Function computeReplacements

src/core/tools/apply-patch/apply.ts:38–95

* Compute the replacements needed to transform originalLines into the new lines. * Each replacement is [startIndex, oldLength, newLines].

(
	originalLines: string[],
	filePath: string,
	chunks: UpdateFileChunk[],
)

 * Each replacement is [startIndex, oldLength, newLines].
 */
function computeReplacements(
	originalLines: string[],
	filePath: string,
	chunks: UpdateFileChunk[],
): Array<[number, number, string[]]> {
	const replacements: Array<[number, number, string[]]> = []
	let lineIndex = 0

	for (const chunk of chunks) {
		// If a chunk has a change_context, find it first
		if (chunk.changeContext !== null) {
			const idx = seekSequence(originalLines, [chunk.changeContext], lineIndex, false)
			if (idx === null) {
				throw new ApplyPatchError(`Failed to find context '${chunk.changeContext}' in ${filePath}`)
			}
			lineIndex = idx + 1
		}

		if (chunk.oldLines.length === 0) {
			// Pure addition (no old lines). Add at the end or before final empty line.
			const insertionIdx =
				originalLines.length > 0 && originalLines[originalLines.length - 1] === ""
					? originalLines.length - 1
					: originalLines.length
			replacements.push([insertionIdx, 0, chunk.newLines])
			continue
		}

		// Try to find the old_lines in the file
		let pattern = chunk.oldLines
		let newSlice = chunk.newLines
		let found = seekSequence(originalLines, pattern, lineIndex, chunk.isEndOfFile)

		// If not found and pattern ends with empty string (trailing newline),
		// retry without it
		if (found === null && pattern.length > 0 && pattern[pattern.length - 1] === "") {
			pattern = pattern.slice(0, -1)
			if (newSlice.length > 0 && newSlice[newSlice.length - 1] === "") {
				newSlice = newSlice.slice(0, -1)
			}
			found = seekSequence(originalLines, pattern, lineIndex, chunk.isEndOfFile)
		}

		if (found !== null) {
			replacements.push([found, pattern.length, newSlice])
			lineIndex = found + pattern.length
		} else {
			throw new ApplyPatchError(
				`Failed to find expected lines in ${filePath}:\n${chunk.oldLines.join("\n").substring(0, 200)}${chunk.oldLines.join("\n").length > 200 ? "..." : ""}`,
			)
		}
	}

	// Sort replacements by start index
	replacements.sort((a, b) => a[0] - b[0])

	return replacements
}