Hammer

Single-bar bullish reversal candidate. A Hammer has a small real body sitting near the top of the bar, a long lower shadow at least twice the body, and a short or absent upper shadow. Traditionally read as rejection of lower prices — sellers tried to push price down, buyers pushed it back up.

Quick reference

Item	Value
Family	Candlestick Patterns
Input type	Candle
Output type	f64 — +1.0 if Hammer, 0.0 otherwise
Output range	{0.0, +1.0}
Default parameters	none — Hammer::new()
Warmup period	1
Interpretation	Bullish reversal candidate (in downtrend context)

Formula

body         = |close - open|
upper_shadow = high - max(open, close)
lower_shadow = min(open, close) - low

hammer = (lower_shadow >= 2 · body)
       AND (upper_shadow <= body)
       AND (body > 0)

Output is +1.0 when the shape matches, 0.0 otherwise. See crates/wickra-core/src/indicators/hammer.rs.

Parameters

None — Hammer::new() takes no arguments. The shape rules use fixed ratios (lower shadow ≥ 2 · body, upper shadow ≤ body).

Signed ±1 encoding

A Hammer is bullish by definition, so under the uniform candlestick sign convention (+1.0 bullish, −1.0 bearish, 0.0 none) it emits +1.0 when the shape matches and 0.0 otherwise — never −1.0. The same geometry read at the top of an uptrend is the bearish HangingMan, which carries the opposite sign.

Inputs / Outputs

Indicator<Input = Candle, Output = f64>. Python: Hammer().batch(open, high, low, close) returns a 1-D np.ndarray. Node: update(candle) returns number | null.

Warmup

warmup_period() == 1. Stateless; every bar emits.

Edge cases

• Zero-body bar. body == 0 fails the body > 0 clause — pure Doji is not a Hammer.
• Pure rejection (close == high). upper_shadow == 0, passes the ≤ body clause; common Hammer shape.
• Trend-context filter. This indicator only checks shape — it does not verify that the prior trend was down. Combine with a trend indicator for actionable signals.
• Reset. Stateless.

Examples

Rust

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

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Hammer: o=100, h=101, l=95, c=100.5 — small body, long lower shadow
    let c = Candle::new(100.0, 101.0, 95.0, 100.5, 1.0, 0)?;
    let mut h = Hammer::new();
    println!("hammer = {:?}", h.update(c));  // +1.0
    Ok(())
}

Python

import numpy as np
import wickra as ta

o = np.array([100.0])
h = np.array([101.0])
l = np.array([ 95.0])
c = np.array([100.5])

ha = ta.Hammer()
print(ha.batch(o, h, l, c))  # [1.0]

Node

const wickra = require('wickra');
const ha = new wickra.Hammer();
console.log(ha.batch([100], [101], [95], [100.5]));

Streaming with trend filter

use wickra::{Candle, Hammer, Indicator, Sma};

let mut h = Hammer::new();
let mut sma = Sma::new(50).unwrap();
let candle_stream: Vec<wickra::Candle> = Vec::new(); // your live OHLCV candle feed
for bar in candle_stream {
    let trend = sma.update(bar.close);
    if h.update(bar) == Some(1.0) {
        // Only act if trend is down (price below long SMA)
        if matches!(trend, Some(s) if bar.close < s) {
            // bullish reversal candidate
        }
    }
}

Interpretation

• Bullish reversal. Most powerful at the bottom of a downtrend or at established support. Without that context, a Hammer is just one of many candle shapes.
• Pair with confirmation. Many traders require the next bar to close above the Hammer's high before entering — see ThreeOutside for the confirmed three-bar variant of the broader engulfing pattern.

Common pitfalls

• Treating Hammer alone as a signal. False-positive rate is very high without trend context.
• Confusing with Hanging Man. Same shape, opposite trend context — Hammer at the bottom is bullish; the same shape at the top of an uptrend is a Hanging Man (bearish). See HangingMan.

References

• Steve Nison, Japanese Candlestick Charting Techniques (1991).

See also

• HangingMan — same shape, bearish context.
• InvertedHammer — mirror with long upper shadow.
• ShootingStar — Inverted Hammer's bearish twin.
• Indicators-Overview — full taxonomy.