Function sliceContext

src/utils/readEditContext.ts:171–227

* Given an absolute match offset, read ±contextLines around it and return * the decoded slice with its starting line number. Reuses `scratch` (the * caller's scan buffer) for back/forward/output reads — zero new allocs * when the context fits, one alloc otherwise.

(
  handle: FileHandle,
  scratch: Buffer,
  matchStart: number,
  matchLen: number,
  contextLines: number,
  linesBeforeMatch: number,
)

 * when the context fits, one alloc otherwise.
 */
async function sliceContext(
  handle: FileHandle,
  scratch: Buffer,
  matchStart: number,
  matchLen: number,
  contextLines: number,
  linesBeforeMatch: number,
): Promise<EditContext> {
  // Scan backward from matchStart to find contextLines prior newlines.
  const backChunk = Math.min(matchStart, CHUNK_SIZE)
  const { bytesRead: backRead } = await handle.read(
    scratch,
    0,
    backChunk,
    matchStart - backChunk,
  )
  let ctxStart = matchStart
  let nlSeen = 0
  for (let i = backRead - 1; i >= 0 && nlSeen <= contextLines; i--) {
    if (scratch[i] === NL) {
      nlSeen++
      if (nlSeen > contextLines) break
    }
    ctxStart--
  }
  // Compute lineOffset now, before scratch is overwritten by the forward read.
  const walkedBack = matchStart - ctxStart
  const lineOffset =
    linesBeforeMatch -
    countNewlines(scratch, backRead - walkedBack, backRead) +
    1

  // Scan forward from matchEnd to find contextLines trailing newlines.
  const matchEnd = matchStart + matchLen
  const { bytesRead: fwdRead } = await handle.read(
    scratch,
    0,
    CHUNK_SIZE,
    matchEnd,
  )
  let ctxEnd = matchEnd
  nlSeen = 0
  for (let i = 0; i < fwdRead; i++) {
    ctxEnd++
    if (scratch[i] === NL) {
      nlSeen++
      if (nlSeen >= contextLines + 1) break
    }
  }

  // Read the exact context range. Reuse scratch if it fits.
  const len = ctxEnd - ctxStart
  const out = len <= scratch.length ? scratch : Buffer.allocUnsafe(len)
  const { bytesRead: outRead } = await handle.read(out, 0, len, ctxStart)

  return { content: normalizeCRLF(out, outRead), lineOffset, truncated: false }
}