fix(utils): throw errors for unexpected situations (#197)

Author: ikatyang

src/utils.ts

@@ -234,15 +234,29 @@ export function fitSpans(
 export function findFrontChar(regex: RegExp, index: number, text: string) {
   let i = index;
   while (!regex.test(text[i])) {
-    i--;
+    // istanbul ignore next
+    if (--i < 0) {
+      throw new Error(
+        `Cannot find front char ${regex} from index ${index} in ${JSON.stringify(
+          text,
+        )}`,
+      );
+    }
   }
   return i;
 }
 
 export function findBackChar(regex: RegExp, index: number, text: string) {
   let i = index;
   while (!regex.test(text[i])) {
-    i++;
+    // istanbul ignore next
+    if (++i >= text.length) {
+      throw new Error(
+        `Cannot find back char ${regex} from index ${index} in ${JSON.stringify(
+          text,
+        )}`,
+      );
+    }
   }
   return i;
 }