RSI

Relative Strength Index — Wilder's bounded momentum oscillator that maps the ratio of average gains to average losses onto the [0, 100] range.

Quick reference

Field	Value
Family	Momentum Oscillators
Input type	f64 (close)
Output type	f64
Output range	[0, 100]
Default parameters	period = 14 (Python)
Warmup period	period + 1 (15 for period = 14)
Interpretation	overbought above 70, oversold below 30 (Wilder's thresholds)

Formula

diff_t   = close_t − close_{t-1}
gain_t   = max(diff_t, 0)
loss_t   = max(−diff_t, 0)

Seed (Wilder, at t = period):
  avg_gain_p = (gain_1 + … + gain_p) / p
  avg_loss_p = (loss_1 + … + loss_p) / p

Recursive smoothing (t > period), with α = 1 / period:
  avg_gain_t = (avg_gain_{t-1} · (period − 1) + gain_t) / period
  avg_loss_t = (avg_loss_{t-1} · (period − 1) + loss_t) / period

RS_t  = avg_gain_t / avg_loss_t
RSI_t = 100 − 100 / (1 + RS_t)

When avg_loss_t == 0 and avg_gain_t > 0, RSI is 100 directly; when both are zero (a perfectly flat series) the implementation returns the standard 50 convention.

Parameters

Name	Type	Default (Python)	Valid range	Description
period	usize	14	>= 1	Wilder smoothing length. Rsi::new(0) returns Error::PeriodZero.

Inputs / Outputs

From impl Indicator for Rsi in crates/wickra-core/src/indicators/rsi.rs:

use wickra::{Indicator, Rsi};
// Rsi: Input = f64, Output = f64
const _: fn(&mut Rsi, f64) -> Option<f64> = <Rsi as Indicator>::update;

The output is a scalar in [0, 100]. In Python batch(prices) returns a 1-D np.ndarray of float64, with NaN in the warmup positions. In Node batch(prices) returns a flat number[], also NaN during warmup.

Warmup

warmup_period() returns period + 1. The reason is that RSI consumes diffs, not prices: with period prices you only have period − 1 diffs, so you need exactly one extra price before Wilder's seed average is well defined. The Rust test warmup_period_is_period_plus_one pins this:

use wickra::{Indicator, Rsi};
let rsi = Rsi::new(14).unwrap();
assert_eq!(rsi.warmup_period(), 15);

In streaming terms, the first period calls to update() return None; the (period + 1)-th call returns the first Some(value).

Edge cases

• Flat input. When every input price is identical, every gain and every loss is zero, so avg_loss == avg_gain == 0. The implementation returns 50.0 by convention (see Rsi::rsi_from_avgs). The unit test flat_series_yields_rsi_50 pins this behaviour.
• Pure uptrend / pure downtrend. avg_loss == 0 with avg_gain > 0 short-circuits to 100; the mirror case returns 0. Tests pure_uptrend_yields_rsi_100 and pure_downtrend_yields_rsi_0 cover this.
• Non-finite input. update() returns the previously emitted value (or None if no value has been emitted yet) when the input is NaN or infinite — the internal state is not advanced.
• Reset. reset() returns the indicator to the freshly-constructed state: prev_close, both seed buffers, both averages, and last_value are cleared.

Examples

Rust

use wickra::{BatchExt, Indicator, Rsi};

let prices = [
    44.34, 44.09, 44.15, 43.61, 44.33, 44.83, 45.10, 45.42,
    45.84, 46.08, 45.89, 46.03, 45.61, 46.28, 46.28, 46.00,
    46.03, 46.41, 46.22, 45.64,
];
let mut rsi = Rsi::new(14)?;
let out = rsi.batch(&prices);
println!("first = {}", out[14].unwrap());
println!("last  = {}", out[19].unwrap());
# Ok::<(), wickra::Error>(())

Verified output:

first = 70.46413502109705
last  = 57.91502067008556

Python

import numpy as np
import wickra as ta

prices = np.array([
    44.34, 44.09, 44.15, 43.61, 44.33, 44.83, 45.10, 45.42,
    45.84, 46.08, 45.89, 46.03, 45.61, 46.28, 46.28, 46.00,
    46.03, 46.41, 46.22, 45.64,
], dtype=float)
rsi = ta.RSI(14)
v = rsi.batch(prices)
print("warmup:", rsi.warmup_period())
print("first :", float(v[14]))
print("last  :", float(v[-1]))

Verified output:

warmup: 15
first : 70.46413502109705
last  : 57.91502067008556

Node

const wickra = require('wickra');

const rsi = new wickra.RSI(14);
const prices = [
  44.34, 44.09, 44.15, 43.61, 44.33, 44.83, 45.10, 45.42,
  45.84, 46.08, 45.89, 46.03, 45.61, 46.28, 46.28, 46.00,
  46.03, 46.41, 46.22, 45.64,
];
const v = rsi.batch(prices);
console.log('warmup:', rsi.warmupPeriod());
console.log('first :', v[14]);
console.log('last  :', v[19]);

Verified output:

warmup: 15
first : 70.46413502109705
last  : 57.91502067008556

Interpretation

• Overbought / oversold zones. Wilder's classic thresholds are 70 (overbought) and 30 (oversold). Many crypto and FX desks tighten them to 80 / 20 for trending markets and loosen to 60 / 40 for range-bound markets.
• Midline cross. A move through 50 is sometimes used as a directional signal; above 50 means average gains exceed average losses over the smoothing window.
• Divergence. A higher price high paired with a lower RSI high (bearish divergence) is a classic Wilder signal; the symmetric pattern at lows is bullish.

Common pitfalls

• RSI on flat input is 50, not undefined. The implementation returns 50.0 when both averages are zero. Do not interpret this as a neutral signal — it is a placeholder that means "the indicator has no opinion yet". Pair RSI with a volatility filter (e.g. ATR) if your strategy is sensitive to ranging markets.
• period + 1 warmup, not period. A common bug is sizing the result array against period and indexing into the warmup region. The first Some arrives at the (period + 1)-th update; in batch form, indices 0..period are None/NaN. See Warmup Periods.
• Non-finite inputs are absorbed silently. update(f64::NAN) does not advance the state and returns the previous value. If you depend on a 1:1 input-to-output mapping, pre-validate your data before feeding it in.

References

• J. Welles Wilder, New Concepts in Technical Trading Systems, Trend Research, 1978. The original publication that defines both RSI and the Wilder smoothing scheme used internally.

See also

• Indicator: MacdIndicator — also momentum, but trend-following and unbounded.
• Indicator: Stochastic — sibling bounded oscillator, faster and noisier than RSI.
• Warmup Periods — the canonical period + 1 off-by-one explained.
• Quickstart: Python — full RSI batch / streaming walk-through.