Improve computational time complexity for printing fill() (#16595)

src/document/printer.js

@@ -32,7 +32,7 @@ const MODE_FLAT = Symbol("MODE_FLAT");
 
 const CURSOR_PLACEHOLDER = Symbol("cursor");
 
-const IS_MUTABLE_FILL = Symbol("IS_MUTABLE_FILL");
+const DOC_FILL_PRINTED_LENGTH = Symbol("DOC_FILL_PRINTED_LENGTH");
 
 function rootIndent() {
   return { value: "", length: 0, queue: [] };
@@ -463,12 +463,15 @@ function printDocToString(doc, options) {
       case DOC_TYPE_FILL: {
         const rem = width - pos;
 
+        const offset = doc[DOC_FILL_PRINTED_LENGTH] ?? 0;
         const { parts } = doc;
-        if (parts.length === 0) {
+        const length = parts.length - offset;
+        if (length === 0) {
           break;
         }
 
-        const [content, whitespace] = parts;
+        const content = parts[offset + 0];
+        const whitespace = parts[offset + 1];
         /** @type {Command} */
         const contentFlatCmd = { ind, mode: MODE_FLAT, doc: content };
         /** @type {Command} */
@@ -482,7 +485,7 @@ function printDocToString(doc, options) {
           true,
         );
 
-        if (parts.length === 1) {
+        if (length === 1) {
           if (contentFits) {
             cmds.push(contentFlatCmd);
           } else {
@@ -496,7 +499,7 @@ function printDocToString(doc, options) {
         /** @type {Command} */
         const whitespaceBreakCmd = { ind, mode: MODE_BREAK, doc: whitespace };
 
-        if (parts.length === 2) {
+        if (length === 2) {
           if (contentFits) {
             cmds.push(whitespaceFlatCmd, contentFlatCmd);
           } else {
@@ -505,26 +508,14 @@ function printDocToString(doc, options) {
           break;
         }
 
-        const secondContent = parts[2];
-
-        // At this point we've handled the first pair (context, separator)
-        // and will create a new *mutable* fill doc for the rest of the content.
-        // Copying all the elements to a new array would make this algorithm quadratic,
-        // which is unusable for large arrays (e.g. large texts in JSX).
-        // https://github.com/prettier/prettier/issues/3263#issuecomment-344275152
-        let remainingDoc = doc;
-        if (doc[IS_MUTABLE_FILL]) {
-          parts.splice(0, 2);
-        } else {
-          remainingDoc = {
-            ...doc,
-            parts: parts.slice(2),
-            [IS_MUTABLE_FILL]: true,
-          };
-        }
+        const secondContent = parts[offset + 2];
 
         /** @type {Command} */
-        const remainingCmd = { ind, mode, doc: remainingDoc };
+        const remainingCmd = {
+          ind,
+          mode,
+          doc: { ...doc, [DOC_FILL_PRINTED_LENGTH]: offset + 2 },
+        };
 
         /** @type {Command} */
         const firstAndSecondContentFlatCmd = {

tests/unit/doc-printer.js

@@ -40,6 +40,31 @@ test("`printDocToString` should not manipulate docs", () => {
 
     const lines = formatted.split("\n");
     expect(lines.length).toBeGreaterThan(1000);
-    expect(endTime - startTime).toBeLessThan(1000);
+    expect(endTime - startTime).toBeLessThan(100);
+  }
+});
+
+describe("`printDocToString` has linear time complexity at most to print fill()", () => {
+  const baseSize = 3_000;
+  const relativeMargin = 0.3;
+  const baseTime = time(makeFill(baseSize));
+  test.each([10_000, 20_000, 40_000])("numWords=%d", (numWords) => {
+    const doc = makeFill(numWords);
+    const ellapsed = time(doc);
+    const ratio = numWords / baseSize;
+    expect(ellapsed).toBeLessThan(baseTime * ratio * (1 + relativeMargin));
+  });
+
+  function makeFill(numWords) {
+    const WORD = "word";
+    const parts = Array.from({ length: numWords }, () => WORD);
+    return fill(join(line, parts));
+  }
+
+  function time(doc) {
+    const printOptions = { printWidth: 40, tabWidth: 2 };
+    const start = performance.now();
+    printDocToString(doc, printOptions);
+    return performance.now() - start;
   }
 });