feat: function to calculate increment

src/functions/increment.spec.ts

@@ -21,5 +21,25 @@ describe('Increment', () => {
     expect(counterThree()).toBe(9)
     expect(counterThree()).toBe(8)
     expect(counterThree()).toBe(8)
+
+    const counterFour = increment({
+      increment: (current, count) => {
+        return 10 - count
+      }
+    })
+
+    expect(counterFour()).toBe(0)
+    expect(counterFour()).toBe(9)
+    expect(counterFour()).toBe(17)
+
+    const counterFive = decrement({
+      increment: (current, count) => {
+        return 10 - count
+      }
+    })
+
+    expect(counterFive()).toBe(0)
+    expect(counterFive()).toBe(-9)
+    expect(counterFive()).toBe(-17)
   })
 })

src/functions/increment.ts

@@ -4,7 +4,7 @@ export interface IncrementOptions {
   /** Initial Value for the counter, defaults to `0` */
   initial: number
   /** Value to increase counter by on each call, defaults to `1`. */
-  increment: number
+  increment: number | ((current: number, count: number) => number)
   /** Maximum value to increment to. */
   max: number
 }
@@ -27,22 +27,29 @@ export const increment = (
 ): IncrementFunction => {
   const {
     initial,
-    increment: incrementBy,
+    increment: incremental,
     max
   } = defaults<IncrementOptions>(options, {
     increment: 1,
     initial: 0,
     max: 0
   })
 
-  let counter = initial - incrementBy
+  let counter =
+    initial -
+    (typeof incremental === 'number' ? incremental : incremental(initial, 0))
+
+  let count = 0
 
   return () => {
     if (max !== 0 && counter >= max) {
       return counter
     }
 
-    counter += incrementBy
+    counter +=
+      typeof incremental === 'number'
+        ? incremental
+        : incremental(initial, count++)
 
     return counter
   }
@@ -59,22 +66,28 @@ export const decrement = (
 ): IncrementFunction => {
   const {
     initial,
-    increment: incrementBy,
+    increment: incremental,
     max
   } = defaults<IncrementOptions>(options, {
     increment: 1,
     initial: 0,
     max: 0
   })
 
-  let counter = initial + incrementBy
+  let counter =
+    initial +
+    (typeof incremental === 'number' ? incremental : incremental(initial, 0))
+  let count = 0
 
   return () => {
     if (max !== 0 && counter <= max) {
       return counter
     }
 
-    counter -= incrementBy
+    counter -=
+      typeof incremental === 'number'
+        ? incremental
+        : incremental(initial, count++)
 
     return counter
   }