Fix: Correct handling of identical minItems and maxItems in array schemas when arrayLength option is true (#1784)

* Fix: Correct handling of minItems and maxItems with identical values in array schemas

- Resolved issue where the generated interface was [] when minItems and maxItems were identical
- Now generates a tuple type for such cases
- Added test case for OpenAPI specification with minItems and maxItems set to 2

* chore: run lint fix

Author: yoshi2no

.changeset/thick-geckos-compete.md

@@ -0,0 +1,5 @@
+---
+"openapi-typescript": patch
+---
+
+Fix: Correct handling of identical minItems and maxItems in array schemas when arrayLength option is true

packages/openapi-typescript/src/transform/schema-object.ts

@@ -330,8 +330,14 @@ function transformSchemaObjectCore(schemaObject: SchemaObject, options: Transfor
         (min !== 0 || max !== undefined) &&
         estimateCodeSize < 30 // "30" is an arbitrary number but roughly around when TS starts to struggle with tuple inference in practice
       ) {
-        // if maxItems is set, then return a union of all permutations of possible tuple types
-        if ((schemaObject.maxItems as number) > 0) {
+        if (min === max) {
+          const elements: ts.TypeNode[] = [];
+          for (let i = 0; i < min; i++) {
+            elements.push(itemType);
+          }
+          return tsUnion([ts.factory.createTupleTypeNode(elements)]);
+        } else if ((schemaObject.maxItems as number) > 0) {
+          // if maxItems is set, then return a union of all permutations of possible tuple types
           const members: ts.TypeNode[] = [];
           // populate 1 short of min …
           for (let i = 0; i <= (max ?? 0) - min; i++) {

packages/openapi-typescript/test/transform/schema-object/array.test.ts

@@ -146,6 +146,20 @@ describe("transformSchemaObject > array", () => {
         },
       },
     ],
+    [
+      "options > arrayLength: true > minItems: 2, maxItems: 2",
+      {
+        given: { type: "array", items: { type: "string" }, minItems: 2, maxItems: 2 },
+        want: `[
+    string,
+    string
+]`,
+        options: {
+          ...DEFAULT_OPTIONS,
+          ctx: { ...DEFAULT_OPTIONS.ctx, arrayLength: true },
+        },
+      },
+    ],
     [
       "options > immutable: true",
       {