StructLayoutToolkitCalc

A safe, deterministic expression evaluator for computing bit‑accurate offsets, sizes, and field values within StructLayoutToolkit.

Overview

sltcalc provides a safe and deterministic expression evaluator used across StructLayoutToolkit to compute bit‑accurate offsets, sizes, and derived field values. It parses expressions into Python’s AST and evaluates a restricted subset of operations, ensuring predictable behavior without accessing Python globals or executing arbitrary code. The evaluator supports arithmetic, bitwise operations, conditional expressions, and user‑defined variables supplied through an isolated environment. This module enables dynamic field definitions, computed offsets, variable‑length structures, and protocol‑specific formulas while maintaining strict safety and structural consistency.

Usage

Basic

from sltcalc import SltEval

evaluator = SltEval()
result = evaluator.eval("1 + 2 * 3")
print(result)  # 7

With variables

from sltcalc import SltEval

env = {
    "offset": 8,
    "width": 3,
    "flag": True,
}

evaluator = SltEval(env)

print(evaluator.eval("offset + width * 2"))       # 14
print(evaluator.eval("1 if flag else 0"))         # 1
print(evaluator.eval("99 if offset >= 8 else -1"))  # 99

Error behavior

from sltcalc import SltEval

evaluator = SltEval()

# NameError: undefined variable
evaluator.eval("unknown + 1")

# ValueError: unsafe function
evaluator.eval("sum(1, 2)")

Supported expressions

SltEval evaluates a restricted subset of Python expression AST nodes.

Literals

• Numeric constants (for example: 0, 42, 3.5)
• Boolean constants (True, False)
• List literals (for example: [1, 2, 3])
• Tuple literals (for example: (1, 2, 3))
• Dict literals (for example: {"a": 1, "b": 2})

Variables

• Variable lookup from the evaluator environment (SltEval(env))

Binary operators

• Arithmetic: +, -, *, /, //, %, **
• Bitwise: &, |, ^, <<, >>

Unary operators

• Unary plus: +x
• Unary minus: -x
• Boolean negation: not x

Boolean operators

• and
• or

Comparisons

• ==, !=, <, <=, >, >=, is, is not, in, not in
• Chained comparisons are supported (for example: 1 < 2 < 3)

Conditional expression

• Ternary expression: a if condition else b

Subscript access

• Index/key access (for example: arr[0], mapping["key"])
• Nested access is supported (for example: access key "items" and then index 0)

Function calls

• Built-in allowlist: abs(...), all(...), any(...), len(...), min(...), max(...), round(...), sorted(...), sum(...)
• Callables passed through environment are allowed (for example: SltEval({"triple": triple}) then triple(4))

Not supported

• Attribute access (for example: obj.value)
• Set literals in expressions
• Non-name call targets (for example: (lambda x: x)(1))

Unsupported constructs raise ValueError with an Unsupported expression message.