Fix default fromIndex in String#lastIndexOf (#165)

std/assembly/string.ts

@@ -229,17 +229,17 @@ export class String {
     return -1;
   }
 
-  lastIndexOf(searchString: String, fromIndex: i32 = 0): i32 {
+  lastIndexOf(searchString: String, fromIndex: i32 = i32.MAX_VALUE): i32 {
     assert(this !== null);
     if (searchString === null) searchString = changetype<String>("null");
     var len: isize = this.length;
     var searchLen: isize = searchString.length;
     if (!searchLen) return len;
     if (!len) return -1;
-    var start = clamp<isize>(fromIndex - searchLen, 0, len);
+    var start = clamp<isize>(fromIndex, 0, len - searchLen);
 
     // TODO: multiple char codes
-    for (let k: isize = len - 1; k >= start; --k) {
+    for (let k = start; k >= 0; --k) {
       if (!compare_memory(
         changetype<usize>(this) + HEADER_SIZE + (k << 1),
         changetype<usize>(searchString) + HEADER_SIZE,

tests/compiler/std/string.ts

@@ -28,9 +28,12 @@ assert("".lastIndexOf("hi") == -1);
 assert(str.lastIndexOf("") == str.length);
 assert(str.lastIndexOf(",") == 2);
 assert(str.lastIndexOf("x") == -1);
+assert(str.lastIndexOf("g") == 15);
 assert(str.lastIndexOf(",", 2) == 2);
 assert(str.lastIndexOf(",", 3) == 2);
-assert(str.lastIndexOf(", I", -1) == 2);
+assert(str.lastIndexOf(", I", -1) == -1);
+assert(str.lastIndexOf("i", 0) == -1);
+assert(str.lastIndexOf("hi", 0) == 0);
 
 export function getString(): string {
   return str;