import { describe, expect, it } from "vitest";
import {
  createFrameRateGovernor,
  resetFrameRateGovernor,
  stepFrameRateGovernor,
} from "./frameRateGovernor";

function renderCount(targetFps: 30 | 60, sourceFps: number, durationMs = 1_000): number {
  let state = createFrameRateGovernor(targetFps);
  let renders = 0;
  for (let timestamp = 0; timestamp <= durationMs; timestamp += 1_000 / sourceFps) {
    const step = stepFrameRateGovernor(state, timestamp);
    state = step.state;
    if (step.shouldRender) renders += 1;
  }
  return renders;
}

describe("frame-rate governor", () => {
  it("holds capped modes near their target on high-refresh displays", () => {
    expect(renderCount(30, 144)).toBeGreaterThanOrEqual(30);
    expect(renderCount(30, 144)).toBeLessThanOrEqual(31);
    expect(renderCount(60, 240)).toBeGreaterThanOrEqual(60);
    expect(renderCount(60, 240)).toBeLessThanOrEqual(61);
  });

  it("renders every source callback in display mode", () => {
    let state = createFrameRateGovernor("display", 4);
    for (const timestamp of [1_000, 1_008, 1_016]) {
      const step = stepFrameRateGovernor(state, timestamp);
      state = step.state;
      expect(step.shouldRender).toBe(true);
    }
    expect(state.elapsedSeconds).toBeCloseTo(4.016);
  });

  it("skips missed deadlines without emitting a catch-up burst", () => {
    let state = createFrameRateGovernor(60);
    state = stepFrameRateGovernor(state, 0).state;
    const delayed = stepFrameRateGovernor(state, 100);
    expect(delayed.shouldRender).toBe(true);
    expect(delayed.state.nextDeadlineMs).toBeGreaterThan(100);
    const sameTimestamp = stepFrameRateGovernor(delayed.state, 100);
    expect(sameTimestamp.shouldRender).toBe(false);
  });

  it("preserves elapsed time while resetting the wall-clock baseline", () => {
    let state = createFrameRateGovernor(30, 2);
    state = stepFrameRateGovernor(state, 1_000).state;
    state = stepFrameRateGovernor(state, 1_034).state;
    const reset = resetFrameRateGovernor(state, 60);
    const resumed = stepFrameRateGovernor(reset, 90_000);
    expect(resumed.shouldRender).toBe(true);
    expect(resumed.elapsedSeconds).toBeCloseTo(state.elapsedSeconds);
  });

  it("never moves elapsed time backwards", () => {
    let state = createFrameRateGovernor(60);
    state = stepFrameRateGovernor(state, 20).state;
    state = stepFrameRateGovernor(state, 40).state;
    const backwards = stepFrameRateGovernor(state, 10);
    expect(backwards.elapsedSeconds).toBe(state.elapsedSeconds);
  });
});