Even faster source map generation

Summary:
This replaces a polymorphic method with three separate methods that support the different used invocations.

In isolation, this leads to a 25% speedup.

Shoutout to trueadm for teaching me about this.

Reviewed By: trueadm

Differential Revision: D4436781

fbshipit-source-id: 3dce83556debf19d5305c4566a56b9e9565e85bc

react-packager/src/Bundler/source-map/Generator.js

@@ -73,22 +73,10 @@ class Generator {
   }
 
   /**
-   * Add a mapping that contains the first 2, 4, or all of the following values:
-   *
-   * 1. line offset in the generated source
-   * 2. column offset in the generated source
-   * 3. line offset in the original source
-   * 4. column offset in the original source
-   * 5. name of the symbol in the original source.
+   * Adds a mapping for generated code without a corresponding source location.
    */
-  addMapping(
-    generatedLine: number,
-    generatedColumn: number,
-    sourceLine?: number,
-    sourceColumn?: number,
-    name?: string,
-  ): void {
-    var {last} = this;
+  addSimpleMapping(generatedLine: number, generatedColumn: number): void {
+    const last = this.last;
     if (this.source === -1 ||
         generatedLine === last.generatedLine &&
         generatedColumn < last.generatedColumn ||
@@ -107,13 +95,20 @@ class Generator {
 
     this.builder.startSegment(generatedColumn - last.generatedColumn);
     last.generatedColumn = generatedColumn;
-
-    if (sourceLine != null) {
-      if (sourceColumn == null) {
-        throw new Error(
-          'Received mapping with source line, but without source column');
   }
 
+  /**
+   * Adds a mapping for generated code with a corresponding source location.
+   */
+  addSourceMapping(
+    generatedLine: number,
+    generatedColumn: number,
+    sourceLine: number,
+    sourceColumn: number,
+  ): void {
+    this.addSimpleMapping(generatedLine, generatedColumn);
+
+    const last = this.last;
     this.builder
       .append(this.source - last.source)
       .append(sourceLine - last.sourceLine)
@@ -122,14 +117,26 @@ class Generator {
     last.source = this.source;
     last.sourceColumn = sourceColumn;
     last.sourceLine = sourceLine;
+  }
 
-      if (name != null) {
+  /**
+   * Adds a mapping for code with a corresponding source location + symbol name.
+   */
+  addNamedSourceMapping(
+    generatedLine: number,
+    generatedColumn: number,
+    sourceLine: number,
+    sourceColumn: number,
+    name: string,
+  ): void {
+    this.addSourceMapping(
+      generatedLine, generatedColumn, sourceLine, sourceColumn);
+
+    const last = this.last;
     const nameIndex = this.names.indexFor(name);
     this.builder.append(nameIndex - last.name);
     last.name = nameIndex;
   }
-    }
-  }
 
   /**
    * Return the source map as object.

react-packager/src/Bundler/source-map/source-map.js

@@ -17,7 +17,7 @@ import type ModuleTransport from '../../lib/ModuleTransport';
 import type {RawMapping as BabelRawMapping} from 'babel-generator';
 
 type GeneratedCodeMapping = [number, number];
-type SourceMapping = [number, number, number, number, void];
+type SourceMapping = [number, number, number, number];
 type SourceMappingWithName =  [number, number, number, number, string];
 
 export type RawMapping =
@@ -59,15 +59,10 @@ function compactMapping(mapping: BabelRawMapping): RawMapping {
   }
 
   if (typeof name !== 'string') {
-    /* $FlowFixMe(>=0.38.0 site=react_native_fb,react_native_oss) - Flow error
-     * detected during the deployment of v0.38.0. To see the error, remove this
-     * comment and run flow */
-    return ([line, column, original.line, original.column]: SourceMapping);
+    return [line, column, original.line, original.column];
   }
 
-  return (
-    [line, column, original.line, original.column, name]: SourceMappingWithName
-  );
+  return [line, column, original.line, original.column, name];
 }
 
 function addMappingsForFile(generator, mappings, module, carryOver) {
@@ -75,27 +70,32 @@ function addMappingsForFile(generator, mappings, module, carryOver) {
 
   const columnOffset = module.code.indexOf('{') + 1;
   for (let i = 0, n = mappings.length; i < n; ++i) {
-    const mapping = mappings[i];
-    generator.addMapping(
-      mapping[0] + carryOver,
-      // lines start at 1, columns start at 0
-      mapping[0] === 1 && mapping[1] + columnOffset || mapping[1],
-      /* $FlowFixMe(>=0.38.0 site=react_native_fb,react_native_oss) - Flow error
-       * detected during the deployment of v0.38.0. To see the error, remove
-       * this comment and run flow */
-      mapping[2],
-      /* $FlowFixMe(>=0.38.0 site=react_native_fb,react_native_oss) - Flow error
-       * detected during the deployment of v0.38.0. To see the error, remove
-       * this comment and run flow */
-      mapping[3],
-      //$FlowIssue #15417846
-      mapping[4],
-    );
+    addMapping(generator, mappings[i], carryOver, columnOffset);
   }
+
   generator.endFile();
 
 }
 
+function addMapping(generator, mapping, carryOver, columnOffset) {
+  const n = mapping.length;
+  const line = mapping[0] + carryOver;
+  // lines start at 1, columns start at 0
+  const column = mapping[0] === 1 ? mapping[1] + columnOffset : mapping[1];
+  if (n === 2) {
+    generator.addSimpleMapping(line, column);
+  } else if (n === 4) {
+    // $FlowIssue #15579526
+    generator.addSourceMapping(line, column, mapping[2], mapping[3]);
+  } else if (n === 5) {
+    generator.addNamedSourceMapping(
+      // $FlowIssue #15579526
+      line, column, mapping[2], mapping[3], mapping[4]);
+  } else {
+    throw new Error(`Invalid mapping: [${mapping.join(', ')}]`);
+  }
+}
+
 function countLines(string) {
   return string.split('\n').length;
 }