buildPreview function

String buildPreview(String body, { required String spillPath, required SpillsConfig config, })

Builds the symmetric preview for body — exactly what the session stores and the model sees (issue #678 Decision). Pinned sectioning (blank-line separated): header lines / head / omitted marker / tail + read hint:

[spilled: <totalChars> chars, <totalLines> lines (threshold <T> chars)]
[spill file: <spillPath>]

<head — at most headChars, codepoint-safe cut>

[... <omitted> chars omitted ...]

<tail — at most tailChars, codepoint-safe cut>
[read the spill file above for the full output]

Char-bounded (never line-counted — a single giant line stays bounded, AC3), UTF-16 surrogate-pair-safe (AC4), and the omitted marker ([... N chars omitted ...]) carries the TRUE omitted size (AC10). Pure function.

Implementation

String buildPreview(
  String body, {
  required String spillPath,
  required SpillsConfig config,
}) {
  final head = _headExcerpt(body, config.headChars);
  final tail = _tailExcerpt(body, config.tailChars);
  final omitted = body.length - head.length - tail.length;
  final lines = '\n'.allMatches(body).length + 1;
  return '[spilled: ${body.length} chars, $lines lines '
      '(threshold ${config.threshold} chars)]\n'
      '[spill file: $spillPath]\n'
      '\n'
      '$head\n'
      '\n'
      '[... $omitted chars omitted ...]\n'
      '\n'
      '$tail\n'
      '$spillReadHint';
}