intercept-bounce

An Interception Tools filter designed to eliminate keyboard chatter (also known as switch bounce) while providing detailed statistics and diagnostics.

It reads raw Linux input_event structs from standard input, filters out rapid duplicate key press/release events based on a configurable time threshold, and writes the filtered events to standard output. Comprehensive statistics about dropped events, timings, and near-misses are printed to standard error on exit or periodically.

Features

Configurable Debouncing: Filters rapid duplicate key press/release events within a specified time window (--debounce-time, default: 25ms). Key repeats (value=2) are never filtered.
Near-Miss Tracking: Identifies and reports key events that pass the filter but occur just slightly after the debounce window closes (--near-miss-threshold-time, default: 100ms). Useful for diagnosing keys with inconsistent timing.
Detailed Statistics (Human-Readable & JSON):
- Overall counts (key events processed, passed, dropped).
- Overall histograms showing the distribution of bounce and near-miss timings.
- Per-key statistics:
  - Total processed, passed, dropped counts, and drop rate (%).
  - Bounce time statistics (min/avg/max) for dropped press/release events.
  - Near-miss time statistics (min/avg/max) for passed press/release events.
  - Detailed histograms for bounce and near-miss timings per key/state (JSON only).
Flexible Logging:
- --log-all-events: Log details ([PASS]/[DROP]) for (almost) every event.
- --log-bounces: Log details only for dropped (bounced) key events.
- --verbose: Enable DEBUG level internal logging.
- RUST_LOG environment variable for fine-grained tracing filter control (overrides --verbose).
Per-Key Controls: Use --debounce-key KEY_ENTER to limit debouncing to specific keys (multiple instances allowed), or --ignore-key KEY_VOLUMEDOWN to exempt controls entirely; both accept names or numeric codes. If a key appears in both lists, --debounce-key wins so the key is still debounced.
Periodic Reporting: Dump statistics periodically (--log-interval, default: 15m).
JSON Output: Output statistics in JSON format (--stats-json) for machine parsing.
Graceful Shutdown: Handles SIGINT, SIGTERM, SIGQUIT to ensure final statistics are reported.
Device Listing: List available input devices with keyboard capabilities (--list-devices).
Debugging Ring Buffer: Optionally store the last N passed events in memory for debugging complex issues (--ring-buffer-size).
OpenTelemetry Export: Optionally export metrics to an OTLP endpoint (--otel-endpoint).
Interception Tools Integration: Designed for use in standard Interception Tools pipelines (intercept | intercept-bounce | uinput).
Robust Testing: Includes unit tests, integration tests (assert_cmd), property tests (proptest), and fuzzing (cargo-fuzz).
Benchmarking: Core filter logic and channel communication can be benchmarked (cargo bench).

Installation

From Crates.io

cargo install intercept-bounce

From Source (Git Repository)

git clone https://github.com/sinity/intercept-bounce.git
cd intercept-bounce
cargo install --path .

Using Nix

With Nix flakes enabled:

# Build and run directly
nix run github:sinity/intercept-bounce -- --help

# Build the package
nix build github:sinity/intercept-bounce

# Install into your Nix profile
nix profile install github:sinity/intercept-bounce

NixOS System (Declarative, using flake)

Since intercept-bounce is not yet packaged in the official nixpkgs repository, the recommended way to install it declaratively on NixOS is by adding this repository as a flake input to your system or Home Manager configuration.

Add the flake input: Modify your top-level flake.nix (e.g., /etc/nixos/flake.nix or ~/.config/home-manager/flake.nix) to include intercept-bounce:

# In your flake.nix
{
  description = "Your system configuration";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable"; # Or your preferred channel

    # Add intercept-bounce flake input
    intercept-bounce.url = "github:sinity/intercept-bounce";
    # Optional: Pin to a specific commit or tag for stability
    # intercept-bounce.inputs.nixpkgs.follows = "nixpkgs"; # Ensure it uses your nixpkgs
    # intercept-bounce.rev = "YOUR_COMMIT_HASH_HERE";
  };

  outputs = { self, nixpkgs, intercept-bounce, ... }@inputs:
    let
      system = "x86_64-linux"; # Or your system architecture
      pkgs = import nixpkgs { inherit system; };
    in
    {
      # Example for configuration.nix:
      nixosConfigurations.your-hostname = nixpkgs.lib.nixosSystem {
        inherit system;
        specialArgs = { inherit inputs; }; # Pass inputs to your config module
        modules = [
          ./configuration.nix # Your main configuration file
          # ... other modules
        ];
      };

      # Example for Home Manager (within a Home Manager flake):
      # homeConfigurations."your-username" = home-manager.lib.homeManagerConfiguration {
      #   inherit pkgs;
      #   extraSpecialArgs = { inherit inputs; }; # Pass inputs to your config module
      #   modules = [
      #     ./home.nix # Your main Home Manager config file
      #     # ... other modules
      #   ];
      # };
    };
}

NixOS module

This repository also ships a reusable NixOS module that renders the CLI invocation, exposes every flag as a typed option, and (optionally) adds the executable to environment.systemPackages. After adding the flake input, enable the module and tune the options that matter to your keyboard pipeline:

{
  imports = [ inputs.intercept-bounce.nixosModules.default ];

  services.interceptBounce = {
    enable = true;
    debounceTime = "40ms";
    logInterval = "6h";
    logBounces = true;
    statsJson = true;
    extraArgs = [ "--debounce-key" "KEY_ENTER" ];
  };
}

The module exposes the resolved command in both list (services.interceptBounce.command) and string (services.interceptBounce.commandString) form, making it straightforward to slot into udevmon pipelines or higher-level abstractions.

Add the package to your configuration: In the NixOS or Home Manager module file referenced above (e.g., configuration.nix or home.nix), add the package to your desired list, referencing it via the inputs:

# In /etc/nixos/configuration.nix or ~/.config/home-manager/home.nix
{ pkgs, inputs, ... }: # Ensure 'inputs' is available here

{
  # Example for NixOS system packages:
  environment.systemPackages = with pkgs; [
    # ... other packages

    # Reference the default package from the intercept-bounce flake input
    inputs.intercept-bounce.packages.${pkgs.system}.default
  ];

  # Example for Home Manager packages:
  # home.packages = with pkgs; [
  #   inputs.intercept-bounce.packages.${pkgs.system}.default
  # ];

  # ... rest of your configuration
}

Rebuild your configuration: Apply the changes using sudo nixos-rebuild switch (for NixOS) or home-manager switch (for Home Manager).

User Profile (Non-NixOS or User-Local)

If you are not using NixOS or prefer to install the tool only for your user, you can install it directly into your Nix profile from the flake URL:

nix profile install github:sinity/intercept-bounce

This command is declarative for your user profile but does not integrate the package into the NixOS system configuration.

Building and Running Directly

You can also build or run the package directly without installing it permanently:

# Build the package (output in ./result)
nix build github:sinity/intercept-bounce

# Run directly without installing
nix run github:sinity/intercept-bounce -- --help

The Nix flake also provides a development shell (nix develop) with necessary tools (see Development).

Usage

intercept-bounce is designed to be used within an Interception Tools pipeline.

Basic Pipeline

The most common usage involves capturing events from a physical keyboard, filtering them with intercept-bounce, and creating a new virtual keyboard with the filtered output using uinput.

# Find your keyboard device first (e.g., using 'intercept-bounce --list-devices' or 'intercept -L')
# Example device path: /dev/input/by-id/usb-My_Awesome_Keyboard-event-kbd

# Run the pipeline (requires root/sudo)
sudo sh -c 'intercept -g /dev/input/by-id/usb-My_Awesome_Keyboard-event-kbd \
           | intercept-bounce --debounce-time 15ms \
           | uinput -d /dev/input/by-id/usb-My_Awesome_Keyboard-event-kbd'
# Note: Using the virtual device created by uinput requires configuration. See the "Integration" section below.

Important:

Replace /dev/input/by-id/usb-My_Awesome_Keyboard-event-kbd with the actual path to your keyboard device. Using paths from /dev/input/by-id/ is recommended as they are stable.
The device path provided to intercept -g must be the same as the one provided to uinput -d.
This command creates a new virtual device. Your desktop environment (Xorg/Wayland) needs to use this new device instead of the original physical one. See the Integration section for details.

Filtering Only Certain Keys

Use --debounce-key when you only want chatter protection on a handful of controls. Any keys not listed pass straight through.

sudo sh -c 'intercept -g $DEVNODE \
           | intercept-bounce --debounce-time 100ms \
                               --debounce-key KEY_ENTER \
                               --debounce-key KEY_SPACE \
           | uinput -d $DEVNODE'

You can still supply --ignore-key for the allowlisted set—--debounce-key wins if both flags mention the same code—so it’s safe to keep shared configs that exempt volume wheels without losing an explicit per-key allowlist.

udevmon Integration (Recommended)

Using udevmon (part of Interception Tools) is the recommended way to manage the pipeline automatically when the device is connected/disconnected. Add a job to your /etc/interception/udevmon.yaml (or user-specific config):

- JOB: intercept -g $DEVNODE | intercept-bounce --debounce-time 15ms | uinput -d $DEVNODE
  DEVICE:
    LINK: /dev/input/by-id/usb-My_Awesome_Keyboard-event-kbd # <-- Change this!

Remember to replace the LINK with the correct path for your keyboard and restart the udevmon service (sudo systemctl restart interception-udevmon or similar).

Command-Line Options

Usage: intercept-bounce [OPTIONS]

Options:
  -t, --debounce-time <DURATION>
          Debounce time threshold (e.g., "25ms", "0.01s"). [default: 25ms]
      --near-miss-threshold-time <DURATION>
          Threshold for logging "near-miss" events (e.g., "100ms"). [default: 100ms]
      --log-interval <DURATION>
          Periodically dump statistics to stderr (e.g., "15m", "60s", "0s" to disable). [default: 15m]
      --log-all-events
          Log details of *every* incoming event ([PASS]/[DROP]).
      --log-bounces
          Log details of *only dropped* (bounced) key events.
      --list-devices
          List available input devices and their capabilities (requires root).
      --stats-json
          Output statistics as JSON format to stderr.
      --verbose
          Enable verbose logging (DEBUG level).
      --ring-buffer-size <SIZE>
          Size of the ring buffer for storing recently passed events (0 to disable). [default: 0]
      --debounce-key <KEY>
          Key codes or names to debounce. When present, only these keys are debounced (all others pass through). Repeat the flag to list multiple keys.
      --ignore-key <KEY>
          Key codes or names to never debounce unless they are also provided via `--debounce-key`.
      --otel-endpoint <URL>
          OTLP endpoint URL for exporting traces and metrics (e.g., "http://localhost:4317").
  -h, --help
          Print help
  -V, --version
          Print version

For detailed explanations of each option, see man intercept-bounce (if installed) or intercept-bounce --help.

How it Works

Debouncing

intercept-bounce filters key chatter by remembering the timestamp of the last passed event for each unique combination of key code (e.g., KEY_A) and key state (press=1, release=0).

When a new key event arrives, its timestamp is compared to the last passed timestamp for the same key code and state.
If the time difference is less than the configured --debounce-time, the new event is considered a bounce and is dropped.
If the time difference is greater than or equal to the --debounce-time, or if the event has a different key code or state, the event is passed through, and its timestamp becomes the new "last passed" time for that specific key/state.
Key repeat events (value=2) are always passed without debouncing.
Non-key events (mouse, sync, etc.) are always passed.

Near-Miss Tracking

This feature helps diagnose keys with inconsistent timing just outside the debounce window.

When a key event passes the debounce filter, the time difference since the previous passed event for the same key/state is calculated.
If this difference is less than or equal to the --near-miss-threshold-time, the event is recorded as a "near-miss" in the statistics.
High near-miss counts for a key might indicate a failing switch or that the --debounce-time needs adjustment.

Statistics

intercept-bounce collects detailed statistics, printed to stderr on exit (Ctrl+C) or periodically (--log-interval).

Human-Readable Format (Default)

Overall Statistics: Total key events processed, passed, dropped, and overall drop percentage.
Overall Histograms: Visual distribution of bounce timings and near-miss timings across all keys.
Dropped Event Statistics Per Key: For each key with activity:
- Summary: Total processed, passed, dropped, drop %.
- Details per state (Press/Release/Repeat): Processed, Passed, Dropped, Drop Rate (%), Bounce Time (Min/Avg/Max) if drops occurred.
Passed Event Near-Miss Statistics: For each key/state with near-misses: Count, Near-Miss Time (Min/Avg/Max).

JSON Format (`--stats-json`)

Provides a machine-readable JSON object containing all the information from the human-readable report, plus raw timing data arrays and detailed histogram bucket counts. Key top-level fields:

report_type: "Cumulative" or "Periodic".
runtime_us: Total runtime (cumulative only).
Configuration values (debounce_time_us, near_miss_threshold_us, etc.).
Overall counts (key_events_processed, key_events_passed, key_events_dropped).
overall_bounce_histogram, overall_near_miss_histogram: Detailed histogram objects.
per_key_stats: Array of objects per key, including detailed stats per state (press/release/repeat) with sampled timings_us, min_us/max_us/avg_us, and a bounce_histogram.
per_key_near_miss_stats: Array of objects per key/state with sampled timings_us, summary fields, and a near_miss_histogram. Sample arrays retain only the most recent timings to avoid unbounded memory growth.

Refer to the StatsCollector::print_stats_json implementation or the man page for the exact structure.

Histograms

Histograms show the distribution of timings (bounce or near-miss) in milliseconds across predefined buckets (e.g., <1ms, 1-2ms, 2-4ms, ..., >=128ms). They help visualize the typical duration of bounces or near-misses. The average timing is also calculated.

Logging

Logging messages are printed to stderr.

--log-all-events: Logs [PASS] or [DROP] for almost every event, showing type, code, value, key name, and timing info. (Skips EV_SYN/EV_MSC for clarity). Performance impact!
--log-bounces: Logs only [DROP] messages for key events, including bounce time. Less verbose than --log-all-events.
--verbose: Enables DEBUG level logging, showing internal state, thread activity, etc. Sets default filter to intercept_bounce=debug if RUST_LOG is not set.
RUST_LOG Environment Variable: Provides fine-grained control using the tracing_subscriber::EnvFilter format (e.g., RUST_LOG=info, RUST_LOG=intercept_bounce=trace, RUST_LOG=warn,intercept_bounce::filter=debug). Overrides --verbose.

Performance Note: High logging verbosity (--log-all-events, RUST_LOG=trace) can significantly impact performance and may cause log messages to be dropped if the logger thread cannot keep up. A warning ("Logger channel full...") will be printed if this happens.

Integration with Interception Tools

Pipeline: The standard usage is intercept -g <device> | intercept-bounce [OPTIONS] | uinput -d <device>.
udevmon: Recommended for managing the pipeline automatically. See Usage.
Virtual Device: uinput creates a new virtual input device (e.g., /dev/input/eventX). Your Desktop Environment (Xorg/Wayland) must be configured to use this new virtual device. The original physical device still emits raw events. Configuration methods vary; sometimes automatic, sometimes requiring DE-specific settings (e.g., Xorg InputClass sections, Wayland compositor settings). Use tools like libinput list-devices to identify the virtual device (often contains "Uinput" or "intercept-bounce" in the name).
Wayland/Xorg: Interception Tools generally work more reliably under Xorg. Wayland compositors often restrict global input grabbing. Using intercept-bounce under Wayland might require specific compositor support or configuration to recognize and prioritize the uinput virtual device.

Troubleshooting

Permission Denied: Running intercept and uinput requires root privileges or specific group memberships (input group for reading /dev/input/event*, potentially custom udev rules for /dev/uinput write access). Using sudo sh -c '...' for the whole pipeline is common. intercept-bounce --list-devices also needs read access.
Incorrect Device Path: Ensure the path used for intercept -g and uinput -d is identical and correct. Use stable paths from /dev/input/by-id/. Use intercept-bounce --list-devices or intercept -L to find devices.
Filter Not Working / No Output:
- Check pipeline order and permissions.
- Verify device paths match.
- Check udevmon status and logs (sudo systemctl status interception-udevmon, journalctl -u interception-udevmon).
- Run intercept-bounce with --verbose or --log-all-events to check processing and stderr for errors.
- Confirm your DE is using the virtual device created by uinput.
Too Much Filtering (Missed Keystrokes): Lower --debounce-time.
Too Little Filtering (Chatter Still Occurs): Increase --debounce-time. Use --log-bounces or statistics (bounce timings/histograms) with a low debounce time first to measure the chatter duration, then set the time slightly higher.
Mixed Output in Terminal: Redirect stderr (2> log.txt) or use udevmon.
"Logger channel full..." Warning: Logger thread can't keep up (heavy logging, slow OTLP endpoint, high load). Log messages/stats may be lost. Reduce logging verbosity or disable OTLP if problematic.
JSON Stats Errors: Check stderr for non-JSON error messages printed before the JSON output.

Development

Building

cargo build
cargo build --release

Testing

# Run all tests (unit, integration, property)
cargo test --all-targets --all-features

# Run specific integration test
cargo test --test sanity -- --nocapture drops_bounce

# Run property tests only
cargo test --test property_tests

Benchmarking

cargo bench

Linting & Formatting

# Check formatting
cargo fmt --check

# Apply formatting
cargo fmt

# Run clippy lints
cargo clippy --all-targets --all-features -- -D warnings

xtasks

Common development tasks are available via a separate xtask crate. Run them using cargo run --package xtask -- <command>:

# Generate man page and shell completions (outputs to docs/)
cargo run --package xtask -- generate-docs

# Run checks (equivalent to cargo check)
cargo run --package xtask -- check
# Run tests (equivalent to cargo test)
cargo run --package xtask -- test

# Run clippy (equivalent to cargo clippy -- -D warnings)
cargo run --package xtask -- clippy

# Check formatting (equivalent to cargo fmt --check)
cargo run --package xtask -- fmt-check

Nix Development Shell

If you have Nix installed with flakes enabled, use nix develop to enter a shell with all necessary development tools (Rust toolchain, cargo-fuzz, cargo-audit, interception-tools, man, etc.) and useful aliases (ct for test, cl for clippy, cf for fmt, xt for xtask).

Fuzzing

Requires cargo-fuzz:

cargo install cargo-fuzz

# List fuzz targets
cargo fuzz list

# Run the stats fuzzer
cargo fuzz run fuzz_target_stats

Contributing

Contributions are welcome! Please feel free to open an issue or submit a pull request on GitHub.

License

Licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Name		Name	Last commit message	Last commit date
Latest commit History 394 Commits
.github/workflows		.github/workflows
benches		benches
docs		docs
fuzz		fuzz
nix/modules		nix/modules
src		src
test-helpers		test-helpers
tests		tests
xtask		xtask
.envrc		.envrc
.gitignore		.gitignore
.pre-commit-config.yaml		.pre-commit-config.yaml
AGENTS.md		AGENTS.md
Cargo.lock		Cargo.lock
Cargo.toml		Cargo.toml
README.md		README.md
build.rs		build.rs
dev.sh		dev.sh
flake.lock		flake.lock
flake.nix		flake.nix

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intercept-bounce

Features

Installation

From Crates.io

From Source (Git Repository)

Using Nix

NixOS System (Declarative, using flake)

NixOS module

User Profile (Non-NixOS or User-Local)

Building and Running Directly

Usage

Basic Pipeline

Filtering Only Certain Keys

udevmon Integration (Recommended)

Command-Line Options

How it Works

Debouncing

Near-Miss Tracking

Statistics

Human-Readable Format (Default)

JSON Format (--stats-json)

Histograms

Logging

Integration with Interception Tools

Troubleshooting

Development

Building

Testing

Benchmarking

Linting & Formatting

xtasks

Nix Development Shell

Fuzzing

Contributing

License

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JSON Format (`--stats-json`)

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