perf(ngcc): store the position of SegmentMarkers to avoid unnecessary computation (#36027)

Previously, calculations related to the position of and difference between
SegmentMarkers required extensive computation based around the line,
line start positions and columns of each segment.

PR Close #36027

packages/compiler-cli/ngcc/src/sourcemaps/segment_marker.ts

@@ -16,6 +16,7 @@
 export interface SegmentMarker {
   readonly line: number;
   readonly column: number;
+  readonly position: number;
   next: SegmentMarker|undefined;
 }
 
@@ -26,20 +27,7 @@ export interface SegmentMarker {
  * and zero if they are at the same position.
  */
 export function compareSegments(a: SegmentMarker, b: SegmentMarker): number {
-  return a.line === b.line ? a.column - b.column : a.line - b.line;
-}
-
-/**
- * Compute the difference between two segment markers in a source file.
- *
- * @param startOfLinePositions the position of the start of each line of content of the source file
- * where we are computing the difference
- * @param a the start marker
- * @param b the end marker
- * @returns the number of characters between the two segments `a` and `b`
- */
-export function segmentDiff(startOfLinePositions: number[], a: SegmentMarker, b: SegmentMarker) {
-  return startOfLinePositions[b.line] - startOfLinePositions[a.line] + b.column - a.column;
+  return a.position - b.position;
 }
 
 /**
@@ -51,19 +39,19 @@ export function segmentDiff(startOfLinePositions: number[], a: SegmentMarker, b:
  * @param offset the number of character to offset by.
  */
 export function offsetSegment(
-    startOfLinePositions: number[], marker: SegmentMarker, offset: number) {
+    startOfLinePositions: number[], marker: SegmentMarker, offset: number): SegmentMarker {
   if (offset === 0) {
     return marker;
   }
 
   let line = marker.line;
-  const newPos = startOfLinePositions[line] + marker.column + offset;
-  while (line < startOfLinePositions.length - 1 && startOfLinePositions[line + 1] <= newPos) {
+  const position = marker.position + offset;
+  while (line < startOfLinePositions.length - 1 && startOfLinePositions[line + 1] <= position) {
     line++;
   }
-  while (line > 0 && startOfLinePositions[line] > newPos) {
+  while (line > 0 && startOfLinePositions[line] > position) {
     line--;
   }
-  const column = newPos - startOfLinePositions[line];
-  return {line, column, next: undefined};
+  const column = position - startOfLinePositions[line];
+  return {line, column, position, next: undefined};
 }

packages/compiler-cli/ngcc/src/sourcemaps/source_file.ts

@@ -9,7 +9,7 @@ import {removeComments, removeMapFileComments} from 'convert-source-map';
 import {SourceMapMappings, SourceMapSegment, decode, encode} from 'sourcemap-codec';
 import {AbsoluteFsPath, dirname, relative} from '../../../src/ngtsc/file_system';
 import {RawSourceMap} from './raw_source_map';
-import {SegmentMarker, compareSegments, offsetSegment, segmentDiff} from './segment_marker';
+import {SegmentMarker, compareSegments, offsetSegment} from './segment_marker';
 
 export function removeSourceMapComments(contents: string): string {
   return removeMapFileComments(removeComments(contents)).replace(/\n\n$/, '\n');
@@ -89,7 +89,7 @@ export class SourceFile {
    * source files with no transitive source maps.
    */
   private flattenMappings(): Mapping[] {
-    const mappings = parseMappings(this.rawMap, this.sources);
+    const mappings = parseMappings(this.rawMap, this.sources, this.startOfLinePositions);
     ensureOriginalSegmentLinks(mappings);
     const flattenedMappings: Mapping[] = [];
     for (let mappingIndex = 0; mappingIndex < mappings.length; mappingIndex++) {
@@ -277,8 +277,7 @@ export function mergeMappings(generatedSource: SourceFile, ab: Mapping, bc: Mapp
   // segment-marker" of B->C (4*): `1 - 4 = -3`.
   // Since it is negative we must increment the "generated segment-marker" with `3` to give [3,2].
 
-  const diff =
-      segmentDiff(ab.originalSource.startOfLinePositions, ab.originalSegment, bc.generatedSegment);
+  const diff = compareSegments(bc.generatedSegment, ab.originalSegment);
   if (diff > 0) {
     return {
       name,
@@ -303,7 +302,8 @@ export function mergeMappings(generatedSource: SourceFile, ab: Mapping, bc: Mapp
  * in the `sources` parameter.
  */
 export function parseMappings(
-    rawMap: RawSourceMap | null, sources: (SourceFile | null)[]): Mapping[] {
+    rawMap: RawSourceMap | null, sources: (SourceFile | null)[],
+    generatedSourceStartOfLinePositions: number[]): Mapping[] {
   if (rawMap === null) {
     return [];
   }
@@ -330,11 +330,13 @@ export function parseMappings(
         const generatedSegment: SegmentMarker = {
           line: generatedLine,
           column: generatedColumn,
+          position: generatedSourceStartOfLinePositions[generatedLine] + generatedColumn,
           next: undefined,
         };
         const originalSegment: SegmentMarker = {
           line,
           column,
+          position: originalSource.startOfLinePositions[line] + column,
           next: undefined,
         };
         mappings.push({name, generatedSegment, originalSegment, originalSource});