Frontend: useIntersectionObserver Hook (Threshold Logic)

MediumTypeScript

Description

> This problem is paper-only - see explanation for the canonical React solution. The IntersectionObserver `threshold` option fires the callback when the visible ratio crosses any of the given thresholds. The auto-grader validates the threshold-crossing logic: Given a previous `intersectionRatio` and a new one, determine which configured threshold values were "crossed" between them (in either direction). Return the crossed thresholds in any order, deduplicated and sorted ascending. A threshold `t` is crossed iff `min(prev, cur) <= t <= max(prev, cur)` AND `prev !== cur`. **For the auto-grader:** implement `crossedThresholds(prev, cur, thresholds)`.

Example 1

Input: prev = 0, cur = 0.6, thresholds = [0, 0.25, 0.5, 0.75, 1]

Output: [0,0.25,0.5]

Explanation: 0 (start), 0.25 and 0.5 are within the 0..0.6 range.

Example 2

Input: prev = 0.5, cur = 0.5, thresholds = [0, 0.5, 1]

Output: []

Explanation: No movement, no crossing.

Example 3

Input: prev = 1, cur = 0, thresholds = [0, 0.5, 1]

Output: [0,0.5,1]

Explanation: Direction doesn't matter.

Constraints

- 0 <= prev, cur <= 1 - thresholds: array of distinct numbers in [0, 1].

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