Fix the implementation of length validation for strings

The Smithy spec specifies that string length is defined as the number of code
points. JavaScript's String.length returns the number of code units, leading to
inaccurate validation when a string contains characters that use multiple code
units to represent one code point.

Fixes smithy-lang#509

Author: adamthom-amzn

smithy-typescript-ssdk-libs/server-common/src/validation/validators.spec.ts

@@ -114,6 +114,10 @@ describe("length validation", () => {
       });
     });
   }
+
+  it("properly assesses string length", () => {
+    expect(new LengthValidator(3, 3).validate("👍👍👍", "threeEmojis")).toBeNull();
+  });
 });
 
 describe("pattern validation", () => {

smithy-typescript-ssdk-libs/server-common/src/validation/validators.ts

@@ -174,7 +174,7 @@ export class EnumValidator implements SingleConstraintValidator<string, EnumVali
   }
 }
 
-type LengthCheckable = { length: number } | { [key: string]: any };
+type LengthCheckable = string | { length: number } | { [key: string]: any };
 
 export class LengthValidator implements SingleConstraintValidator<LengthCheckable, LengthValidationFailure> {
   private readonly min?: number;
@@ -194,7 +194,9 @@ export class LengthValidator implements SingleConstraintValidator<LengthCheckabl
     }
 
     let length: number;
-    if (LengthValidator.hasLength(input)) {
+    if (typeof input === "string") {
+      length = [...input].length; // string length is defined by the number of code points
+    } else if (LengthValidator.hasLength(input)) {
       length = input.length;
     } else {
       length = Object.keys(input).length;