How to implement adaptive layout of Y-axis position in web annotation?

Detailed explanation of the Y-axis position adaptive algorithm for web page annotation

This article discusses how to build a web annotation function similar to Word documents, focusing on solving the problem of overlapping comments and realizing an adaptive layout of the Y-axis position of the annotation. Ideally, annotations should be closely arranged to avoid overlapping, while maintaining reasonable spacing between annotations.

The core challenge is to design an algorithm that automatically calculates its Y-axis position when adding new annotations. An effective solution is to use absolute positioning and combine data structures and algorithms to manage annotation locations.

Data structure:

We use an array to store the information for each annotation, each element containing top (initial top position) and height (height) properties. For example:

 [
  { top: 100, height: 200 },
  { top: 800, height: 200 },
  { top: 820, height: 200 },
  { top: 1020, height: 200 },
]

Adaptive algorithm:

We use an algorithm similar to waterfall flow layout, use the reduce method to iterate the array, and calculate the final Y-axis position of each annotation ( currentTop ). The core of the algorithm is to compare the initial top value of the current annotation with the bottom position of the previous annotation, and select a larger value as the final currentTop of the current annotation, thereby avoiding overlap.

 const arr = [
  { top: 100, height: 200 },
  { top: 800, height: 200 },
  { top: 820, height: 200 },
  { top: 1020, height: 200 },
  { top: 1430, height: 180 },
];

arr.reduce((s, n, i) => {
  n.currentTop = Math.max(n.top, (s.currentTop || s.top) s.height);
  return n;
});

console.log(arr);

Math.max(n.top, (s.currentTop || s.top) s.height) This line of code is the core of the algorithm. s.currentTop || s.top handles the case of the first annotation.

application:

The currentTop value calculated by this algorithm can be applied to the CSS style of the annotation element, such as top: ${currentTop}px; , thereby realizing the adaptive layout of the annotation, effectively solving the annotation overlap problem, and achieving an effect similar to the annotation spacing of Word documents. This algorithm ensures that the annotations are arranged closely, while avoiding mutual obstruction, achieving a smooth web annotation experience.

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