1 files changed, 87 insertions, 0 deletions
diff --git a/js/type_directives.ts b/js/type_directives.ts
new file mode 100644
index 000000000..3e903a80b
--- /dev/null
+++ b/js/type_directives.ts
@@ -0,0 +1,87 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+
+interface DirectiveInfo {
+  path: string;
+  start: number;
+  end: number;
+}
+
+/** Remap the module name based on any supplied type directives passed. */
+export function getMappedModuleName(
+  moduleName: string,
+  containingFile: string,
+  typeDirectives?: Record<string, string>
+): string {
+  if (containingFile.endsWith(".d.ts") && !moduleName.endsWith(".d.ts")) {
+    moduleName = `${moduleName}.d.ts`;
+  }
+  if (!typeDirectives) {
+    return moduleName;
+  }
+  if (moduleName in typeDirectives) {
+    return typeDirectives[moduleName];
+  }
+  return moduleName;
+}
+
+/** Matches directives that look something like this and parses out the value
+ * of the directive:
+ *
+ *      // @deno-types="./foo.d.ts"
+ *
+ * [See Diagram](http://bit.ly/31nZPCF)
+ */
+const typeDirectiveRegEx = /@deno-types\s*=\s*(["'])((?:(?=(\\?))\3.)*?)\1/gi;
+
+/** Matches `import` or `export from` statements and parses out the value of the
+ * module specifier in the second capture group:
+ *
+ *      import * as foo from "./foo.js"
+ *      export { a, b, c } from "./bar.js"
+ *
+ * [See Diagram](http://bit.ly/2GSkJlF)
+ */
+const importExportRegEx = /(?:import|export)\s+[\s\S]*?from\s+(["'])((?:(?=(\\?))\3.)*?)\1/;
+
+/** Parses out any Deno type directives that are part of the source code, or
+ * returns `undefined` if there are not any.
+ */
+export function parseTypeDirectives(
+  sourceCode: string | undefined
+): Record<string, string> | undefined {
+  if (!sourceCode) {
+    return;
+  }
+
+  // collect all the directives in the file and their start and end positions
+  const directives: DirectiveInfo[] = [];
+  let maybeMatch: RegExpExecArray | null = null;
+  while ((maybeMatch = typeDirectiveRegEx.exec(sourceCode))) {
+    const [matchString, , path] = maybeMatch;
+    const { index: start } = maybeMatch;
+    directives.push({
+      path,
+      start,
+      end: start + matchString.length
+    });
+  }
+  if (!directives.length) {
+    return;
+  }
+
+  // work from the last directive backwards for the next `import`/`export`
+  // statement
+  directives.reverse();
+  const directiveRecords: Record<string, string> = {};
+  for (const { path, start, end } of directives) {
+    const searchString = sourceCode.substring(end);
+    const maybeMatch = importExportRegEx.exec(searchString);
+    if (maybeMatch) {
+      const [, , fromPath] = maybeMatch;
+      directiveRecords[fromPath] = path;
+    }
+    sourceCode = sourceCode.substring(0, start);
+  }
+
+  return directiveRecords;
+}