Function swizzle 

pub fn swizzle(index: usize, num_steps: usize, step_length: u32) -> Coords2d

Maps a linear index to 2D zigzag coordinates (x, y) within horizontal or vertical strips.

Each strip is made of num_steps steps, each of length step_length. Strips alternate direction: even strips go top-down, odd strips bottom-up. Steps alternate direction: even steps go left-to-right, odd steps right-to-left.

• Prefer odd num_steps for smoother transitions between strips.
• Prefer power-of-two step_length for better performance.

Parameters

• index: linear input index
• num_steps: number of snaking steps in a strip
• step_length: number of elements in each step (must be > 0)

Returns

(x, y) coordinates after swizzling