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check-similarity-mbt

Testing & Quality
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Detect duplicate MoonBit code using AST-based similarity analysis. Use when working with .mbt files and looking for code duplication, refactoring opportunities, or enforcing code quality.

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MoonBit Code Similarity Detection

What to do

Run similarity-mbt on the target MoonBit project to detect duplicate functions, then analyze the results and propose a refactoring plan.

If similarity-mbt is not installed:

cargo install similarity-mbt

Step 1: Run similarity analysis

Run with the arguments provided, or use sensible defaults:

similarity-mbt $ARGUMENTS

If no arguments are given, scan the current directory:

similarity-mbt . --threshold 0.85 --min-lines 5

Step 2: Analyze the results

Read the output and categorize duplicate pairs:

High-priority refactoring targets

  • 100% similarity: Exact structural duplicates. Extract into a shared function with parameters.
  • 95-100% similarity: Near-identical. Usually differ by 1-2 identifiers or values. Parameterize the difference.
  • X/Y axis pairs (e.g. get_width vs get_height, offset_x vs offset_y): Common in layout/graphics code. Abstract with an axis parameter or generic.

Medium-priority

  • 85-95% similarity: Same algorithm with minor structural differences. Consider extracting the common pattern.
  • async/sync variants: Same logic duplicated for async and sync versions. Consider a shared core function.
  • _with_options variants (e.g. parse_rule vs parse_rule_with_diagnostics): Use optional parameters or builder pattern.

Low-priority / acceptable

  • Protocol handler boilerplate: Large sets of nearly identical dispatch functions. Better solved with code generation or macros if available.
  • Small accessors (< 5 lines, 85-90%): Short functions naturally look similar. Usually not worth refactoring.

Step 3: Propose refactoring

For each high-priority pair, suggest a concrete refactoring:

  1. Show the two duplicate functions
  2. Propose the unified version
  3. Explain what callers need to change

Key CLI Options

OptionDescription
--threshold <0-1>Similarity threshold (default: 0.85)
--min-lines <n>Skip functions shorter than n lines (default: 3)
--min-tokens <n>Skip functions with fewer than n AST nodes
--printShow actual code snippets in output
--filter-function <name>Only show pairs matching function name
--filter-function-body <text>Only show pairs whose body contains text
--fail-on-duplicatesExit code 1 if duplicates found (CI use)
--rename-cost <0-1>APTED rename cost (default: 0.3, lower = more tolerant of renames)
--no-size-penaltyDisable penalty for differently-sized functions

Effective Thresholds

  • 0.95+: Nearly identical (variable renames only)
  • 0.85-0.95: Same algorithm, minor differences
  • 0.75-0.85: Similar structure, different details
  • 0.70-0.75: Related logic, worth investigating

Workflow tips

  • Start with --threshold 0.9 to find obvious duplicates, then lower to 0.85
  • Use --print to see actual code for top pairs
  • Use --min-lines 5 to skip trivial accessors
  • Use --filter-function <name> to focus on a specific function
  • For CI: similarity-mbt . --threshold 0.95 --fail-on-duplicates --min-lines 5

Common MoonBit refactoring patterns

Axis parameterization

// Before: two functions
fn get_width(self) -> Double { ... }
fn get_height(self) -> Double { ... }

// After: one function with axis parameter
fn get_dimension(self, axis : Axis) -> Double { ... }

Operation abstraction

// Before: set_attribute / remove_attribute with identical structure
// After: shared body with operation parameter or closure
fn modify_attribute(self, node, name, op : (Map[String, String], String) -> Unit) -> Result[Unit, Error] { ... }

Optional parameter consolidation

// Before: parse_rule / parse_rule_with_diagnostics
// After: single function with optional diagnostics collector
fn parse_rule(self, diagnostics~ : Array[Diagnostic]? = None) -> Rule { ... }