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Swiss Army Knife Indicator

Ehlers DSP filter ehlers dsp multi-purpose smoothing

A versatile indicator that can be configured as EMA, SMA, Gaussian, Butterworth, High Pass, Band Pass, or Band Stop filter.

Visual Example

Swiss Army Knife Indicator — annotated preview mapping to core implementation

Synthetic ideal per library logic. Generated 2026-07-01 IST via docs/generate_all_previews.py (reproducible; maps to core Next<T> implementation).

Description

A versatile indicator that can be configured as EMA, SMA, Gaussian, Butterworth, High Pass, Band Pass, or Band Stop filter.

Use as a single configurable filter that can emulate SMA, EMA, Gaussian, Butterworth, or bandpass responses by switching mode flags. Ideal for prototyping different filter designs without code changes.

Part of QuantWave's Ehlers digital signal processing suite. Designed for low-lag cycle and trend work — pair with Roofing Filter or SuperSmoother on noisy inputs.

Ehlers presents the Swiss Army Knife Indicator in Cycle Analytics for Traders as a unified filter framework. A set of boolean flags selects the operating mode, making it possible to compare multiple filter responses on the same data by simply toggling flags rather than reimplementing each filter.

Typical applications:

  • Use for cycle timing in mean-reverting regimes
  • Gate with Hurst exponent or ADX before taking cycle signals
  • Allow BandPass+ bars warm-up for filter state to stabilise
  • Chain with Roofing Filter when input is noisy

QuantWave implements this via the universal Next<T> trait — bit-identical across Rust streaming, Python streaming, and Polars .ta() batch plugins.

Formula / Specification

Implementation (quantwave-core/src/indicators/swiss_army_knife.rs):

\[ Filt = c_0(b_0 x_t + b_1 x_{t-1} + b_2 x_{t-2}) + a_1 Filt_{t-1} + a_2 Filt_{t-2} - c_1 x_{t-N} \]

Gold-standard parity vectors: quantwave-core/tests/gold_standard/swiss_army_knife.json.

Parameters

Parameter Default Description
mode BandPass Filter mode (EMA, SMA, Gauss, Butter, Smooth, HP, 2PHP, BP, BS)
period 20 Filter period
delta 0.1 Bandwidth parameter for BP and BS modes

Usage Examples

Streaming (Rust)

use quantwave_core::indicators::SWISS_ARMY_KNIFE;
use quantwave_core::traits::Next;

let mut ind = SWISS_ARMY_KNIFE::new(BandPass);
for price in &prices {
    let value = ind.next(price);
}

Streaming (Python)

from quantwave import SWISS_ARMY_KNIFE

ind = SWISS_ARMY_KNIFE(BandPass)
for price in prices:
    value = ind.next(price)

Polars Batch (Python)

import polars as pl
import quantwave as qw

def apply_swiss_army_knife_indicator(series: pl.Series) -> pl.Series:
    ind = qw.SWISS_ARMY_KNIFE(BandPass)
    return pl.Series([ind.next(float(v)) for v in series.to_list()])

df = (
    pl.read_csv('ohlcv.csv')
    .lazy()
    .with_columns(
        pl.col("close").map_batches(apply_swiss_army_knife_indicator, return_dtype=pl.Float64).alias("swiss_army_knife_indicator")
    )
    .collect()
)

All surfaces are bit-identical via the single Next<T> implementation and proptests.

Edge Cases & Limitations

  • Recursive DSP filters require a warm-up period; first N bars may be unstable or raw-pass-through.
  • Designed for cyclic/mean-reverting regimes; trending markets can produce lag or drift.
  • Parameter period (or equivalent) controls cutoff — too small adds noise, too large adds lag.
  • Prefer chaining with other Ehlers tools (Roofing Filter, SuperSmoother) on noisy inputs.
  • Validated via proptests against gold-standard vectors where available.
  • No look-ahead bias; suitable for live streaming and batch feature pipelines.

Boundary Behavior

Condition Behavior
Warm-up Leading bars return NaN until warmup_bars is satisfied.
period > len When period exceeds series length, output is all NaN.
NaN inputs NaN in input propagates to output (NaN out).
Invalid params Non-positive period or missing required params raise ValueError.
Empty data Empty input returns an empty result series.

Sources & References

Primary Source: https://github.com/lavs9/quantwave/blob/main/references/Ehlers%20Papers/SwissArmyKnifeIndicator.pdf

Implementation: quantwave-core/src/indicators/swiss_army_knife.rs (SWISS_ARMY_KNIFE / SWISS_ARMY_KNIFE_METADATA). Parity: quantwave-core/tests/gold_standard/swiss_army_knife.json

Provenance: Standards bulk upgrade 2026-07-01 IST — see docs/DOCUMENTATION_STANDARDS.md.