mut self, number: u16) -> Option<u64> {
            (number < 40).then_some(u64::from(number) * 10)
        }

        fn data_model(&mut self) -> DataModel {
            self.model
        }

        fn read(&mut self, address: u64, width: Width) -> Option<u64> {
            let start = usize::try_from(address.checked_sub(0x100)?).ok()?;
            let bytes = self.bytes.get(start..start + width.bytes() as usize)?;
            Some(
                bytes
                    .iter()
                    .rev()
                    .fold(0, |value, &byte| value << 8 | u64::from(byte)),
            )
        }
    }

    fn run(code: &[u8]) -> Result<u64, Failed> {
        let mut fake = Fake {
            bytes: vec![0x78, 0x56, 0x34, 0x12, 0xFE, 0xFF, b'h', b'i', 0],
            model: DataModel::Ilp32,
        };
        evaluate(code, &mut fake, &mut Vec::new())
    }

    const MINUS_ONE: u64 = u64::MAX;

    /// Each bytecode "Bytecode Descriptions" gives, against what it leaves on the stack.
    #[test]
    fn each_opcode_does_what_f_2_says() {
        use code::*;
        let c8 = |n: u8| [CONST8, n];
        let both = |a: u8, b: u8, op: u8| [CONST8, a, CONST8, b, op, END];
        for (code, expected) in [
            (both(7, 5, ADD).to_vec(), 12),
            (both(7, 5, SUB).to_vec(), 2),
            (both(5, 7, SUB).to_vec(), (-2i64).cast_unsigned()),
            (both(7, 5, MUL).to_vec(), 35),
            (both(7, 2, DIV_UNSIGNED).to_vec(), 3),
            (both(7, 2, REM_UNSIGNED).to_vec(), 1),
            (both(1, 4, LSH).to_vec(), 16),
            (both(16, 4, RSH_UNSIGNED).to_vec(), 1),
            (both(6, 3, BIT_AND).to_vec(), 2),
            (both(6, 3, BIT_OR).to_vec(), 7),
            (both(6, 3, BIT_XOR).to_vec(), 5),
            (both(6, 6, EQUAL).to_vec(), 1),
            (both(6, 7, EQUAL).to_vec(), 0),
            (both(6, 7, LESS_UNSIGNED).to_vec(), 1),
            ([&c8(0)[..], &[LOG_NOT, END]].concat(), 1),
            ([&c8(9)[..], &[LOG_NOT, END]].concat(), 0),
            ([&c8(0)[..], &[BIT_NOT, END]].concat(), MINUS_ONE),
            ([&c8(0xFF)[..], &[EXT, 8, END]].concat(), MINUS_ONE),
            ([&c8(0x7F)[..], &[EXT, 8, END]].concat(), 0x7F),
            (
                [&[CONST16, 0x12, 0x34][..], &[ZERO_EXT, 8, END]].concat(),
                0x34,
            ),
            (vec![CONST32, 0x80, 0, 0, 0, END], 0x8000_0000),
            (
                vec![CONST64, 1, 2, 3, 4, 5, 6, 7, 8, END],
                0x0102_0304_0506_0708,
            ),
            (vec![REG, 0, 3, END], 30),
            (vec![CONST16, 1, 0, REF8, END], 0x78),
            (vec![CONST16, 1, 0, REF16, END], 0x5678),
            (vec![CONST16, 1, 0, REF32, END], 0x1234_5678),
            (
                vec![CONST16, 1, 4, REF16, EXT, 16, END],
                (-2i64).cast_unsigned(),
            ),
            (both(1, 2, SWAP).to_vec(), 1),
            ([&c8(1)[..], &[DUP, ADD, END]].concat(), 2),
            ([&c8(1)[..], &c8(2), &[POP, END]].concat(), 1),
            ([&c8(1)[..], &c8(2), &c8(3), &[PICK, 2, END]].concat(), 1),
            ([&c8(1)[..], &c8(2), &c8(3), &[ROT, END]].concat(), 2),
            ([&c8(1)[..], &c8(2), &c8(3), &[ROT, POP, END]].concat(), 1),
            (
                [&c8(1)[..], &[IF_GOTO, 0, 8, CONST8, 9, END, CONST8, 4, END]].concat(),
                4,
            ),
            (
                [&c8(0)[..], &[IF_GOTO, 0, 8, CONST8, 9, END, CONST8, 4, END]].concat(),
                9,
            ),
            (vec![GOTO, 0, 5, CONST8, 9, CONST8, 4, END], 4),
            (vec![END], 0),
        ] {
            assert_eq!(run(&code), Ok(expected), "{code:02x?}");
        }
        let minus = |n: u8| [CONST8, n.wrapping_neg(), EXT, 8];
        let least = [&[CONST64][..], &i64::MIN.to_be_bytes()].concat();
        let minus_one = [CONST8, 0xFF, EXT, 8];
        for (op, expected) in [(DIV_SIGNED, i64::MIN), (REM_SIGNED, 0)] {
            let code = [&least[..], &minus_one, &[op, END]].concat();
            assert_eq!(run(&code), Ok(expected.cast_unsigned()), "{op:#x} wraps");
        }
        for (op, expected) in [
            (DIV_SIGNED, -3i64),
            (REM_SIGNED, -1),
            (LESS_SIGNED, 1),
            (RSH_SIGNED, -2),
        ] {
            let code = [&minus(7)[..], &[CONST8, 2, op, END]].concat();
            assert_eq!(run(&code), Ok(expected.cast_unsigned()), "{op:#x}");
        }
    }

    /// An expression that can't finish fails, and says why.
    #[test]
    fn an_expression_that_cant_finish_fails() {
        use code::*;
        assert_eq!(
            run(&[CONST8, 1, CONST8, 0, DIV_UNSIGNED, END]),
            Err(Failed::DivideByZero)
        );
        assert_eq!(
            run(&[CONST8, 1, CONST8, 0, REM_SIGNED, END]),
            Err(Failed::DivideByZero)
        );
        assert_eq!(
            run(&[CONST16, 0x20, 0, REF32, END]),
            Err(Failed::Memory(0x2000))
        );
        assert_eq!(run(&[REG, 0, 99, END]), Err(Failed::Register(99)));
        assert_eq!(run(&[ADD, END]), Err(Failed::Stack));
        assert_eq!(run(&[0x0C, END]), Err(Failed::Bytecode(0x0C)), "trace");
        assert_eq!(run(&[0x01, END]), Err(Failed::Bytecode(0x01)), "float");
        assert_eq!(run(&[CONST16, 1]), Err(Failed::Truncated));
        assert_eq!(run(&[CONST8, 1]), Err(Failed::Truncated), "no end");
        assert_eq!(run(&[GOTO, 0, 0]), Err(Failed::TooLong));
        let full: Vec<u8> = std::iter::repeat_n([CONST8, 1], STACK + 1)
            .flatten()
            .collect();
        assert_eq!(run(&full), Err(Failed::Stack));
    }

    /// A `long` is as wide as the family's data model has it: 32 bits on one with 32-bit pointers,
    /// so `%ld` takes the argument's low word, and 64 on one with 64-bit pointers.
    #[test]
    fn a_long_is_as_wide_as_the_data_model_says() {
        let print = |model| {
            let mut fake = Fake {
                bytes: Vec::new(),
                model,
            };
            let mut printed = Vec::new();
            format_into(b"%ld", &[0x1_0000_0001], &mut fake, &mut printed).unwrap();
            String::from_utf8(printed).unwrap()
        };
        assert_eq!(print(DataModel::Ilp32), "1");
        assert_eq!(print(DataModel::Lp64), "4294967297");
    }

    /// `printf` pops the channel and the function, then its arguments, the first on top, and
    /// prints them as C's printf would, the format's escapes as C source writes them.
    #[test]
    fn printf_formats_as_c_does() {
        let print = |format: &str, arguments: &[u64]| {
            let mut code = Vec::new();
            for &argument in arguments.iter().rev() {
                code.push(code::CONST64);
                code.extend_from_slice(&argument.to_be_bytes());
            }
            code.extend([code::CONST8, 0, code::CONST8, 0, code::PRINTF]);
            let format = [format.as_bytes(), &[0]].concat();
            code.push(u8::try_from(arguments.len()).unwrap());
            code.extend_from_slice(&u16::try_from(format.len()).unwrap().to_be_bytes());
            code.extend(format);
            code.push(code::END);
            let mut fake = Fake {
                bytes: b"\0\0\0\0\0\0hi\0".to_vec(),
                model: DataModel::Ilp32,
            };
            let mut printed = Vec::new();
            evaluate(&code, &mut fake, &mut printed).map(|_| printed)
        };
        let printed = |format: &str, arguments: &[u64]| {
            String::from_utf8(print(format, arguments).unwrap()).unwrap()
        };
        assert_eq!(printed("tick %d\\n", &[5]), "tick 5\n");
        assert_eq!(printed("%d %i", &[u64::MAX, 0xFFFF_FFFF]), "-1 -1");
        assert_eq!(printed("%u %lld", &[u64::MAX, u64::MAX]), "4294967295 -1");
        assert_eq!(
            printed("%x %#X %08x %o", &[255, 255, 0xAB, 8]),
            "ff 0XFF 000000ab 10"
        );
        assert_eq!(printed("%hhd %hu", &[0xFF, 0x1_0002]), "-1 2");
        assert_eq!(
            printed("[%5d|%-5d|%+d]", &[42, 42, 42]),
            "[   42|42   |+42]"
        );
        assert_eq!(
            printed("%+u %+x % u", &[7, 7, 7]),
            "7 7 7",
            "a sign is for d and i"
        );
        assert_eq!(
            printed("%#o %#o %#.0o %#5o", &[0, 8, 0, 8]),
            "0 010 0   010",
            "the first digit a zero, once"
        );
        assert_eq!(printed("%c%s%%", &[u64::from(b'>'), 0x106]), ">hi%");
        assert_eq!(print("%c", &[0xE9]).unwrap(), [0xE9], "a byte as it is");
        assert_eq!(printed("%.1s|%4s", &[0x106, 0x106]), "h|  hi");
        assert_eq!(printed("%p", &[0x2000_0000]), "0x20000000");
        assert_eq!(printed("\\t\\101\\x42\\\\", &[]), "\tAB\\");
        assert_eq!(print("%s", &[0x3000]), Err(Failed::Memory(0x3000)));
        assert_eq!(
            print("%d", &[]),
            Err(Failed::Format),
            "a conversion with no argument"
        );
        assert_eq!(print("%y", &[1]), Err(Failed::Format));
        assert_eq!(
            printed(&format!("%{WIDEST}d"), &[1]).len(),
            WIDEST,
            "as wide as it goes"
        );
        for too_wide in ["%99999999999d", "%4097d", "%.4294967295s", "%.4097d"] {
            assert_eq!(print(too_wide, &[0x106]), Err(Failed::Format), "{too_wide}");
        }
    }

    proptest::proptest! {
        /// Whatever bytes come as an expression, evaluating them ends, with a value or a failure,
        /// and prints no more than its steps could.
        #[test]
        fn any_expression_ends(code in proptest::collection::vec(proptest::prelude::any::<u8>(), 0..512)) {
            let mut fake = Fake {
                bytes: vec![0x41; 64],
                model: DataModel::Ilp32,
            };
            let mut printed = Vec::new();
            evaluate(&code, &mut fake, &mut printed).ok();
            proptest::prop_assert!(printed.len() <= STEPS as usize * (WIDEST + 512));
        }
    }
}