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External Mergesort

Previous sorts all assume ...
  • efficient random access to Items
  • which suggests that data is in arrays in memory
  • which limits sortable data to what fits in memory
When the data is in disk files
  • random access is inefficient (files are sequential access)
  • but max data size is far less constrained
Because mergesort makes multiple sequential passes, it adapts well as a sorting approach for files.