Add startLocation option

FEATURE: The new `startLocation` option can now be used to tell the parser the
line and column the parse starts at (mostly useful with `parseExpressionAt`).

Issue '#1448

Author: marijnh

acorn/README.md

@@ -137,6 +137,11 @@ required):
   one-based line and zero-based column numbers in `{line, column}`
   form. Default is `false`.
 
+- **startLocation**: An optional `{line, column}` object to use for
+  the start of the parse. This is mostly useful when using
+  `parseExpressionAt` with `locations: true`, to prevent the parser
+  from having to determine the line position at the start position.
+
 - **onToken**: If a function is passed for this option, each found
   token will be passed in same format as tokens returned from
   `tokenizer().getToken()`.

acorn/src/acorn.d.ts

@@ -693,6 +693,14 @@ export interface Options {
    */
   locations?: boolean
 
+  /**
+   * Pass an optional `{line, column}` object to use for the start of
+   * the parse. This is mostly useful when using `parseExpressionAt`
+   * with `locations: true`, to prevent the parser from having to
+   * determine the line position at the start position.
+   */
+  startLocation?: {line: number, column: number}
+
   /**
    * a callback that will cause Acorn to call that export function with object in the same
    * format as tokens returned from `tokenizer().getToken()`. Note

acorn/src/options.js

@@ -60,6 +60,11 @@ export const defaultOptions = {
   // line being 1-based and column 0-based) will be attached to the
   // nodes.
   locations: false,
+  // Pass an optional `{line, column}` object to use for the start of
+  // the parse. This is mostly useful when using `parseExpressionAt`
+  // with `locations: true`, to prevent the parser from having to
+  // determine the line position at the start position.
+  startLocation: null,
   // A function can be passed as `onToken` option, which will
   // cause Acorn to call that function with object in the same
   // format as tokens returned from `tokenizer().getToken()`. Note

acorn/src/state.js

@@ -32,14 +32,17 @@ export class Parser {
     // Set up token state
 
     // The current position of the tokenizer in the input.
+    this.pos = startPos || 0
     this.curLine = 1
-    if (startPos) {
-      this.pos = startPos
+    if (options.startLocation) {
+      this.lineStart = this.pos - options.startLocation.column
+      this.curLine = options.startLocation.line
+    } else if (startPos) {
       this.lineStart = this.input.lastIndexOf("\n", startPos - 1) + 1
       if (this.options.locations)
         this.curLine = this.input.slice(0, this.lineStart).split(lineBreak).length
     } else {
-      this.pos = this.lineStart = 0
+      this.lineStart = 0
     }
 
     // Properties of the current token: