CONCATENATE LINES OF built an intermediate string[] for every row and then called join(), and re-evaluated respectingBlanks/separator conditions per row. The plain CONCATENATE path was already converted to direct string building in #1388; this applies the same treatment to the lines: true path.

Runtime (packages/runtime/src/statements/concatenate.ts)

• Build the result string directly instead of array + join()
• Index-based loop over tab.array() instead of an iterator
• Hoist respectingBlanks and separator-presence checks out of the loop

let result = "";
const array = tab.array();
const respectingBlanks = input.respectingBlanks === true;
const hasSeparator = sep.length > 0;
for (let i = 0; i < array.length; i++) {
  if (hasSeparator === true && i > 0) {
    result += sep;
  }
  const value = array[i].get();
  result += respectingBlanks === true ? value : value.trimEnd();
}
input.target.set(result);

Behaviour is unchanged: separator only between lines, trailing blanks trimmed unless RESPECTING BLANKS, empty table yields an empty result.

Measurements

100k-row table of strings, 20 iterations:

Tests

• performance/test43.ts (+ registration): 43: CONCATENATE LINES OF
• Unit tests for empty table, single line (no dangling separator), and a c LENGTH 5 table with/without separator

A trailing-whitespace fast path (skipping trimEnd() based on the last char code) was also benchmarked but regressed the trimming case, so it was left out.

• Fixes optimize CONCATENATE LINES OF #1389