calculate_eta: Calculate and format ETA for batch operations
Description
Helper function to calculate estimated time of arrival based on elapsed time
and progress through a batch operation.
Usage
calculate_eta(current_item, total_items, start_time)
Value
character string with formatted ETA and completion time or empty string if not calculable
Arguments
- current_item
current item number (1-based)
- total_items
total number of items to process
- start_time
POSIXct start time of the operation