fix: channel validator

Closes: #371

Author: opatiny

src/__tests__/Image.test.ts

@@ -279,7 +279,7 @@ test('fill channel 2', () => {
 test('fill channel invalid channel', () => {
   const img = new Image(1, 2);
   expect(() => img.fillChannel(4, 50)).toThrow(
-    /invalid channel: 4. It must be a positive integer smaller than 4/,
+    /invalid channel: 4. It must be a positive integer smaller than 3/,
   );
 });
 

src/draw/__tests__/drawPolygonOnImage.test.ts

@@ -280,7 +280,7 @@ test('stroke color not compatible with image', () => {
       fillColor: [2, 5],
     });
   }).toThrow(
-    'invalid channel: 2. It must be a positive integer smaller than 2',
+    'invalid channel: 1. It must be a positive integer smaller than 1',
   );
 });
 

src/utils/validators/validators.ts

@@ -18,11 +18,9 @@ export function validateChannels(channels: number[], image: Image): void {
  * @param image - The image being processed.
  */
 export function validateChannel(channel: number, image: Image): void {
-  if (!Number.isInteger(channel) || channel > image.channels || channel < 0) {
+  if (!Number.isInteger(channel) || channel >= image.channels || channel < 0) {
     throw new RangeError(
-      `invalid channel: ${channel}. It must be a positive integer smaller than ${
-        image.channels + 1
-      }`,
+      `invalid channel: ${channel}. It must be a positive integer smaller than ${image.channels}`,
     );
   }
 }
@@ -79,6 +77,6 @@ export function validateForComparison(
  * @param image - Image.
  */
 export function validateColor(color: number[], image: Image): void {
-  validateChannel(color.length, image);
+  validateChannel(color.length - 1, image);
   validateValues(color, image);
 }