Feature Contract

Generate an implementation contract for a FastLED change. The contract must be reviewed before any code is written.

$ARGUMENTS

Why Use a Contract

FastLED runs on 100+ microcontroller platforms. A change that seems simple can break:

• Platform-specific driver behavior (ESP32, Teensy, AVR, STM32, RP2040, nRF52...)
• Public API compatibility (users pin to specific versions)
• Timing-sensitive LED protocols (a single wrong bit ruins the strip)
• Build-system portability (meson, fbuild, Arduino IDE all involved)

The contract documents what changes, what risks exist, and how to prove it works — before a line of code is written.

Steps

1. Understand the Request

Determine:

• What is the requested change? (feature / bug fix / refactor / platform port / driver addition)
• Which LED chipsets or platforms are affected?
• What is the expected behavior after the change?
• Are there hardware constraints? (specific ESP32 variant, specific LED protocol, pin count)
• Is this a public API change or internal implementation?

2. Define Scope

Document:

• Summary: One-paragraph description of the change
• Non-goals: What this change explicitly does NOT do
• Affected files: Every file to be created, modified, or deleted
• APIs touched: Public functions, types, or macros that will change

3. Assess Platform Impact

For each affected area, evaluate impact:

4. Assess Risks

Evaluate each risk category:

Timing Risks (critical for LED protocols):

• Does the change affect bit-timing (T0H, T1H, reset pulse)?
• Could added latency cause LED flicker or color errors?
• Is the change tested on hardware, not just WASM/emulation?

API Compatibility Risks:

• Does this change any public header or function signature?
• Will existing user sketches compile without modification?
• Is a deprecation path needed?

Memory Risks:

• Does this add to SRAM usage (critical for AVR with 2KB)?
• Does this add to flash usage?
• Are heap allocations properly checked for NULL?

Concurrency Risks (ESP32 / RTOS platforms):

• Shared state accessed from multiple tasks or ISR?
• DMA buffer lifetime during transfer?
• Interrupt-safe API used where required?

Build System Risks:

• Does this require changes to meson.build, library.properties, or component.mk?
• Is the new code compiled only on relevant platforms (guards correct)?

5. Define Test Plan

Specify the test layers that apply:

List specific test cases:

• Positive: Expected behavior with valid inputs
• Negative: Error handling, boundary values
• Regression: Existing tests that must still pass

6. Define Rollback Strategy

Document:

• How to revert the change (git revert, feature flag, etc.)
• Whether any NVS/EEPROM data migration is needed
• Whether users need to update their code if the API changes

7. Generate Contract Document

# Implementation Contract: [Title]

## Change Summary
[One-paragraph description of what changes and why]

## Non-Goals
- [What this explicitly does NOT include]

## Affected Files
| File | Action | Description |
|------|--------|-------------|
| src/platforms/esp/32/foo.h | MODIFY | [What changes] |
| src/fl/bar.h | CREATE | [Purpose] |

## APIs Touched
- `FastLED.addLeds<...>()` — [How it changes, if at all]
- `CRGB` — [Impact]

## Platform Impact
| Platform | Impact | Rationale |
|---------|--------|-----------|
| AVR | None | No new code on AVR path |
| ESP32 | High | New RMT encoder implementation |
| ... | ... | ... |

## Risk Assessment

### Timing: [LOW / MED / HIGH]
[Details — will LED protocols still work?]

### API Compatibility: [LOW / MED / HIGH]
[Details — will existing user code still compile and run?]

### Memory: [LOW / MED / HIGH]
[Details — RAM/flash impact, especially for AVR]

### Concurrency: [LOW / MED / HIGH]
[Details — ISR safety, DMA lifetime, task interaction]

### Build System: [LOW / MED / HIGH]
[Details — meson.build, guards, new dependencies]

## Test Plan
- [ ] Host unit test: [test name / description]
- [ ] WASM compile: `bash compile wasm --examples Blink`
- [ ] Platform compile: `bash compile <platform>`
- [ ] Hardware validation: [specific test on which board]
- [ ] Regression: `bash test --cpp` — all existing tests pass

## Rollback Strategy
[Steps to safely revert this change]

## Acceptance Criteria
- [ ] [Specific, measurable criterion]
- [ ] All platform compiles pass
- [ ] No regressions in host test suite
- [ ] [Hardware test evidence if applicable]

Rules

1. Never skip the contract for non-trivial changes. A "small" change to a shared header can break 100 platform builds.
2. Define platform impact explicitly. Unknown = HIGH risk. Investigate before marking LOW.
3. Hardware validation is required for any driver or timing change. WASM compilation is not sufficient proof.
4. Public API changes need a compatibility note. FastLED users should not be surprised by breaking changes.
5. Update the contract if scope changes. Scope creep must be re-reviewed.

What Makes a Good Acceptance Criterion

Good: "WS2812 output on ESP32 shows correct colors at 1000 LEDs without flicker, confirmed by hardware observation" Bad: "It works"

Good: "All existing host tests pass with bash test --cpp" Bad: "Tests pass"

Good: "Sketch compiles on Arduino IDE 2.x with no warnings" Bad: "Compiles somewhere"