AIKeyboard — HeliBoard + Gemma 4 On-Device AI Correction

HeliBoard fork that adds on-device spell and grammar correction powered by Gemma 4 E2B-it running locally via Google's LiteRT-LM SDK.

After you finish typing a sentence (., !, ?), the model silently checks it and — if it finds an error — shows the corrected sentence in the suggestion strip. Tap it once to replace the original text. Everything runs on-device, no network, no cloud API.

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How it works

User types "guten rag."
          ↓
InputLogic detects sentence-ending punctuation
          ↓
AiTriggerHook (500 ms debounce) extracts last sentence
          ↓
AiCorrectionEngine.correctSentence()
  → LiteRT-LM Engine (GPU first, CPU fallback)
  → Gemma 4 E2B-it .litertlm model
          ↓
AiSuggestionManager.postSuggestion("Guten Tag.", "guten rag.")
          ↓
Suggestion strip shows "Guten Tag." in italic/accent color
          ↓
User taps → original text replaced with correction

Components

File	Role
app/…/ai/AiCorrectionEngine.kt	LiteRT-LM wrapper; GPU→CPU fallback
app/…/ai/AiTriggerHook.kt	Sentence detection + debounce
app/…/ai/AiSuggestionManager.kt	Posts corrected text to suggestion strip
patches/0001-InputLogic-ai-hook.patch	Hook in InputLogic.java after commitCodePoint
patches/0002-LatinIME-ai-lifecycle.patch	AI object lifecycle in LatinIME.java
patches/0003-SuggestionStrip-ai-style.patch	Italic + accent color for AI suggestions

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Requirements

• Android device with ARM64 (arm64-v8a), Android 11+ (API 31)
• Android Studio Ladybug or newer / Gradle 8+
• NDK 28.0.13004108
• Gemma 4 E2B-it model in .litertlm format (~2.6 GB)
  Download: https://huggingface.co/litert-community/gemma-4-E2B-it-litert-lm

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Setup

1. Clone and prepare HeliBoard sources

git clone http://172.17.2.68:3001/nova/AIKeyboard.git
cd AIKeyboard
bash setup_heliboard.sh

setup_heliboard.sh clones HeliBoard from GitHub and applies the three AI integration patches automatically.

2. Push the model to the device

adb push gemma-4-E2B-it.litertlm /data/local/tmp/gemma-4-E2B-it.litertlm
adb shell chmod 644 /data/local/tmp/gemma-4-E2B-it.litertlm

3. Build and install

./gradlew installDebug

Enable AIKeyboard as your input method in Android Settings → System → Languages & input → On-screen keyboard.

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Patch details

The three patches modify HeliBoard source files in heliboard/:

0001 — InputLogic hook
After every committed code point, checks AiTriggerHook.isSentenceEnder(). On a sentence-ender, reads up to 500 chars before the cursor and calls onSentenceEndDetected().

0002 — LatinIME lifecycle
Instantiates AiCorrectionEngine, AiSuggestionManager, and AiTriggerHook in onCreate(), cleans them up in onDestroy(). Overrides showAiSuggestion() to bypass the normal suggestions-enabled gate and intercepts pickSuggestionManually() to do a clean sentence replacement (finish composing → delete original → commit corrected text).

0003 — SuggestionStrip styling
Detects the KIND_AI_FLAG bit (0x10000) on a SuggestedWordInfo and applies italic style + auto-correct accent color to visually distinguish AI suggestions from normal word predictions.

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Model placement

The model must be at /data/local/tmp/gemma-4-E2B-it.litertlm on the device. This path is configured in AiCorrectionEngine.MODEL_PATH.

The model file is not tracked in this repository (2.6 GB, .litertlm is in .gitignore).

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Backend selection

AiCorrectionEngine tries GPU (WebGPU/Vulkan via LiteRT-LM) first. If GPU initialization fails, it falls back to CPU immediately. If a GPU inference error occurs at runtime, it permanently switches to CPU for the session and recreates the engine.

Tested on:

• Google Pixel 7 (Tensor G2) — CPU backend (OpenCL not available)
• Devices with Mali-G710 — GPU via WebGPU/Vulkan

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License

The AI integration layer (app/src/main/java/helium314/keyboard/latin/ai/) is original work added to this project.

HeliBoard itself is licensed under GPL-3.0-only. See heliboard/LICENSE after running setup_heliboard.sh.