import { describe, expect, it } from 'vitest';

import { isNearBottom, isNearTop, resolveInitialAnchor } from './scrollController';

const m = (scrollTop: number, scrollHeight: number, clientHeight: number) => ({
  scrollTop,
  scrollHeight,
  clientHeight,
});

describe('isNearBottom', () => {
  it('true exactly at the bottom', () => {
    expect(isNearBottom(m(900, 1000, 100), 64)).toBe(true);
  });
  it('true within threshold', () => {
    expect(isNearBottom(m(860, 1000, 100), 64)).toBe(true);
  });
  it('false beyond threshold', () => {
    expect(isNearBottom(m(800, 1000, 100), 64)).toBe(false);
  });
});

describe('isNearTop', () => {
  it('true at top', () => {
    expect(isNearTop(m(0, 1000, 100), 64)).toBe(true);
  });
  it('false past threshold', () => {
    expect(isNearTop(m(200, 1000, 100), 64)).toBe(false);
  });
});

describe('resolveInitialAnchor', () => {
  it('anchors to last row at end by default (no saved position)', () => {
    expect(resolveInitialAnchor(null, 50)).toEqual({ index: 49, align: 'end' });
  });
  it('anchors to bottom when saved position stuck to bottom', () => {
    expect(resolveInitialAnchor({ topmostIndex: 10, stickToBottom: true }, 50)).toEqual({
      index: 49,
      align: 'end',
    });
  });
  it('restores the saved row at the top when scrolled up', () => {
    expect(resolveInitialAnchor({ topmostIndex: 12, stickToBottom: false }, 50)).toEqual({
      index: 12,
      align: 'start',
    });
  });
  it('clamps a stale saved index to the current row count', () => {
    expect(resolveInitialAnchor({ topmostIndex: 999, stickToBottom: false }, 50)).toEqual({
      index: 49,
      align: 'start',
    });
  });
  it('handles an empty list', () => {
    expect(resolveInitialAnchor(null, 0)).toEqual({ index: 0, align: 'end' });
  });
});