From cc65750423bd77f2547224075bb08a9735bed737 Mon Sep 17 00:00:00 2001
From: Cody Casterline <cody@nextdata.com>
Date: Sun, 16 Jul 2023 12:56:09 -0700
Subject: [PATCH] Add joinToString()

---
 mod.ts             | 27 +++++++++++++++++++++++++++
 tests/main_test.ts | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/mod.ts b/mod.ts
index 2791339..fa22d3f 100644
--- a/mod.ts
+++ b/mod.ts
@@ -216,6 +216,9 @@ interface LazyShared<T> {
     /** Flattens a Lazy<Iterable<T>> to a Lazy<T> */
     flatten(): LazyShared<Flattened<T>>
 
+    /** Joins multiple string values into a string. */
+    joinToString(args?: JoinToStringArgs): Awaitable<JoinedToString<T>>
+
     /** Fold values. See example in {@link LazyShared#sum} */
     fold<I>(initialValue: I, foldFn: (i: I, t: T) => I): Awaitable<I>
 
@@ -469,6 +472,12 @@ export class Lazy<T> implements Iterable<T>, LazyShared<T> {
         }())
     }
 
+    /** Joins multiple string values into a string. */
+    joinToString(args?: JoinToStringArgs): JoinedToString<T> {
+        const sep = args?.sep ?? ", "
+        return this.toArray().join(sep) as JoinedToString<T>
+    }
+
     /** Collect all items into an array. */
     toArray(): Array<T> {
         return [... this.#inner]
@@ -874,6 +883,12 @@ export class LazyAsync<T> implements AsyncIterable<T>, LazyShared<T> {
         }())
     }
 
+    /** Joins multiple string values into a string. */
+    async joinToString(args?: JoinToStringArgs): Promise<JoinedToString<T>> {
+        const sep = args?.sep ?? ", "
+        return (await this.toArray()).join(sep) as JoinedToString<T>
+    }
+
     /** Collect all items into an array. */
     async toArray(): Promise<T[]> {
         let out: T[] = []
@@ -1070,6 +1085,18 @@ export interface RangeArgs {
     inclusive?: boolean
 }
 
+export interface JoinToStringArgs {
+    /** The separator to use. Default: ", " */
+    sep?: string
+
+    // Can add more join options here if people want. 
+}
+
+/**
+ * Only `string` values can be joined to string
+ */
+export type JoinedToString<T> = T extends string ? string : never
+
 const rangeArgsDefaults: Required<RangeArgs> = {
     from: 0,
     to: Number.MAX_SAFE_INTEGER,
diff --git a/tests/main_test.ts b/tests/main_test.ts
index 0b10d83..e3e11d0 100644
--- a/tests/main_test.ts
+++ b/tests/main_test.ts
@@ -286,3 +286,45 @@ Deno.test(async function avg(t) {
         assertEquals(await iter.avg(), 4.5)
     })
 })
+
+Deno.test(async function joinToString(t) {
+    const input = () => range({from: 14, to: 21})
+    await testBoth(t, input, async (iter) => {
+        let result = await iter
+            .map(num => `${num}: ${fizzBuzz(num)}`)
+            .joinToString()
+        assertEquals(result, "14: 14, 15: FizzBuzz, 16: 16, 17: 17, 18: Fizz, 19: 19, 20: Buzz")
+    })
+})
+
+Deno.test(async function joinToStringWithSep(t) {
+    const input = () => range({from: 14, to: 21})
+    await testBoth(t, input, async (iter) => {
+        let result = await iter
+            .map(num => `${num}: ${fizzBuzz(num)}`)
+            .joinToString({sep: "X"})
+        assertEquals(result, "14: 14X15: FizzBuzzX16: 16X17: 17X18: FizzX19: 19X20: Buzz")
+    })
+})
+
+Deno.test(async function joinToStringEmpty(t) {
+    await testBoth(t, [], async (iter) => {
+        let result = await iter
+            .joinToString()
+
+        assertEquals(result, "")
+    })
+})
+
+function fizzBuzz(num: number) {
+    if (num % 15 == 0) {
+        return "FizzBuzz"
+    }
+    if (num % 5 == 0) {
+        return "Buzz"
+    }
+    if (num % 3 == 0) {
+        return "Fizz"
+    }
+    return `${num}`
+}
\ No newline at end of file