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* @file scanner.js
* @description Source-position primitives shared by every part of the bundler.
*
* ## Why Avenx scans rather than parses
*
* Everything else in this compiler reads source with a purpose-built scanner —
* the HTML tokenizer, the declaration reader, the Atlas reference scanner — and
* the bundler follows the same rule for the same reason: a dependency-free
* build is a property of the project, not an accident of what was convenient.
*
* The bundler does not need a full ECMAScript parser. It needs to know three
* things about a module: which `import`/`export` declarations it contains,
* where each top-level statement begins and ends, and which top-level names it
* declares. All three are answerable from a scanner that knows where source
* text *is not* code.
*
* ## What "is not code" means here
*
* String literals, template literals (including nested `${}` expressions),
* regular-expression literals, line comments and block comments. Every one of
* them can contain the characters this file searches for — a `{` inside a
* string would otherwise corrupt every depth calculation downstream, and the
* word `import` inside a comment would otherwise become a phantom dependency.
*
* Distinguishing `/` as division from `/` as the start of a regex is the one
* genuinely ambiguous case in JavaScript lexing. {@link regexAllowedAfter}
* resolves it the way every hand-written JS lexer does: by looking at the last
* significant token.
* @module lib/bundler/scanner
*/
/**
* Characters that can legally precede a regular-expression literal.
*
* After any of these, a `/` starts a regex; after an identifier, a number, or a
* closing bracket it is division. The exceptions to "closing bracket means
* division" are keywords such as `return` and `typeof`, handled by
* {@link KEYWORDS_BEFORE_REGEX}.
* @type {Set<string>}
*/
const PUNCTUATORS_BEFORE_REGEX = new Set([
'(', ',', '=', ':', '[', '!', '&', '|', '?', '{', '}', ';', '+', '-', '*', '%', '^', '~', '<', '>',
]);
/**
* Keywords after which a `/` begins a regular expression rather than division.
* @type {Set<string>}
*/
const KEYWORDS_BEFORE_REGEX = new Set([
'return', 'typeof', 'instanceof', 'in', 'of', 'new', 'delete', 'void', 'throw',
'case', 'do', 'else', 'yield', 'await',
]);
/**
* Decides whether a `/` at `index` opens a regular-expression literal.
* @param {string} source - The full source text.
* @param {number} index - Offset of the `/`.
* @returns {boolean} True when a regex literal starts here.
*/
export function regexAllowedAfter(source, index) {
let i = index - 1;
while (i >= 0 && /\s/.test(source[i])) {
i -= 1;
}
if (i < 0) {
return true;
}
const char = source[i];
if (PUNCTUATORS_BEFORE_REGEX.has(char)) {
return true;
}
if (/[\w$]/.test(char)) {
let start = i;
while (start >= 0 && /[\w$]/.test(source[start])) {
start -= 1;
}
return KEYWORDS_BEFORE_REGEX.has(source.slice(start + 1, i + 1));
}
return false;
}
/**
* A single lexical region the scanner recognises.
* @typedef {object} Region
* @property {'code'|'string'|'template'|'regex'|'line-comment'|'block-comment'} kind - What the region is.
* @property {number} start - Inclusive start offset.
* @property {number} end - Exclusive end offset.
*/
/**
* Walks a source and reports every non-code region.
*
* Template literals are reported as a single region spanning the whole literal,
* including any `${}` substitutions. Code inside a substitution is therefore
* not scanned at all — which is correct for this bundler's purposes, because an
* `import` declaration cannot appear inside an expression and a top-level
* statement boundary cannot fall inside a template literal.
* @param {string} source - The source text.
* @returns {Region[]} Regions in ascending order of `start`.
*/
export function scanRegions(source) {
/** @type {Region[]} */
const regions = [];
const length = source.length;
let i = 0;
while (i < length) {
const char = source[i];
if (char === '/' && source[i + 1] === '/') {
const start = i;
while (i < length && source[i] !== '\n') {
i += 1;
}
regions.push({ kind: 'line-comment', start, end: i });
continue;
}
if (char === '/' && source[i + 1] === '*') {
const start = i;
i += 2;
while (i < length && !(source[i] === '*' && source[i + 1] === '/')) {
i += 1;
}
i = Math.min(i + 2, length);
regions.push({ kind: 'block-comment', start, end: i });
continue;
}
if (char === '"' || char === "'") {
const start = i;
const quote = char;
i += 1;
while (i < length) {
if (source[i] === '\\') {
i += 2;
continue;
}
if (source[i] === quote) {
i += 1;
break;
}
// An unterminated string literal cannot span a line. Stopping at the
// newline keeps a malformed file from swallowing the rest of the
// module and reporting nonsense about its imports.
if (source[i] === '\n') {
break;
}
i += 1;
}
regions.push({ kind: 'string', start, end: i });
continue;
}
if (char === '`') {
const start = i;
i += 1;
let depth = 0;
while (i < length) {
if (source[i] === '\\') {
i += 2;
continue;
}
if (depth === 0 && source[i] === '$' && source[i + 1] === '{') {
depth += 1;
i += 2;
continue;
}
if (depth > 0) {
if (source[i] === '{') depth += 1;
else if (source[i] === '}') depth -= 1;
else if (source[i] === '`') {
// A nested template inside a substitution. Skip it whole so its
// own braces cannot unbalance this one.
const nested = scanRegions(source.slice(i));
const first = nested[0];
i += first && first.kind === 'template' && first.start === 0 ? first.end : 1;
continue;
}
i += 1;
continue;
}
if (source[i] === '`') {
i += 1;
break;
}
i += 1;
}
regions.push({ kind: 'template', start, end: i });
continue;
}
if (char === '/' && regexAllowedAfter(source, i)) {
const start = i;
i += 1;
let inClass = false;
while (i < length) {
const current = source[i];
if (current === '\\') {
i += 2;
continue;
}
if (current === '\n') {
break;
}
if (current === '[') inClass = true;
else if (current === ']') inClass = false;
else if (current === '/' && !inClass) {
i += 1;
while (i < length && /[a-z]/i.test(source[i])) {
i += 1;
}
break;
}
i += 1;
}
regions.push({ kind: 'regex', start, end: i });
continue;
}
i += 1;
}
return regions;
}
/**
* Mask value for a comment: not code, and its whitespace carries no meaning.
* @type {number}
*/
const COMMENT = 1;
/**
* Mask value for a string, template or regex literal: not code, and its text
* is data that must survive verbatim.
* @type {number}
*/
const LITERAL = 2;
/**
* A lookup answering "is this offset inside a string, comment or literal?".
*
* Built once per module and consulted many times. A flat `Uint8Array` costs one
* byte per source character and turns every query into an array read, which
* matters because the export scanner asks the question for every identifier in
* a module.
*/
export class CodeMask {
/**
* @param {string} source - The source the mask describes.
*/
constructor(source) {
/** @type {string} */
this.source = source;
/** @type {Region[]} */
this.regions = scanRegions(source);
// One byte per character: 0 for code, 1 for a comment, 2 for a literal
// whose text is data. Distinguishing the last two is what lets a minifier
// blank a comment's whitespace and leave a template literal's alone, and it
// has to be a lookup rather than a search -- the question is asked once per
// character of the bundle.
/** @type {Uint8Array} */
this.mask = new Uint8Array(source.length);
for (const region of this.regions) {
const end = Math.min(region.end, source.length);
const value =
region.kind === 'string' || region.kind === 'template' || region.kind === 'regex' ? LITERAL : COMMENT;
for (let i = region.start; i < end; i += 1) {
this.mask[i] = value;
}
}
}
/**
* Whether an offset holds executable code rather than literal or comment text.
* @param {number} index - The offset to test.
* @returns {boolean} True when the offset is code.
*/
isCode(index) {
return index >= 0 && index < this.mask.length && this.mask[index] === 0;
}
/**
* Whether an offset sits inside a literal whose text is data.
*
* Distinct from {@link CodeMask#isCode}: a comment is not code, but its
* whitespace is still safe to remove, whereas a string or template literal
* carries its spacing as a value. Anything that rewrites whitespace has to
* tell those two apart.
* @param {number} index - The offset to test.
* @returns {boolean} True inside a string, template or regex literal.
*/
isLiteral(index) {
return index >= 0 && index < this.mask.length && this.mask[index] === LITERAL;
}
/**
* Returns the source with every comment replaced by equivalent whitespace.
*
* Offsets are preserved, so a position found in the blanked text is valid in
* the original. String and template contents are left alone: they are values,
* not noise, and blanking them would corrupt the module.
* @returns {string} The comment-free source, same length as the original.
*/
withoutComments() {
const chars = this.source.split('');
for (const region of this.regions) {
if (region.kind !== 'line-comment' && region.kind !== 'block-comment') {
continue;
}
for (let i = region.start; i < Math.min(region.end, chars.length); i += 1) {
chars[i] = chars[i] === '\n' ? '\n' : ' ';
}
}
return chars.join('');
}
}
/**
* Finds the offset just past a balanced bracket run starting at `index`.
* @param {string} source - The source text.
* @param {CodeMask} mask - Mask for the same source.
* @param {number} index - Offset of the opening bracket.
* @returns {number} Offset one past the matching close, or `source.length`.
*/
export function matchBracket(source, mask, index) {
const open = source[index];
const close = open === '(' ? ')' : open === '[' ? ']' : '}';
let depth = 0;
for (let i = index; i < source.length; i += 1) {
if (!mask.isCode(i)) continue;
const char = source[i];
if (char === open) depth += 1;
else if (char === close) {
depth -= 1;
if (depth === 0) return i + 1;
}
}
return source.length;
}
/**
* Finds the end of the top-level statement that begins at `start`.
*
* "End" means the offset one past its terminating semicolon, or one past the
* closing brace of a block-bodied declaration, or the end of the line for an
* ASI-terminated statement. Only bracket depth at code positions is consulted,
* so a `;` inside a string or an object literal never terminates anything.
* @param {string} source - The source text.
* @param {CodeMask} mask - Mask for the same source.
* @param {number} start - Offset of the statement's first character.
* @returns {number} Exclusive end offset.
*/
export function statementEnd(source, mask, start) {
let depth = 0;
let i = start;
while (i < source.length) {
if (!mask.isCode(i)) {
i += 1;
continue;
}
const char = source[i];
if (char === '(' || char === '[' || char === '{') {
depth += 1;
} else if (char === ')' || char === ']' || char === '}') {
depth -= 1;
if (depth === 0) {
// A declaration whose body is a block ends at that block's close,
// unless a semicolon or an operator follows on the same statement.
let j = i + 1;
while (j < source.length && /[ \t]/.test(source[j])) j += 1;
if (source[j] === ';') return j + 1;
if (j >= source.length || source[j] === '\n' || source[j] === '\r') {
return i + 1;
}
}
if (depth < 0) {
return i;
}
} else if (char === ';' && depth === 0) {
return i + 1;
}
i += 1;
}
return source.length;
}
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