diff.Diff optimization if one input is empty.

[shichuzhu]

godebug/diff/diff.go

Lines 37 to 40 in e693023

	// Diff returns a string containing a line-by-line unified diff of the linewise
	// changes required to make A into B. Each line is prefixed with '+', '-', or
	// ' ' to indicate if it should be added, removed, or is correct respectively.
	func Diff(A, B string) string {

It seems if one input of Diff is empty, then the algorithm uses O(N^2) space, where N is the size of the other input.

The result is I see out of memory error if I diff an empty chunk with a non-empty chunk.

It'll be a good improvement if we can just do an empty check before calling the Diff algorithm. And generating diffstring in such case should not be very hard.

[shichuzhu]

Clarify: In this case, I'm calling pretty.Compare(a, b)
where a is an empty Struct (zero value), where b is not.