Enhance WhitespaceUtil with LCS-based diff algorithm for improved change detection

- Implemented a Longest Common Subsequence (LCS) algorithm to accurately compute line differences, enhancing the detection of whitespace-only changes.
- Refactored the CountLineDifferences function to utilize LCS for both exact and normalized comparisons, improving performance and clarity.
- Added normalization option to handle whitespace discrepancies effectively, ensuring more precise diff results.

plugin/src/WhitespaceUtil.lua

@@ -1,5 +1,7 @@
 --[[
 	Utility for detecting whitespace-only changes in source code.
+	Uses LCS (Longest Common Subsequence) based diff algorithm for accurate
+	change detection that properly handles insertions and deletions.
 ]]
 
 local WhitespaceUtil = {}
@@ -16,7 +18,52 @@ local function NormalizeLine(line: string): string
 	return string.gsub(string.gsub(line, "\r+$", ""), "[ \t]+$", "")
 end
 
--- Counts lines that differ between two strings (capped at 100)
+-- Computes the length of the Longest Common Subsequence between two line arrays
+-- If normalize is true, lines are compared after whitespace normalization
+local function ComputeLcsLength(linesA: { string }, linesB: { string }, normalize: boolean): number
+	local m, n = #linesA, #linesB
+	if m == 0 or n == 0 then
+		return 0
+	end
+
+	-- Precompute comparison lines (normalized if requested)
+	local compareA, compareB = linesA, linesB
+	if normalize then
+		compareA = table.create(m)
+		compareB = table.create(n)
+		for i = 1, m do
+			compareA[i] = NormalizeLine(linesA[i])
+		end
+		for j = 1, n do
+			compareB[j] = NormalizeLine(linesB[j])
+		end
+	end
+
+	-- Space-optimized DP for LCS
+	-- prev[j+1] = LCS length for linesA[1..i-1] and linesB[1..j]
+	-- curr[j+1] = LCS length for linesA[1..i] and linesB[1..j]
+	local prev = table.create(n + 1, 0)
+	local curr = table.create(n + 1, 0)
+
+	for i = 1, m do
+		for j = 1, n do
+			if compareA[i] == compareB[j] then
+				curr[j + 1] = prev[j] + 1
+			else
+				curr[j + 1] = math.max(prev[j + 1], curr[j])
+			end
+		end
+		-- Swap rows and reset curr
+		prev, curr = curr, prev
+		for j = 1, n + 1 do
+			curr[j] = 0
+		end
+	end
+
+	return prev[n + 1]
+end
+
+-- Counts lines that differ between two strings using LCS-based diff
 -- Returns: totalDiff, whitespaceDiff (lines that only differ in whitespace)
 -- Returns 10000 for totalDiff if exceeds 100 (frontend displays as "100+")
 -- whitespaceDiff is 10000 only if ALL checked differences were whitespace-only
@@ -27,29 +74,37 @@ function WhitespaceUtil.CountLineDifferences(current: string, incoming: string):
 
 	local currentLines = SplitLines(current)
 	local incomingLines = SplitLines(incoming)
-	local maxLines = math.max(#currentLines, #incomingLines)
+	local m, n = #currentLines, #incomingLines
 
-	local totalDiff, whitespaceDiff = 0, 0
-	local hasContentChange = false
-	for i = 1, maxLines do
-		local currentLine = currentLines[i] or ""
-		local incomingLine = incomingLines[i] or ""
-
-		if currentLine ~= incomingLine then
-			totalDiff += 1
-			if NormalizeLine(currentLine) == NormalizeLine(incomingLine) then
-				whitespaceDiff += 1
-			else
-				hasContentChange = true
-			end
-			if totalDiff > 100 then
-				-- Return 10000 for both only if ALL differences were whitespace-only
-				-- Otherwise return different values so isWhitespaceOnly is false
-				return 10000, if hasContentChange then 0 else 10000
-			end
-		end
+	-- Quick check: if size difference alone exceeds limit
+	local minPossibleDiff = math.abs(m - n)
+	if minPossibleDiff > 100 then
+		return 10000, 0
 	end
 
+	-- Compute LCS with exact matching
+	-- totalDiff = deletions + insertions = (m - lcs) + (n - lcs)
+	local exactLcs = ComputeLcsLength(currentLines, incomingLines, false)
+	local totalDiff = (m - exactLcs) + (n - exactLcs)
+
+	if totalDiff > 100 then
+		-- Compute normalized LCS to determine if changes are whitespace-only
+		local normalizedLcs = ComputeLcsLength(currentLines, incomingLines, true)
+		local normalizedDiff = (m - normalizedLcs) + (n - normalizedLcs)
+		return 10000, if normalizedDiff == 0 then 10000 else 0
+	end
+
+	if totalDiff == 0 then
+		return 0, 0
+	end
+
+	-- Compute LCS with normalized matching to find whitespace-only changes
+	local normalizedLcs = ComputeLcsLength(currentLines, incomingLines, true)
+	local normalizedDiff = (m - normalizedLcs) + (n - normalizedLcs)
+
+	-- whitespaceDiff = changes that disappear after normalization
+	local whitespaceDiff = totalDiff - normalizedDiff
+
 	return totalDiff, whitespaceDiff
 end