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+# @ampproject/remapping
+
+> Remap sequential sourcemaps through transformations to point at the original source code
+
+Remapping allows you to take the sourcemaps generated through transforming your code and "remap"
+them to the original source locations. Think "my minified code, transformed with babel and bundled
+with webpack", all pointing to the correct location in your original source code.
+
+With remapping, none of your source code transformations need to be aware of the input's sourcemap,
+they only need to generate an output sourcemap. This greatly simplifies building custom
+transformations (think a find-and-replace).
+
+## Installation
+
+```sh
+npm install @ampproject/remapping
+```
+
+## Usage
+
+```typescript
+function remapping(
+  map: SourceMap | SourceMap[],
+  loader: (file: string, ctx: LoaderContext) => (SourceMap | null | undefined),
+  options?: { excludeContent: boolean, decodedMappings: boolean }
+): SourceMap;
+
+// LoaderContext gives the loader the importing sourcemap, tree depth, the ability to override the
+// "source" location (where child sources are resolved relative to, or the location of original
+// source), and the ability to override the "content" of an original source for inclusion in the
+// output sourcemap.
+type LoaderContext = {
+ readonly importer: string;
+ readonly depth: number;
+ source: string;
+ content: string | null | undefined;
+}
+```
+
+`remapping` takes the final output sourcemap, and a `loader` function. For every source file pointer
+in the sourcemap, the `loader` will be called with the resolved path. If the path itself represents
+a transformed file (it has a sourcmap associated with it), then the `loader` should return that
+sourcemap. If not, the path will be treated as an original, untransformed source code.
+
+```js
+// Babel transformed "helloworld.js" into "transformed.js"
+const transformedMap = JSON.stringify({
+  file: 'transformed.js',
+  // 1st column of 2nd line of output file translates into the 1st source
+  // file, line 3, column 2
+  mappings: ';CAEE',
+  sources: ['helloworld.js'],
+  version: 3,
+});
+
+// Uglify minified "transformed.js" into "transformed.min.js"
+const minifiedTransformedMap = JSON.stringify({
+  file: 'transformed.min.js',
+  // 0th column of 1st line of output file translates into the 1st source
+  // file, line 2, column 1.
+  mappings: 'AACC',
+  names: [],
+  sources: ['transformed.js'],
+  version: 3,
+});
+
+const remapped = remapping(
+  minifiedTransformedMap,
+  (file, ctx) => {
+
+    // The "transformed.js" file is an transformed file.
+    if (file === 'transformed.js') {
+      // The root importer is empty.
+      console.assert(ctx.importer === '');
+      // The depth in the sourcemap tree we're currently loading.
+      // The root `minifiedTransformedMap` is depth 0, and its source children are depth 1, etc.
+      console.assert(ctx.depth === 1);
+
+      return transformedMap;
+    }
+
+    // Loader will be called to load transformedMap's source file pointers as well.
+    console.assert(file === 'helloworld.js');
+    // `transformed.js`'s sourcemap points into `helloworld.js`.
+    console.assert(ctx.importer === 'transformed.js');
+    // This is a source child of `transformed`, which is a source child of `minifiedTransformedMap`.
+    console.assert(ctx.depth === 2);
+    return null;
+  }
+);
+
+console.log(remapped);
+// {
+//   file: 'transpiled.min.js',
+//   mappings: 'AAEE',
+//   sources: ['helloworld.js'],
+//   version: 3,
+// };
+```
+
+In this example, `loader` will be called twice:
+
+1. `"transformed.js"`, the first source file pointer in the `minifiedTransformedMap`. We return the
+   associated sourcemap for it (its a transformed file, after all) so that sourcemap locations can
+   be traced through it into the source files it represents.
+2. `"helloworld.js"`, our original, unmodified source code. This file does not have a sourcemap, so
+   we return `null`.
+
+The `remapped` sourcemap now points from `transformed.min.js` into locations in `helloworld.js`. If
+you were to read the `mappings`, it says "0th column of the first line output line points to the 1st
+column of the 2nd line of the file `helloworld.js`".
+
+### Multiple transformations of a file
+
+As a convenience, if you have multiple single-source transformations of a file, you may pass an
+array of sourcemap files in the order of most-recent transformation sourcemap first. Note that this
+changes the `importer` and `depth` of each call to our loader. So our above example could have been
+written as:
+
+```js
+const remapped = remapping(
+  [minifiedTransformedMap, transformedMap],
+  () => null
+);
+
+console.log(remapped);
+// {
+//   file: 'transpiled.min.js',
+//   mappings: 'AAEE',
+//   sources: ['helloworld.js'],
+//   version: 3,
+// };
+```
+
+### Advanced control of the loading graph
+
+#### `source`
+
+The `source` property can overridden to any value to change the location of the current load. Eg,
+for an original source file, it allows us to change the location to the original source regardless
+of what the sourcemap source entry says. And for transformed files, it allows us to change the
+relative resolving location for child sources of the loaded sourcemap.
+
+```js
+const remapped = remapping(
+  minifiedTransformedMap,
+  (file, ctx) => {
+
+    if (file === 'transformed.js') {
+      // We pretend the transformed.js file actually exists in the 'src/' directory. When the nested
+      // source files are loaded, they will now be relative to `src/`.
+      ctx.source = 'src/transformed.js';
+      return transformedMap;
+    }
+
+    console.assert(file === 'src/helloworld.js');
+    // We could futher change the source of this original file, eg, to be inside a nested directory
+    // itself. This will be reflected in the remapped sourcemap.
+    ctx.source = 'src/nested/transformed.js';
+    return null;
+  }
+);
+
+console.log(remapped);
+// {
+//   …,
+//   sources: ['src/nested/helloworld.js'],
+// };
+```
+
+
+#### `content`
+
+The `content` property can be overridden when we encounter an original source file. Eg, this allows
+you to manually provide the source content of the original file regardless of whether the
+`sourcesContent` field is present in the parent sourcemap. It can also be set to `null` to remove
+the source content.
+
+```js
+const remapped = remapping(
+  minifiedTransformedMap,
+  (file, ctx) => {
+
+    if (file === 'transformed.js') {
+      // transformedMap does not include a `sourcesContent` field, so usually the remapped sourcemap
+      // would not include any `sourcesContent` values.
+      return transformedMap;
+    }
+
+    console.assert(file === 'helloworld.js');
+    // We can read the file to provide the source content.
+    ctx.content = fs.readFileSync(file, 'utf8');
+    return null;
+  }
+);
+
+console.log(remapped);
+// {
+//   …,
+//   sourcesContent: [
+//     'console.log("Hello world!")',
+//   ],
+// };
+```
+
+### Options
+
+#### excludeContent
+
+By default, `excludeContent` is `false`. Passing `{ excludeContent: true }` will exclude the
+`sourcesContent` field from the returned sourcemap. This is mainly useful when you want to reduce
+the size out the sourcemap.
+
+#### decodedMappings
+
+By default, `decodedMappings` is `false`. Passing `{ decodedMappings: true }` will leave the
+`mappings` field in a [decoded state](https://github.com/rich-harris/sourcemap-codec) instead of
+encoding into a VLQ string.