Implement Williams %R momentum indicator

[Copilot]

Adds Williams %R (Williams Percent Range), a momentum indicator measuring overbought/oversold levels on a -100 to 0 scale. Readings near 0 indicate overbought conditions, readings near -100 indicate oversold.

Implementation

• Formula: %R = (Highest High - Close) / (Highest High - Lowest Low) × -100
• Optimization: Single-pass loop for finding highest/lowest values (O(n) vs O(2n))
• Edge cases: Returns -100 when price range is zero (prevents division by zero)

Usage

import {WilliamsR} from 'trading-signals';

const willR = new WilliamsR(14); // 14-period lookback

willR.add({close: 44.00, high: 44.50, low: 43.00});
willR.add({close: 43.50, high: 44.25, low: 42.75});
// ... add more candles

if (willR.isStable) {
  const value = willR.getResult(); // e.g., -23.5
}

Verification

Test suite confirms Williams %R is arithmetically equivalent to the inverted Stochastic %K oscillator (using parameters 5, 1, 1 for raw %K without smoothing): Williams %R = Stochastic %K - 100

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Williams %R indicator?</issue_title>
<issue_description>Could you please implement "Williams %R" indicator?</issue_description>

<agent_instructions>Implement the Williams %R (Williams Percent Range) indiactor along with some test cases. The %R indicator is arithmetically exactly equivalent to the %K stochastic oscillator, mirrored at the 0%-line, when using the same time interval. Verify that as well in a test case by using the "StochasticOscillator" already provided by the "trading-signals" library.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@bennycode @netandreus are you referring to [Williams Percent Range](https://en.wikipedia.org/wiki/Williams_%25R)?

• Fixes Williams %R indicator? #880

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.