vulcan/vasm/src/vasm_assembler.rs

use crate::ast::{Label, VASMLine};
use crate::vasm_evaluator::{eval, EvalError, Scope};
use std::collections::BTreeMap;
use std::fmt::{Display, Formatter};

#[derive(Debug, Clone, PartialEq)]
pub enum AssembleError<'a> {
    EquResolveError(i32, &'a str, EvalError<'a>),
    EquDuplicateError(i32, &'a str),
    OrgResolveError(i32, EvalError<'a>),
    ArgError(i32, EvalError<'a>),
    NoCode,
}

impl<'a> Display for AssembleError<'a> {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            AssembleError::EquResolveError(line, name, err) => {
                write!(f, "Cannot resolve .equ {} on line {}: {}", name, line, err)
            }
            AssembleError::EquDuplicateError(line, name) => {
                write!(f, "Duplicate .equ {} on line {}", name, line)
            }
            AssembleError::OrgResolveError(line, err) => {
                write!(f, "Cannot resolve .org on line {}: {}", line, err)
            }
            AssembleError::ArgError(line, err) => {
                write!(f, "Cannot calculate argument on line {}: {}", line, err)
            }
            AssembleError::NoCode => {
                write!(f, "No output would be generated by this code")
            }
        }
    }
}

/// This will solve all the .equ directives and return a symbol table of them.
/// .equ directives must be able to be solved in order, that is, in terms of
/// only preceding .equ directives. Anything else is an error.
pub fn solve_equs<'a>(lines: &[VASMLine<'a>]) -> Result<Scope<'a>, AssembleError<'a>> {
    let mut scope: Scope = Scope::new();
    let line_nums: BTreeMap<i32, i32> = BTreeMap::new();
    for (line_idx, line) in lines.iter().enumerate() {
        let line_num = (line_idx + 1) as i32;
        if let VASMLine::Equ(Label(name), expr) = line {
            let value = eval(expr, line_num as i32, &line_nums, &scope)
                .map_err(|e| AssembleError::EquResolveError(line_num as i32, name, e))?;

            if let Some(_old_value) = scope.insert(name, value) {
                return Err(AssembleError::EquDuplicateError(line_num as i32, name));
            }
        }
    }
    Ok(scope)
}

type LineLengths = BTreeMap<i32, usize>;
type LineAddresses = BTreeMap<i32, i32>;
type LineArgs = BTreeMap<i32, i32>;

fn arg_length(val: i32) -> usize {
    if val < 0 {
        3
    } else if val < 256 {
        1
    } else if val < 65536 {
        2
    } else {
        3
    }
}

/// This figures out the instruction lengths. We'll do this naively; if we can't
/// immediately tell that an instruction needs only a 0/1/2 byte argument (because it's
/// a constant, or a .equ that we've solved, or something) then we'll assume it's a
/// full 24-bit argument.
///
/// - Lines that don't represent output (.equ, .org, etc) have length 0
/// - .db directives are either strings (set aside the length of the string), or
///   numbers (set aside three bytes. If it's shorter than that it still may be a variable,
///   which might grow to be larger).
/// - Opcodes with no argument are 1 byte long.
/// - Opcodes with an argument, if that argument is a constant or decidable solely with
///   what we know right now (.equs), are however long that argument is. If we don't
///   know right now (based on a label, say) then we'll set aside the full 3 bytes (so it's
///   4 bytes long, with the instruction byte).
pub fn measure_instructions<'a>(lines: &[VASMLine<'a>], scope: &Scope) -> LineLengths {
    let line_nums: BTreeMap<i32, i32> = BTreeMap::new();
    let mut lengths = LineLengths::new();
    for (line_idx, line) in lines.iter().enumerate() {
        let line_num = (line_idx + 1) as i32;
        match line {
            VASMLine::Instruction(_, _, None) => {
                lengths.insert(line_num, 1);
            }
            VASMLine::Instruction(_, _, Some(node)) => {
                let len = eval(node, line_num as i32, &line_nums, scope).map_or(3, arg_length);
                lengths.insert(line_num, len + 1);
            }
            VASMLine::Db(_, _) => {
                lengths.insert(line_num, 3);
            }
            VASMLine::StringDb(_, value) => {
                lengths.insert(line_num, value.len());
            }
            VASMLine::Org(_, _) | VASMLine::Equ(_, _) | VASMLine::LabelDef(_) => {
                lengths.insert(line_num, 0);
            }
        }
    }
    lengths
}

/// Time to start placing labels. The tricky part here is the .org directives, which can have
/// expressions as their arguments. We'll compromise a little bit and say that a .org directive
/// can only refer to labels that precede it, so, you can use .orgs to generate (say) a jump table
/// but still make it easy for me to figure out what refers to what.
///
/// We'll go through the lines, adding each one's length (calculated in measure_instructions) to it.
/// If it has a label, we'll store that label's new value to the scope.
///
/// But, we'll skip labels that come before .equs: that would make every .equ set to its address,
/// rather than the argument.
fn place_labels<'a>(
    lines: &[VASMLine<'a>],
    scope: Scope<'a>,
    lengths: &LineLengths,
) -> Result<(LineAddresses, Scope<'a>), AssembleError<'a>> {
    let mut scope = scope;
    let mut address = 0;
    let mut addresses = LineAddresses::new();
    for (line_idx, line) in lines.iter().enumerate() {
        let line_num = (line_idx + 1) as i32;

        if let VASMLine::Org(_, expr) = line {
            address = eval(expr, line_num, &addresses, &scope)
                .map_err(|err| AssembleError::OrgResolveError(line_num, err))?;
            addresses.insert(line_num, address);
        }

        if let Some(Label(label)) = line.label() {
            if !scope.contains_key(label) {
                scope.insert(label, address as i32);
            }
        }

        match line {
            VASMLine::Org(_, _) => {}
            _ => {
                addresses.insert(line_num, address);
                address += *lengths.get(&line_num).unwrap_or(&0) as i32;
            }
        }
    }
    Ok((addresses, scope))
}

/// At this point we have everything we need to calculate the arguments, so we'll do that.
/// This will take the lines, addresses, and scope we've calculated so far, and iterate through
/// each line. If the line has an argument, evaluate it based on the scope, and store the number
/// that it evaluates to
fn calculate_args<'a>(
    lines: &[VASMLine<'a>],
    scope: Scope,
    line_addresses: &LineAddresses,
) -> Result<LineArgs, AssembleError<'a>> {
    let mut args = LineArgs::new();

    for (line_idx, line) in lines.iter().enumerate() {
        let line_num = (line_idx + 1) as i32;

        match line {
            VASMLine::Instruction(_, _, Some(arg)) | VASMLine::Db(_, arg) => {
                let num = eval(arg, line_num, line_addresses, &scope)
                    .map_err(|err| AssembleError::ArgError(line_num, err))?;
                args.insert(line_num, num);
            }
            _ => {}
        }
    }

    Ok(args)
}

fn poke_word(code: &mut Vec<u8>, at: usize, word: i32) {
    let [low, mid, high, _] = word.to_le_bytes();
    code[at] = low;
    code[at + 1] = mid;
    code[at + 2] = high;
}

/// Find the lower and upper bounds where this program will place memory
fn code_bounds<'a>(
    lines: &[VASMLine<'a>],
    line_addresses: &LineAddresses,
    line_lengths: &LineLengths,
) -> Result<(usize, usize), AssembleError<'a>> {
    let mut actual_lines = lines
        .iter()
        .enumerate()
        .filter(|(_, line)| !line.zero_length());
    let (first_idx, _) = actual_lines.next().ok_or(AssembleError::NoCode)?;
    let start = line_addresses[&(first_idx as i32 + 1)] as usize;

    let actual_lines = lines
        .iter()
        .enumerate()
        .filter(|(_, line)| !line.zero_length());
    let (last_idx, _) = actual_lines.last().unwrap();
    let end = line_addresses[&(last_idx as i32 + 1)] as usize;
    let end_length = line_lengths[&(last_idx as i32 + 1)];

    Ok((start, end + end_length - 1))
}

/// At this point all lines have addresses and lengths, and all arguments are reduced to
/// numeric constants. It's time to generate code.
///
/// - Make an array of zeroes, length (end - start)
/// - Go through the list of instructions, generating code for them:
/// - .db instructions turn into byte values starting at `address - start`
/// - Opcodes turn into instruction bytes at `address - start` followed (maybe) by
///   arguments.
/// - Any other directive is skipped (even .orgs, we already have the addresses calculated)
///
/// The instruction bytes are formed of six bits defining the instruction followed by two
/// bits denoting how many bytes of argument follow it.
///
/// Vulcan is a little-endian architecture: multi-byte arguments / .dbs will store the
/// least-significant byte at the lowest address, then the more significant bytes following.
fn generate_code<'a>(
    lines: &[VASMLine<'a>],
    scope: Scope,
    line_addresses: LineAddresses,
    line_lengths: LineLengths,
) -> Result<Vec<u8>, AssembleError<'a>> {
    let line_args = calculate_args(lines, scope, &line_addresses)?;
    let (start, end) = code_bounds(lines, &line_addresses, &line_lengths)?;
    let mut code = vec![0u8; end - start + 1];
    let mut current_addr = start;

    for (line_idx, line) in lines.iter().enumerate() {
        let line_num = (line_idx + 1) as i32;
        match line {
            VASMLine::Instruction(_, opcode, None) => {
                code[current_addr] = u8::from(*opcode) << 2;
                current_addr += 1;
            }
            VASMLine::Instruction(_, opcode, Some(_)) => {
                let arg = line_args[&line_num];
                let len = arg_length(arg);
                let instr = (u8::from(*opcode) << 2) + len as u8;
                code[current_addr - start] = instr;
                let [low, mid, high, _] = arg.to_le_bytes();
                code[current_addr - start + 1] = low;
                if len > 1 {
                    code[current_addr - start + 2] = mid
                }
                if len > 2 {
                    code[current_addr - start + 3] = high
                }
                current_addr += len + 1;
            }
            VASMLine::Db(_, _) => {
                let arg = line_args[&line_num];
                poke_word(&mut code, current_addr - start, arg);
                current_addr += 3;
            }
            VASMLine::StringDb(_, string) => {
                for ch in string.as_bytes() {
                    code[current_addr - start] = *ch;
                    current_addr += 1;
                }
            }
            VASMLine::Org(_, _) => {
                current_addr = line_addresses[&(line_num + 1)] as usize;
            }
            VASMLine::Equ(_, _) | VASMLine::LabelDef(_) => {}
        }
    }

    Ok(code)
}

#[cfg(test)]
mod test {
    use super::AssembleError::*;
    use super::EvalError::*;
    use super::*;
    use crate::ast::VASMLine;
    use crate::vasm_parser::parse_vasm_line;

    fn parse<'a>(lines: &'a [&str]) -> Vec<VASMLine<'a>> {
        lines
            .iter()
            .map(|line| parse_vasm_line(*line).unwrap())
            .collect()
    }

    fn place_labels_pass<'a>(
        lines: &'a [&str],
    ) -> Result<(LineAddresses, Scope<'a>), AssembleError<'a>> {
        let lines = parse(lines);
        let scope = solve_equs(&lines).unwrap();
        let lengths = measure_instructions(&lines, &scope);
        place_labels(&lines, scope, &lengths)
    }

    fn calculate_args_pass<'a>(lines: &'a [&str]) -> Result<LineArgs, AssembleError<'a>> {
        let (line_addresses, scope) = place_labels_pass(lines)?;
        let lines = parse(lines);
        calculate_args(&lines, scope, &line_addresses)
    }

    fn bounds<'a>(lines: &'a [&str]) -> Result<(usize, usize), AssembleError<'a>> {
        let (line_addresses, scope) = place_labels_pass(lines)?;
        let lines = parse(lines);
        let lengths = measure_instructions(&lines, &scope);
        code_bounds(&lines, &line_addresses, &lengths)
    }

    fn generate_code_pass<'a>(lines: &'a [&str]) -> Result<Vec<u8>, AssembleError<'a>> {
        let lines = parse(lines);
        let scope = solve_equs(&lines).unwrap();
        let lengths = measure_instructions(&lines, &scope);
        let (line_addresses, scope) = place_labels(&lines, scope, &lengths)?;
        generate_code(&lines, scope, line_addresses, lengths)
    }

    #[test]
    fn test_equs() {
        assert_eq!(
            solve_equs(&parse(&["blah: .equ 5+3"])),
            Ok([("blah", 8)].into())
        );
        assert_eq!(
            solve_equs(&parse(&["blah: .equ 5", "foo: .equ 3"])),
            Ok([("blah", 5), ("foo", 3)].into())
        );
        assert_eq!(
            solve_equs(&parse(&["blah: .equ 5", "foo: .equ blah + 7"])),
            Ok([("blah", 5), ("foo", 12)].into())
        );
        assert_eq!(
            solve_equs(&parse(&["add", "blah: .equ 5"])),
            Ok([("blah", 5)].into())
        );
    }

    #[test]
    fn test_unsolvable_equs() {
        assert_eq!(
            solve_equs(&parse(&["blah: .equ 5", "foo: .equ banana"])),
            Err(EquResolveError(2, "foo", MissingLabel("banana")))
        );
        assert_eq!(
            solve_equs(&parse(&["blah: .equ foo+3", "foo: .equ 7"])),
            Err(EquResolveError(1, "blah", MissingLabel("foo")))
        );
        assert_eq!(
            solve_equs(&parse(&["blah: .equ 3", "blah: .equ 7"])),
            Err(EquDuplicateError(2, "blah"))
        );
    }

    #[test]
    fn test_lengths() {
        assert_eq!(
            measure_instructions(
                &parse(&["add", "add 1", "add 500", "add 70000", "add -7"]),
                &[].into()
            ),
            [(1, 1), (2, 2), (3, 3), (4, 4), (5, 4)].into()
        );
        assert_eq!(
            measure_instructions(&parse(&[".db 7", ".db \"hello\\0\""]), &[].into()),
            [(1, 3), (2, 6)].into()
        );
        assert_eq!(
            measure_instructions(&parse(&[".org 256", "blah:", "foo: .equ 7"]), &[].into()),
            [(1, 0), (2, 0), (3, 0)].into()
        );
        assert_eq!(
            measure_instructions(
                &parse(&["add 2 + foo", "add 3 + blah", "jmpr @foo"]),
                &[("blah", 300)].into()
            ),
            [(1, 4), (2, 3), (3, 4)].into()
        );
    }

    #[test]
    fn test_place_labels() {
        assert_eq!(
            place_labels_pass(&["start: .org 256", "add", "dup"]),
            Ok((
                [(1, 256), (2, 256), (3, 257)].into(),
                [("start", 256)].into()
            ))
        );
        assert_eq!(
            place_labels_pass(&["push 70000", "dup"]),
            Ok(([(1, 0), (2, 4)].into(), [].into()))
        );
        assert_eq!(
            place_labels_pass(&["start: .equ 256", "blah: .org start + 4", "add"]),
            Ok((
                [(1, 0), (2, 260), (3, 260)].into(),
                [("blah", 260), ("start", 256)].into()
            ))
        );
        assert_eq!(
            place_labels_pass(&["start: .org 256", "blah: .org start + 10", "add"]),
            Ok((
                [(1, 256), (2, 266), (3, 266)].into(),
                [("blah", 266), ("start", 256)].into()
            ))
        );
    }

    #[test]
    fn test_unresolvable_orgs() {
        assert_eq!(
            place_labels_pass(&[".org 0xffffff - blah"]),
            Err(OrgResolveError(1, EvalError::MissingLabel("blah")))
        );
        assert_eq!(
            place_labels_pass(&["blah: .org blah"]),
            Err(OrgResolveError(1, EvalError::MissingLabel("blah")))
        );
    }

    #[test]
    fn test_calculate_args() {
        assert_eq!(
            calculate_args_pass(&["add 3", "add 5"]),
            Ok([(1, 3), (2, 5)].into())
        );
        assert_eq!(
            calculate_args_pass(&["foo: .equ 4", "add 3+foo", "add foo+7"]),
            Ok([(2, 7), (3, 11)].into())
        );
        assert_eq!(
            calculate_args_pass(&[".org 0x400", "blah:", "jmp blah + 10"]),
            Ok([(3, 1034)].into())
        );
        assert_eq!(
            calculate_args_pass(&[".org 0x400", "blah:", "add -4", "jmp @blah + 10"]),
            Ok([(3, -4), (4, 6)].into()) // blah is 1024, address of line 4 is 1028, so 6
        );
        assert_eq!(
            calculate_args_pass(&[".org 0x400", "sub", "jmp $-1"]),
            Ok([(3, 0x400)].into())
        );
        assert_eq!(
            calculate_args_pass(&[".org 0x400", "sub", "agt -3", "brz @-2"]),
            Ok([(3, -3), (4, -5)].into())
        );
        assert_eq!(
            calculate_args_pass(&["add banana + 3"]),
            Err(AssembleError::ArgError(
                1,
                EvalError::MissingLabel("banana")
            ))
        );
    }

    #[test]
    fn test_bounds() {
        assert_eq!(bounds(&["add"]), Ok((0, 0)));
        assert_eq!(bounds(&["add -4"]), Ok((0, 3)));
        assert_eq!(bounds(&["add 7"]), Ok((0, 1)));
        assert_eq!(bounds(&[".org 0x400", "add"]), Ok((1024, 1024)));
        assert_eq!(
            bounds(&["start: .equ 1024", ".org start", "add"]),
            Ok((1024, 1024))
        );
        assert_eq!(
            bounds(&[".org 0x400", "add", ".org 0x800"]),
            Ok((1024, 1024))
        );
        assert_eq!(
            bounds(&[".org 0x400", "add", ".org 0x800", ".db 5", "blah:"]),
            Ok((1024, 2050))
        );
        assert_eq!(bounds(&[]), Err(AssembleError::NoCode));
        assert_eq!(bounds(&[".org 0x400"]), Err(AssembleError::NoCode));
        assert_eq!(
            bounds(&["foo: .equ 3", ".org 0x400"]),
            Err(AssembleError::NoCode)
        );
    }

    #[test]
    fn test_generate_code() {
        assert_eq!(generate_code_pass(&["add"]), Ok(vec![4]));
        assert_eq!(generate_code_pass(&[".org 0x400", "add 7"]), Ok(vec![5, 7]));
        assert_eq!(generate_code_pass(&[".db 57"]), Ok(vec![57, 0, 0]));
        assert_eq!(generate_code_pass(&[".db \"AZ\0\""]), Ok(vec![65, 90, 0]));
    }
}