Crab

Five-point (X-A-B-C-D) harmonic with the deepest D of the family — a 1.618 extension of XA reached by a very long terminal leg. Bullish (D a swing low) → +1, bearish → -1.

Quick reference

Item	Value
Family	Harmonic Patterns
Input type	Candle (uses high, low)
Output type	f64 (+1 / -1 / 0)
Output range	{-1.0, 0.0, +1.0}
Default parameters	none (swing threshold 5%, baked)
Warmup period	6
Interpretation	Reversal at a deep extended D

Formula

last five pivots X-A-B-C-D:
  AB / XA ∈ [0.382, 0.618]
  BC / AB ∈ [0.382, 0.886]
  CD / BC ∈ [2.24, 3.618]  (a very long terminal leg)
  AD / XA ∈ [1.55, 1.65]   (≈ 1.618 — the defining deep D completion)
direction: D a swing low → +1, a swing high → -1

See crates/wickra-core/src/indicators/crab.rs.

Parameters

None. Swing threshold 0.05 is a baked-in family constant (pattern_swing.rs); the Fibonacci windows are documented detector constants. Crab::new is infallible.

Inputs / Outputs

const _: fn(&mut wickra::Crab, wickra::Candle) -> Option<f64> =
    <wickra::Crab as wickra::Indicator>::update;

• Python. update((o,h,l,c,v,ts)) → float (never None); batch(open, high, low, close) → 1-D ndarray.
• Node. update(open, high, low, close) → number; batch(open, high, low, close) → number[].
• WASM. update(open, high, low, close) → number.

Warmup

warmup_period() == 6. Five confirmed pivots are required. Pinned by test accessors_and_metadata.

Edge cases

• Bullish Crab reports +1 (test bullish_crab_is_plus_one).
• Bearish Crab reports -1 (test bearish_crab_is_minus_one).
• Legs outside the windows report 0.0 (test out_of_ratio_does_not_trigger).
• reset clears state (test reset_clears_state).
• Streaming equals batch (test batch_equals_streaming).

Examples

Rust

use wickra::{Candle, Crab, Indicator};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let bars = [
        (149.85, 150.0, 149.85, 149.85),
        (100.0, 148.5, 100.0, 100.0),
        (101.0, 140.0, 101.0, 101.0),
        (120.0, 138.6, 120.0, 120.0),
        (121.2, 137.5, 121.2, 121.2),
        (75.3, 136.125, 75.3, 75.3),
        (76.053, 82.83, 76.053, 76.053), // deep D confirms → bullish
    ];
    let mut pat = Crab::new();
    let mut last = 0.0;
    for (ts, (o, h, l, c)) in bars.iter().enumerate() {
        last = pat.update(Candle::new(*o, *h, *l, *c, 1.0, ts as i64)?).unwrap();
    }
    println!("{last}"); // 1
    Ok(())
}

Python

import wickra as ta

bars = [
    (149.85, 150.0, 149.85, 149.85, 1.0, 0),
    (100.0, 148.5, 100.0, 100.0, 1.0, 1),
    (101.0, 140.0, 101.0, 101.0, 1.0, 2),
    (120.0, 138.6, 120.0, 120.0, 1.0, 3),
    (121.2, 137.5, 121.2, 121.2, 1.0, 4),
    (75.3, 136.125, 75.3, 75.3, 1.0, 5),
    (76.053, 82.83, 76.053, 76.053, 1.0, 6),
]
pat = ta.Crab()
print([pat.update(b) for b in bars][-1])  # 1.0

Node

const wickra = require('wickra');
const bars = [
  [149.85, 150.0, 149.85, 149.85], [100.0, 148.5, 100.0, 100.0],
  [101.0, 140.0, 101.0, 101.0], [120.0, 138.6, 120.0, 120.0],
  [121.2, 137.5, 121.2, 121.2], [75.3, 136.125, 75.3, 75.3],
  [76.053, 82.83, 76.053, 76.053],
];
const pat = new wickra.Crab();
let last = 0;
for (const [o, h, l, c] of bars) last = pat.update(o, h, l, c);
console.log(last); // 1

Streaming

pat = ta.Crab()
for o, h, l, c, v, ts in candle_feed:
    signal = pat.update((o, h, l, c, v, ts))
    if signal != 0:
        pass  # Crab completed at a deep D — reversal setup

Interpretation

1. Extreme reversal. The 1.618 D is the deepest completion in the family, catching capitulation/blow-off moves. The reward can be large; the stop (beyond D) is correspondingly distant.
2. Long CD leg. The 2.24–3.618 CD is the structural fingerprint — a sharp terminal thrust to D.

Common pitfalls

• Rarity. The deep extension is uncommon; expect few signals.
• Confirmation lag. Non-repainting but lags the visual D.

References

• Carney, S. Harmonic Trading (2010).

See also

• Butterfly, Bat, Gartley.
• Indicators-Overview — full taxonomy.