brackt/app/models/__tests__/playoff-match-odds.test.ts

import { describe, it, expect } from "vitest";
import {
  americanToImpliedProbability,
  impliedProbabilityToAmerican,
  normalizeOdds,
} from "../playoff-match-odds";

describe("americanToImpliedProbability", () => {
  it("converts even odds (+100) to 0.5", () => {
    expect(americanToImpliedProbability(100)).toBeCloseTo(0.5);
  });

  it("converts favourite negative odds (-110) correctly", () => {
    // 110 / (110 + 100) = 110/210 ≈ 0.5238
    expect(americanToImpliedProbability(-110)).toBeCloseTo(110 / 210);
  });

  it("converts heavy favourite (-400) correctly", () => {
    // 400 / 500 = 0.8
    expect(americanToImpliedProbability(-400)).toBeCloseTo(0.8);
  });

  it("converts underdog (+200) correctly", () => {
    // 100 / (200 + 100) = 100/300 ≈ 0.3333
    expect(americanToImpliedProbability(200)).toBeCloseTo(1 / 3);
  });

  it("converts heavy underdog (+500) correctly", () => {
    // 100 / 600 ≈ 0.1667
    expect(americanToImpliedProbability(500)).toBeCloseTo(1 / 6);
  });

  it("returns 0 for zero odds", () => {
    expect(americanToImpliedProbability(0)).toBe(0);
  });

  it("implied probabilities for two-way market sum > 1 (vig included)", () => {
    // Standard -110/-110 line should sum to > 1 due to vig
    const p1 = americanToImpliedProbability(-110);
    const p2 = americanToImpliedProbability(-110);
    expect(p1 + p2).toBeGreaterThan(1);
  });
});

describe("impliedProbabilityToAmerican", () => {
  it("converts 0.5 to even money (+100)", () => {
    // At exactly 0.5, should be -100 (technically even both ways)
    const result = impliedProbabilityToAmerican(0.5);
    expect(result).toBe(-100);
  });

  it("converts a favourite probability to negative odds", () => {
    // 0.8 probability → -(0.8/0.2)*100 = -400
    expect(impliedProbabilityToAmerican(0.8)).toBe(-400);
  });

  it("converts an underdog probability to positive odds", () => {
    // 1/3 ≈ 0.3333 → ((1 - 1/3) / (1/3)) * 100 = 200
    expect(impliedProbabilityToAmerican(1 / 3)).toBeCloseTo(200, 0);
  });

  it("converts a heavy underdog probability to large positive number", () => {
    // 1/6 ≈ 0.1667 → (5/6 / 1/6) * 100 = 500
    expect(impliedProbabilityToAmerican(1 / 6)).toBeCloseTo(500, 0);
  });

  it("throws for probability of 0", () => {
    expect(() => impliedProbabilityToAmerican(0)).toThrow(RangeError);
  });

  it("throws for probability of 1", () => {
    expect(() => impliedProbabilityToAmerican(1)).toThrow(RangeError);
  });

  it("throws for probability > 1", () => {
    expect(() => impliedProbabilityToAmerican(1.5)).toThrow(RangeError);
  });

  it("throws for negative probability", () => {
    expect(() => impliedProbabilityToAmerican(-0.1)).toThrow(RangeError);
  });

  it("roundtrips correctly for typical favourite", () => {
    // -200 → prob → back to -200
    const prob = americanToImpliedProbability(-200);
    const back = impliedProbabilityToAmerican(prob);
    expect(back).toBe(-200);
  });

  it("roundtrips correctly for typical underdog", () => {
    // +150 → prob → back to +150
    const prob = americanToImpliedProbability(150);
    const back = impliedProbabilityToAmerican(prob);
    expect(back).toBe(150);
  });
});

describe("normalizeOdds", () => {
  it("normalized probabilities sum to 1", () => {
    const [p1, p2] = normalizeOdds(-110, -110);
    expect(p1 + p2).toBeCloseTo(1);
  });

  it("equal moneylines give 50/50 after normalization", () => {
    const [p1, p2] = normalizeOdds(-110, -110);
    expect(p1).toBeCloseTo(0.5);
    expect(p2).toBeCloseTo(0.5);
  });

  it("favourite has higher normalized probability", () => {
    const [favProb, underdogProb] = normalizeOdds(-200, +170);
    expect(favProb).toBeGreaterThan(underdogProb);
  });

  it("normalized probabilities are between 0 and 1", () => {
    const [p1, p2] = normalizeOdds(-300, +250);
    expect(p1).toBeGreaterThan(0);
    expect(p1).toBeLessThan(1);
    expect(p2).toBeGreaterThan(0);
    expect(p2).toBeLessThan(1);
  });

  it("removes vig - heavy favourite still less than 1 after normalization", () => {
    // Raw implied prob for -400 = 0.8. Normalized should be < 1 since opponent exists.
    const [p1] = normalizeOdds(-400, +300);
    expect(p1).toBeLessThan(1);
    expect(p1).toBeGreaterThan(0.5);
  });

  it("ordering is consistent with input", () => {
    const [p1First, p2First] = normalizeOdds(-200, +170);
    const [p2Second, p1Second] = normalizeOdds(+170, -200);
    expect(p1First).toBeCloseTo(p1Second);
    expect(p2First).toBeCloseTo(p2Second);
  });
});