Optimize validator

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Optimize Cardano Validator

Guide optimization of Aiken validators for lower execution costs (CPU/memory) and smaller compiled script size. Optimization should only be applied to validators that are already correct and tested.

When to use

• User has a working validator and wants to reduce transaction fees
• Script size exceeds limits or is unnecessarily large
• Execution units (CPU/memory) are higher than expected
• User wants to compare optimization strategies
• Before deploying a reference script (size directly affects storage deposit)
• Transaction is failing due to exceeding execution unit limits

When NOT to use

• User needs to write a new validator (use write-validator)
• User needs a security review (use review-contract)
• The validator has not been tested yet (correctness comes before performance)
• The optimization would remove a security check

Key principles

1. Measure before optimizing: Use aiken build to check script size and aiken bench to measure execution units. Without baseline numbers, you cannot know if changes helped.
2. Script size and execution units are independent costs: A larger script is not necessarily more expensive to execute. Inlining functions increases size but can reduce CPU by avoiding closure allocation.
3. CPU and memory budgets are separate: A transaction can fail by exceeding either limit independently. Identify which resource is the bottleneck before optimizing.
4. The stdlib is well-optimized: Do not rewrite stdlib functions unless profiling shows they are a bottleneck. Custom implementations often end up larger and slower.
5. Correctness over performance: Never sacrifice security checks for performance. Optimize how a check is performed, not whether it runs.

Workflow

Step 1: Establish baseline

Read the validator code and gather current metrics.

Suggest the user run:

# Build and check script sizes
aiken build

# Run benchmarks if available
aiken bench

# Check compiled script sizes
cat plutus.json | jq '.validators[] | {title, size: (.compiledCode | length / 2)}'

Note the following:

• Current script size in bytes (from plutus.json or build output)
• Execution unit estimates per action (from aiken bench)
• Which operations the validator performs (list traversals, value comparisons, datum deserialization)

Step 2: Search Bundled Documentation

Search the bundled documentation for relevant content:

• ${CLAUDE_SKILL_DIR}/../../docs/sources/aiken/ - Aiken language docs
• ${CLAUDE_SKILL_DIR}/../../docs/sources/aiken-stdlib/ - Aiken standard library docs
• ${CLAUDE_SKILL_DIR}/../../docs/sources/plinth/ - Plinth (PlutusTx) docs

Step 3: Identify expensive operations

Search the validator code for known cost centers:

High CPU cost:

• Nested list traversals (O(n*m) comparisons)
• Value comparisons (comparing two Values traverses both nested maps) -- expensive for multi-asset values
• Large datum deserialization
• Cryptographic hashing (blake2b, sha256)
• On-chain signature verification (extremely expensive -- use extra_signatories instead)

High memory cost:

• Building large intermediate data structures
• Repeated datum deserialization
• String concatenation for trace messages
• Map/filter chains that create intermediate lists

Script size bloat:

• Trace messages (string literals embedded in compiled output)
• Dead code (unreachable branches, unused functions)
• Redundant type conversions
• Inlined functions that could be shared

Step 4: Apply optimizations

Execution unit optimizations

Order pattern matching by frequency:

// Most common action first -- UPLC checks branches sequentially
when redeemer is {
  Swap -> check_swap(...)           // 90% of transactions
  AddLiquidity -> check_add(...)    // 8%
  RemoveLiquidity -> check_remove(...)  // 1.5%
  UpdateParams -> check_update(...)     // 0.5%
}

Fail fast -- check cheap conditions before expensive ones:

// GOOD: Cheap signer check (O(small n)) before expensive output scan (O(large n))
list.has(tx.extra_signatories, datum.owner) &&
check_outputs(tx.outputs, expected_value)

Extract common sub-expressions:

// BAD: Resolves own input twice
list.any(tx.outputs, fn(o) { o.address == resolve_own_address(tx, own_ref) }) &&
list.all(tx.outputs, fn(o) {
  o.address != resolve_own_address(tx, own_ref) || check_value(o)
})

// GOOD: Compute once, reuse
expect Some(own_input) = transaction.find_input(tx.inputs, own_ref)
let own_addr = own_input.output.address
list.any(tx.outputs, fn(o) { o.address == own_addr }) &&
list.all(tx.outputs, fn(o) { o.address != own_addr || check_value(o) })

Reduce list traversals:

// BAD: Two separate passes over outputs
let has_script_output = list.any(tx.outputs, fn(o) { o.address == script_addr })
let has_payment = list.any(tx.outputs, fn(o) { o.address == seller })

// GOOD: Single pass with combined check
let (has_script, has_pay) =
  list.foldl(tx.outputs, (False, False), fn(o, acc) {
    let (s, p) = acc
    (s || o.address == script_addr, p || o.address == seller)
  })

Use expect instead of when for single-variant destructuring:

// LARGER (compiled): when with explicit fail
when datum is {
  MyDatum { owner, amount } -> use(owner, amount)
  _ -> fail
}

// SMALLER: expect compiles to direct destructure
expect MyDatum { owner, amount } = datum
use(owner, amount)

Script size optimizations

Remove traces for production builds:

# Development (with traces for debugging)
aiken build

# Production (traces removed, 10-30% smaller)
aiken build --trace-level silent

Extract shared helper functions:

// BAD: Duplicated logic in each branch
when redeemer is {
  Claim -> {
    expect Some(out) = list.find(tx.outputs, fn(o) { o.address == addr })
    assets.lovelace_of(out.value) >= amount && list.has(tx.extra_signatories, owner)
  }
  Update -> {
    expect Some(out) = list.find(tx.outputs, fn(o) { o.address == addr })
    assets.lovelace_of(out.value) >= amount && list.has(tx.extra_signatories, owner)
  }
}

// GOOD: Shared function
fn check_output_and_signer(tx, addr, amount, signer) {
  expect Some(out) = list.find(tx.outputs, fn(o) { o.address == addr })
  assets.lovelace_of(out.value) >= amount && list.has(tx.extra_signatories, signer)
}

Remove dead code and unused imports: Search for functions, types, and imports that are not referenced. Unused code still contributes to script size.

Data structure optimizations

• Use Pair<a, b> instead of 2-element tuples when possible (smaller UPLC representation)
• For small fixed collections, explicit fields are cheaper than lists
• For lookups, sorted lists with early-exit beat unsorted lists
• Smaller datums mean less deserialization cost -- remove fields that can be computed from other fields

Step 5: Consider reference scripts

For scripts over approximately 4 KB that will be used in multiple transactions:

• Store the script as a reference script in a UTxO
• Reference it by hash in subsequent transactions
• The script is paid for once at creation and amortized over many uses
• Reduces per-transaction size significantly

Step 6: Verify and benchmark

After applying optimizations, the user should verify:

1. All existing tests still pass: aiken check
2. Script size delta: Compare compiled sizes before and after
3. CPU units delta: Run aiken bench and compare
4. Memory units delta: Run aiken bench and compare
5. No security checks were removed: Review changes for correctness

Provide before/after comparison format:

                Before        After        Delta
cpu:        2,345,678    1,890,123     -19.4%
mem:          234,567      198,432     -15.4%
size:           4,567        3,890     -14.8%

Step 7: Document trade-offs

If any optimization involves a trade-off (e.g., increased size for lower CPU), document:

• What was changed and why
• The measured impact on each metric
• Any correctness considerations

References

• references/uplc-cost-model.md -- UPLC cost model basics, operation costs, and budget limits
• Search ${CLAUDE_SKILL_DIR}/../../docs/sources/ for benchmark results and performance requirements
• Aiken documentation on optimization: https://aiken-lang.org
• Use aiken build --trace-level silent for production builds
• Use aiken bench for execution unit measurements