FindNearestJob.cs

using Unity.Burst;
using Unity.Collections;
using Unity.Jobs;
using Unity.Mathematics;

[BurstCompile]
public struct FindNearestJob : IJobParallelFor
{
    [ReadOnly] public NativeArray<float3> TargetPositions;
    [ReadOnly] public NativeArray<float3> SeekerPositions;

    public NativeArray<float3> NearestTargetPositions;

    void Search(float3 seekerPos, int startIdx, int endIdx, int step,
        ref float3 nearestTargetPos, ref float nearestDistSq)
    {
        for (int i = startIdx; i != endIdx; i += step)
        {
            float3 targetPos = TargetPositions[i];
            float xdiff = seekerPos.x - targetPos.x;

            // If the square of the x distance is greater than the current nearest, we can stop searching. 
            if ((xdiff * xdiff) > nearestDistSq) break;

            float distSq = math.distancesq(targetPos, seekerPos);

            if (distSq < nearestDistSq)
            {
                nearestDistSq = distSq;
                nearestTargetPos = targetPos;
            }
        }
    }

    public void Execute(int index)
    {
        float3 seekerPos = SeekerPositions[index];

        // Find the target with the closest X coord.
        int startIdx = TargetPositions.BinarySearch(seekerPos, new AxisXComparer { });

        // When no precise match is found, BinarySearch returns the bitwise negation of the last-searched offset.
        // So when startIdx is negative, we flip the bits again, but we then must ensure the index is within bounds. 
        if (startIdx < 0) startIdx = ~startIdx;
        if (startIdx >= TargetPositions.Length) startIdx = TargetPositions.Length - 1;

        // The position of the target with the closest X coord.
        float3 nearestTargetPos = TargetPositions[startIdx];
        float nearestDistSq = math.distancesq(seekerPos, nearestTargetPos);

        // Searching upwards through the array for a closer target. 
        Search(seekerPos, startIdx + 1, TargetPositions.Length, +1, ref nearestTargetPos, ref nearestDistSq);

        // Search downwards through the array for a closer target.
        Search(seekerPos, startIdx - 1, -1, -1, ref nearestTargetPos, ref nearestDistSq);

        NearestTargetPositions[index] = nearestTargetPos;
    }
}