fix: respect desired path on case-insensitive file systems (#397)

Today typescript-transform-paths changes the case of import statements
on a case-insensitive file system. I have a file `Foo` and an import
`from "./Foo"`, yet after the transformation with
typescript-transform-paths, the import looks like `"./foo"`. Even if
that import would work on a case-insensitive file system, it seems
clearly undesirable.

Author: denis-sokolov

Diff for: src/utils/get-relative-path.ts

@@ -46,7 +46,8 @@ function getIsFsCaseSensitive() {
   return isCaseSensitiveFilesystem;
 }
 
-function getMatchPortion(from: string, to: string) {
+/** @private The Export is only for unit tests */
+export function getMatchPortion(from: string, to: string) {
   const lowerFrom = from.toLocaleLowerCase();
   const lowerTo = to.toLocaleLowerCase();
 
@@ -58,7 +59,7 @@ function getMatchPortion(from: string, to: string) {
     i++;
   }
 
-  return from.slice(0, i);
+  return to.slice(0, i);
 }
 
 // endregion

Diff for: test/tests/get-match-portion.test.ts

@@ -0,0 +1,18 @@
+import { getMatchPortion } from "../../src/utils/get-relative-path";
+
+describe(`getMatchPortion`, () => {
+  it("works in a simple case", () => {
+    expect(getMatchPortion("/foo/bar", "/foo/quux")).toBe("/foo/");
+  });
+
+  // We use the function getMatchPortion to generate a new path for “to”, so let’s preserve
+  // the case where possible.
+  // Otherwise we are introducing inconsistency for our users, who may have had import from Foo,
+  // their file is named Foo, but we rewrite the path to foo.
+  // Although the file is still accessible in the file system, other tools might reasonably
+  // complain about the unexpected case mismatch.
+  it("prioritizes the casing of the “to” parameter", () => {
+    expect(getMatchPortion("/foo/bar", "/foO/quux")).toBe("/foO/");
+    expect(getMatchPortion("/foo/bar", "/foo/Bonk")).toBe("/foo/B");
+  });
+});