# rsaeb
`rsaeb` is a Rust 2024 `no_std + alloc`, byte-oriented interpreter for A=B
ordered rewrite programs.
A=B: <https://store.steampowered.com/app/1720850/AB/>
## Unofficial Project Notice
This project is an unofficial, independently developed interpreter library. It
is not affiliated with, endorsed by, or maintained by Artless Games or the
original A=B author.
A=B's compact `lhs=rhs` ordered rewrite system is an unusually elegant
programming-puzzle idea. This crate exists because that design is worth
studying, testing, and reimplementing. If this interpreter interests you,
please support the original game.
## Documentation Map
- This README is the package entry point. It explains the interpreter shape, the
accepted A=B surface, byte-domain boundaries, and release checks.
- The generated rustdoc is the exact API reference and carries the complete
doctested public examples.
- The GitHub Wiki is a short navigation layer for use cases and embedding
boundaries.
The crate root intentionally does not re-export duplicate type paths. Public
types live under their domain modules, such as `source`, `input`, `program`,
`limits`, `execution`, `inspect`, `trace`, and `error`.
## Quick Start
Parse source into an immutable `Program`, validate runtime input, admit it into
a run seed with explicit execution limits, then run:
```rust
use rsaeb::input::{RunSeed, RuntimeInput, RuntimeInputSource};
use rsaeb::limits::{
DEFAULT_MAX_INPUT_LEN, DEFAULT_MAX_RETURN_LEN, DEFAULT_MAX_STATE_LEN,
DEFAULT_MAX_STEPS, DEFAULT_PARSE_LIMITS, ExecutionLimits, RuntimeInputLimits,
};
use rsaeb::program::{Program, RunOutcome};
use rsaeb::source::ProgramSource;
fn main() -> Result<(), Box<dyn std::error::Error>> {
let program = Program::parse(ProgramSource::from_text("a=b"), DEFAULT_PARSE_LIMITS)?;
let input_limits = RuntimeInputLimits::new(DEFAULT_MAX_INPUT_LEN);
let execution_limits = ExecutionLimits::new(
DEFAULT_MAX_STEPS,
DEFAULT_MAX_STATE_LEN,
DEFAULT_MAX_RETURN_LEN,
);
let input = RuntimeInput::validate(RuntimeInputSource::from_bytes(b"a"), input_limits)?;
let seed = RunSeed::admit(input, execution_limits)?;
let result = program.run(seed)?;
if !matches!(
result.outcome(),
RunOutcome::Stable(output) if output.as_slice() == b"b"
) {
return Err("unexpected stable output".into());
}
Ok(())
}
```
`ProgramSource::from_text` and `ProgramSource::from_bytes` only label source
input; `Program::parse` performs source validation. `RuntimeInputSource` and
`RuntimeInput::validate` do the same for runtime input bytes. Reuse parsed
programs freely: a `Program` is immutable, and `(once)` consumption is local to
each execution.
## Execution Shape
The normal host flow is:
1. Load source bytes or text outside the interpreter.
2. Construct `ProgramSource`.
3. Parse with `Program::parse`.
4. Label host input bytes with `RuntimeInputSource::from_bytes`.
5. Validate with `RuntimeInput::validate`.
6. Admit with `RunSeed::admit` and `ExecutionLimits`.
7. Execute through run-to-completion, stepwise execution, tracing, or
rule-attempt stepping.
The crate intentionally contains no filesystem, process, argument parsing,
environment access, stdout/stderr, or lossy display boundary. Hosts perform I/O
outside the interpreter and pass already-loaded bytes into typed boundaries.
`Program::run` is the borrowed run-to-completion API. `Program::start_run` is
the borrowed stepwise API for hosts that keep a reusable parsed program.
`Program::into_run` is the explicit ownership-transfer stepwise API for cases
where the execution session must own the parsed program. Rule-attempt execution
is separate: `Program::start_rule_attempt_run` and `Program::into_rule_attempt_run`
observe executable rule-line attempts, including misses, without changing normal
committed-step semantics.
The exact typestate names, transition variants, owned recovery methods, tracing
events, and error variants are documented in rustdoc.
## A=B Language Reference
A program source is a byte sequence containing one rewrite rule per non-empty
code line:
```text
lhs=rhs
```
Each line is parsed in this order:
1. `#` starts a comment. Everything from `#` to the end of the line is ignored.
2. Non-ASCII bytes are rejected in the remaining code part.
3. ASCII whitespace in the code part is removed completely.
4. Remaining non-whitespace code bytes must be printable ASCII.
5. Empty compact code is ignored.
6. Non-empty compact code must contain exactly one `=`.
7. The left side and right side are parsed as compact rule syntax.
Examples:
```text
a=b# this is parsed as a=b
#a=b this whole line is a comment
a b = b b # this is parsed as ab=bb
```
Comments may contain arbitrary non-ASCII or non-UTF-8 bytes when source is
provided with `ProgramSource::from_bytes`. Executable code outside comments must
be ASCII. ASCII control bytes are invalid in executable code except for ASCII
whitespace that is removed during compaction.
Parse error columns are one-based byte positions in the original source line
before whitespace compaction. Diagnostics point at the user's source text, not
at the internal compacted representation.
### Reserved Characters
The following characters are reserved in program code:
```text
= # ( )
```
Their meanings are fixed:
- `=` separates the left side from the right side.
- `#` starts a comment.
- `(` and `)` are only allowed as part of supported modifier/action tokens.
A second `=` in compact code is a parse error:
```text
a=b=c
```
A second `=` inside a comment is ignored:
```text
a=b#=c
```
Reserved syntax where payload data is expected is always a parse error:
```text
a=b(
a=b)
a=b()
a=()
a=b(start)
a=(once)b
a(once)=b
```
Because whitespace is removed from program code, spaces cannot be represented as
rule data. Because `=`, `#`, `(`, and `)` are reserved, program payloads also
refuse them as rule data.
Runtime input is different. Input bytes are runtime data, not program code.
Input must be ASCII, but it may contain whitespace, ASCII control bytes, and
reserved characters. Ordinary rewrite actions cannot match, create, or delete
those bytes directly.
```text
program: a=b
input: a=()#c
output: b=()#c
```
Rules cannot match across preserved runtime-only bytes:
```text
program: ab=bb
input: a bc
output: a bc
```
`(return)` stops execution and replaces the final output with its return
payload, so runtime-only input bytes are not preserved after a matching return
rule:
```text
program: a=(return)x
input: a=()#c
output: x
```
### Left-Side Modifiers
The left side may start with one repeat modifier and one anchor modifier:
- `(once)`: the rule may be used at most once per runtime execution.
- `(start)`: the rule only matches at the start of the current state.
- `(end)`: the rule only matches at the end of the current state.
Supported modifier order is `(once)` first, then an optional anchor. Duplicated
or unsupported left-side modifier order is a parse error.
Examples:
```text
a=b
(once)a=b
(start)a=b
(end)a=b
(once)(start)a=b
```
Because code whitespace is ignored, this is also valid and equivalent to
`(once)(start)a=b`:
```text
( once ) ( start ) a = b
```
### Right-Side Actions
The right side selects the action for a matching rule:
- `text`: replace the matched left side with `text`.
- `(start)text`: remove the match and insert `text` at the start of the state.
- `(end)text`: remove the match and append `text` to the end of the state.
- `(return)text`: stop execution immediately and output `text`, discarding the
current runtime state.
The action payload is still program data, so it cannot contain whitespace,
reserved characters, non-ASCII bytes, or ASCII control bytes. `(return)` can
therefore output only program-representable bytes, even if the discarded runtime
state contained spaces or reserved characters from the original input.
Examples:
```text
a=b
x=(start)y
x=(end)y
x=(return)y
```
### Empty Sides
The left side and right side may be empty.
An empty right side deletes the matched left side:
```text
a=
```
An empty left side matches an empty byte sequence. For unanchored rules and
`(start)` rules, it matches at the start of the current state:
```text
(once)=x
```
With input `ab`, this inserts `x` at the start and produces `xab`.
For `(end)` rules, an empty left side matches at the end of the current state:
```text
(once)(end)=x
```
With input `ab`, this inserts `x` at the end and produces `abx`.
An unanchored empty-left rule without `(once)`, `(return)`, or some later rule
that makes execution stop can rewrite forever until the step limit is reached.
That is legal syntax; execution remains governed by `ExecutionLimits`.
### Ordered Execution
Execution is ordered and single-step.
On each step, the runtime scans rules from top to bottom and applies the first
rule that matches the current state. For an unanchored non-empty left side, the
leftmost match in the current state is used. After one applied step, scanning
restarts from the first rule.
Example:
```text
program:
aa=x
a=y
input:
aaaa
output:
xx
```
The first rule is preferred over the second rule, and each application rewrites
the leftmost matching `aa`.
## Byte-Domain Boundary
Program source and runtime input are deliberately different byte domains:
- Program code is compact printable ASCII syntax.
- ASCII whitespace in program code is ignored before parsing.
- `#` starts a comment for the rest of the source line.
- Comments may contain non-ASCII or non-UTF-8 bytes.
- Executable code outside comments must be ASCII.
- Program payloads cannot contain whitespace, `=`, `#`, `(`, `)`, non-ASCII
bytes, or ASCII control bytes.
- Runtime input is ASCII data and may contain spaces, ASCII control bytes, and
reserved syntax bytes.
- Normal rewrites preserve runtime-only bytes that program code cannot
construct or match.
- `(return)` stops execution and replaces the whole output with its return
payload.
Internally, parser and runtime phases stay separate instead of passing raw byte
buffers through every stage:
```text
raw line bytes
-> RawSourceLine
-> CodeLine # comment removed, executable code ASCII validated
-> CompactCodeLine # whitespace removed, SourceColumn retained
-> NonEmptyCompactCodeLine # empty compact lines cannot enter rule parsing
-> RuleSyntaxLine # exactly one '=' has been proven
-> LeftSyntax / RightSyntax
-> ProgramByte # bytes that program code may construct and match
runtime input bytes
-> AsciiByte # runtime input domain validation
-> RuntimeByte # private ProgramConstructible(ProgramByte) or Opaque(NonProgramAsciiByte)
-> execution session # consumes RuntimeInput and owns mutable execution state
```
Program payloads are stored as `ProgramByte`, not raw `u8`. Runtime state is
stored as `RuntimeByte`: payload-compatible input and rule output become
editable program bytes, while whitespace, control bytes, and reserved syntax
bytes from input become opaque ASCII bytes. Ordinary rules match only editable
bytes. Opaque input bytes are preserved by surrounding rewrites but cannot be
directly matched, created, or deleted by program payloads.
Public observation crosses explicit materialization boundaries. Runtime state
views materialize to snapshots only when requested, stable run results own final
state bytes, `(return)` outputs use a separate return-output domain, parsed
payload inspection materializes explicitly, and snapshot tracing has its own
byte limit. During execution, the active state and rewrite scratch buffer remain
separate typed buffers until a successful continuation step commits.
`(once)` rules carry private slots assigned during parsing. Each execution
allocates only those slot states, and only a committed application can consume
its slot.
## `no_std + alloc` Boundary
The library crate is `#![no_std]` and uses `alloc` only at owned-buffer
boundaries such as parsed rules, runtime input validation, per-run `(once)` state,
run results, canonical rule source, explicit view materialization, and trace
snapshots. It requires an allocator, but not `std`.
Allocation is explicit and fallible. Parser/runtime paths reserve explicitly
and report `AllocationError` instead of relying on accidental `Vec` growth.
Runtime expansion is budgeted through `ExecutionLimits`; the runtime checks size
limits before allocating oversized states or return outputs. Step budget is
reserved before rewrite or return-output materialization, so an exhausted step
limit cannot allocate a candidate state or return buffer. Trace snapshot
materialization is budgeted separately through `TraceSnapshotByteLimit`.
Owned public values that contain byte buffers intentionally do not implement
`Clone`; copying bytes is an explicit materialization step, not a hidden
infallible API. Parser payload validation is reported before payload storage
allocation, so invalid source bytes are not hidden behind allocation failures.
A downstream `std` application can use the library normally. A downstream
`no_std` application must provide an allocator before calling APIs that
allocate.
## Error Model
The library error model is intentionally split. Parse errors, runtime input
errors, run-admission errors, runtime execution errors, allocation errors, and
trace materialization errors have separate structured types under
`rsaeb::error`.
Allocation failures preserve the allocation boundary as `AllocationContext`.
Reservation failures also report a typed `RequestedCapacity`, so hosts can
distinguish failures while validating input, materializing state views,
building canonical rule source, producing final output, or retaining trace
snapshots without parsing display strings.
Configured byte budgets and step budgets are reported through concrete errors
such as `ParseLimitError`, `RuntimeStateLimitError`, `ReturnOutputLimitError`,
`StepLimitError`, and `RuleAttemptLimitError`. Trace snapshot byte limits are
reported through `TraceSnapshotError`, because snapshot materialization is
outside runtime execution.
Filesystem failures are not part of the library error model. External I/O must
be handled before bytes enter `ProgramSource::from_bytes`,
`ProgramSource::from_text`, or `RuntimeInputSource::from_bytes`.
## Development Checks
Run the public documentation and package checks before publishing changes:
```sh
rustup target add thumbv7em-none-eabihf
cargo fmt --check
cargo check --lib --no-default-features
cargo check --lib --all-features --target thumbv7em-none-eabihf
cargo clippy --all-targets --all-features -- -D warnings
cargo test --all-targets --all-features
cargo test --doc --all-features
latest_rlib="$(find target/debug/deps -maxdepth 1 -name 'librsaeb-*.rlib' -printf '%T@ %p\n' | sort -nr | awk 'NR == 1 { print $2 }')"
rustdoc --edition=2024 --test README.md -L dependency=target/debug/deps --extern "rsaeb=${latest_rlib}"
RUSTDOCFLAGS="-D warnings" cargo doc --all-features --no-deps
cargo package --list
cargo package
```