Function readFileInRangeFast

source code/utils/readFileInRange.ts:130–196

(
  raw: string,
  mtimeMs: number,
  offset: number,
  maxLines: number | undefined,
  truncateAtBytes: number | undefined,
)

// ---------------------------------------------------------------------------

function readFileInRangeFast(
  raw: string,
  mtimeMs: number,
  offset: number,
  maxLines: number | undefined,
  truncateAtBytes: number | undefined,
): ReadFileRangeResult {
  const endLine = maxLines !== undefined ? offset + maxLines : Infinity

  // Strip BOM.
  const text = raw.charCodeAt(0) === 0xfeff ? raw.slice(1) : raw

  // Split lines, strip \r, select range.
  const selectedLines: string[] = []
  let lineIndex = 0
  let startPos = 0
  let newlinePos: number
  let selectedBytes = 0
  let truncatedByBytes = false

  function tryPush(line: string): boolean {
    if (truncateAtBytes !== undefined) {
      const sep = selectedLines.length > 0 ? 1 : 0
      const nextBytes = selectedBytes + sep + Buffer.byteLength(line)
      if (nextBytes > truncateAtBytes) {
        truncatedByBytes = true
        return false
      }
      selectedBytes = nextBytes
    }
    selectedLines.push(line)
    return true
  }

  while ((newlinePos = text.indexOf('\n', startPos)) !== -1) {
    if (lineIndex >= offset && lineIndex < endLine && !truncatedByBytes) {
      let line = text.slice(startPos, newlinePos)
      if (line.endsWith('\r')) {
        line = line.slice(0, -1)
      }
      tryPush(line)
    }
    lineIndex++
    startPos = newlinePos + 1
  }

  // Final fragment (no trailing newline).
  if (lineIndex >= offset && lineIndex < endLine && !truncatedByBytes) {
    let line = text.slice(startPos)
    if (line.endsWith('\r')) {
      line = line.slice(0, -1)
    }
    tryPush(line)
  }
  lineIndex++

  const content = selectedLines.join('\n')
  return {