typescript 7.0.2: Compute source code line starts and map text positions using computeLineStarts

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: Compute source code line starts and map text positions using computeLineStarts
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/typescript@7.0.2
Demonstrate these symbols/APIs:
  - computeLineStarts

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

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{
  "name": "typescript-computelinestarts-sample",
  "version": "1.0.0",
  "description": "Compute source code line starts and map text positions using computeLineStarts",
  "type": "module",
  "main": "src/index.mjs",
  "scripts": {
    "test": "node test/contract.mjs"
  },
  "dependencies": {
    "typescript": "7.0.2"
  },
  "license": "MIT-0"
}

{
  "schemaVersion": 1,
  "goal": "Compute source code line starts and map text positions using computeLineStarts",
  "kind": "HOW",
  "packages": [
    "pkg:npm/typescript@7.0.2"
  ],
  "symbols": [
    "computeLineStarts"
  ]
}

import { computeLineStarts } from 'typescript/unstable/ast';

/**
 * Computes the 0-indexed line start offsets for a given source code string.
 *
 * @param {string} text - Source code text.
 * @returns {number[]} Array of 0-indexed character offsets where each line begins.
 */
export function getLineStarts(text) {
  if (typeof text !== 'string') {
    throw new TypeError('Expected text to be a string');
  }
  return computeLineStarts(text);
}

/**
 * Maps a 0-indexed character position to a 0-indexed line and character (column) position.
 *
 * @param {readonly number[]} lineStarts - Line start offsets computed by computeLineStarts.
 * @param {number} position - 0-indexed character offset in the source text.
 * @returns {{ line: number, character: number }}
 */
export function getPositionLineAndCharacter(lineStarts, position) {
  if (!Array.isArray(lineStarts) || lineStarts.length === 0) {
    throw new Error('Invalid lineStarts array');
  }
  if (typeof position !== 'number' || position < 0) {
    throw new Error('Position must be a non-negative number');
  }

  let low = 0;
  let high = lineStarts.length - 1;

  while (low <= high) {
    const mid = Math.floor((low + high) / 2);
    const start = lineStarts[mid];

    if (start === position) {
      return { line: mid, character: 0 };
    } else if (start < position) {
      low = mid + 1;
    } else {
      high = mid - 1;
    }
  }

  const line = high >= 0 ? high : 0;
  const character = position - lineStarts[line];
  return { line, character };
}

/**
 * Extracts the exact line content for a given 0-indexed line number.
 *
 * @param {string} sourceText - Full source text.
 * @param {readonly number[]} lineStarts - Line start offsets.
 * @param {number} lineIndex - 0-indexed line index.
 * @returns {string} Text of the requested line.
 */
export function extractLineText(sourceText, lineStarts, lineIndex) {
  if (lineIndex < 0 || lineIndex >= lineStarts.length) {
    throw new RangeError(`Line index ${lineIndex} out of bounds`);
  }
  const start = lineStarts[lineIndex];
  const end = lineIndex + 1 < lineStarts.length ? lineStarts[lineIndex + 1] : sourceText.length;
  return sourceText.slice(start, end);
}

/**
 * Returns the total line count for the source text.
 *
 * @param {readonly number[]} lineStarts - Line start offsets.
 * @returns {number} Number of lines.
 */
export function getLineCount(lineStarts) {
  return lineStarts.length;
}

export { computeLineStarts };

import assert from 'node:assert/strict';
import {
  computeLineStarts,
  getLineStarts,
  getPositionLineAndCharacter,
  extractLineText,
  getLineCount,
} from '../src/index.mjs';

// Contract 1: computeLineStarts calculates 0-indexed line start offsets for multiline source code
{
  const source = 'function greet(name: string): string {\n  return `Hello, ${name}!`;\n}\n';
  const lineStarts = computeLineStarts(source);
  assert.ok(Array.isArray(lineStarts), 'Line starts must be an array');
  assert.strictEqual(lineStarts[0], 0, 'First line starts at offset 0');
  assert.strictEqual(lineStarts[1], 39, 'Second line starts after first newline');
  assert.strictEqual(lineStarts[2], 67, 'Third line starts after second newline');
  assert.strictEqual(lineStarts[3], 69, 'Fourth line starts after third newline');
  assert.strictEqual(getLineCount(lineStarts), 4, 'Total line count should match array length');
}

// Contract 2: computeLineStarts handles various newline delimiters including CRLF and LF
{
  const unixText = 'first\nsecond\nthird';
  const unixStarts = computeLineStarts(unixText);
  assert.deepStrictEqual(unixStarts, [0, 6, 13], 'Unix LF line starts match');

  const winText = 'first\r\nsecond\r\nthird';
  const winStarts = computeLineStarts(winText);
  assert.deepStrictEqual(winStarts, [0, 7, 15], 'Windows CRLF line starts match');

  const singleLine = 'hello world';
  const singleStarts = computeLineStarts(singleLine);
  assert.deepStrictEqual(singleStarts, [0], 'Single line without newline has start at 0');

  const emptyText = '';
  const emptyStarts = computeLineStarts(emptyText);
  assert.deepStrictEqual(emptyStarts, [0], 'Empty text has one line start at 0');
}

// Contract 3: getPositionLineAndCharacter maps character offsets to line and character positions
{
  const source = 'line zero\nline one\nline two';
  const lineStarts = getLineStarts(source);

  const pos0 = getPositionLineAndCharacter(lineStarts, 0);
  assert.strictEqual(pos0.line, 0);
  assert.strictEqual(pos0.character, 0);

  const pos5 = getPositionLineAndCharacter(lineStarts, 5);
  assert.strictEqual(pos5.line, 0);
  assert.strictEqual(pos5.character, 5);

  const pos10 = getPositionLineAndCharacter(lineStarts, 10);
  assert.strictEqual(pos10.line, 1);
  assert.strictEqual(pos10.character, 0);

  const pos15 = getPositionLineAndCharacter(lineStarts, 15);
  assert.strictEqual(pos15.line, 1);
  assert.strictEqual(pos15.character, 5);

  const pos19 = getPositionLineAndCharacter(lineStarts, 19);
  assert.strictEqual(pos19.line, 2);
  assert.strictEqual(pos19.character, 0);
}

// Contract 4: extractLineText retrieves the exact line content for a given line index
{
  const source = 'const x: number = 42;\nconst y: string = "test";\nconsole.log(x, y);\n';
  const lineStarts = getLineStarts(source);

  const line0 = extractLineText(source, lineStarts, 0);
  assert.strictEqual(line0, 'const x: number = 42;\n');

  const line1 = extractLineText(source, lineStarts, 1);
  assert.strictEqual(line1, 'const y: string = "test";\n');

  const line2 = extractLineText(source, lineStarts, 2);
  assert.strictEqual(line2, 'console.log(x, y);\n');

  const line3 = extractLineText(source, lineStarts, 3);
  assert.strictEqual(line3, '');

  assert.throws(() => {
    extractLineText(source, lineStarts, -1);
  }, RangeError);

  assert.throws(() => {
    extractLineText(source, lineStarts, 99);
  }, RangeError);
}

console.log('All pkg:npm/typescript@7.0.2 computeLineStarts contracts passed.');