From 43d56786977ab929af485f1c947ad6ee1aca8ed3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C3=ABl=20Lemaire?= <michael@thunderk.net>
Date: Thu, 26 Aug 2021 23:36:25 +0200
Subject: [PATCH] Add imapfilter function

---
 README.md   |  1 +
 doc/use.md  |  1 +
 mod.test.ts | 13 ++++++++++++-
 mod.ts      | 20 ++++++++++++++++++++
 4 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index b81898e..21334c0 100644
--- a/README.md
+++ b/README.md
@@ -64,6 +64,7 @@ iloop(i1); // 1 2 3 1 2 3 ...
 iskip(i3, 2); // -2 -3 -4 -5 -6 ...
 imap(i1, (x) => x * 2); // 2 4 6
 ifilter(i3, (x) => x % 4 == 0); // 0 -4 -8 -12 ...
+imapfilter(i1, (x) => x != 4 ? x * 2 : null); // 2 6
 ipartition(i3, (x) => x % 4 == 0); // [0 -4 -8 ..., -1 -2 -3 -5 ...]
 iunique(iarray([1, 4, 2, 4, 3, 1, 5])); // 1 4 2 3 5
 
diff --git a/doc/use.md b/doc/use.md
index 757a9b6..616e6b3 100644
--- a/doc/use.md
+++ b/doc/use.md
@@ -30,6 +30,7 @@ iloop(i1); // 1 2 3 1 2 3 ...
 iskip(i3, 2); // -2 -3 -4 -5 -6 ...
 imap(i1, (x) => x * 2); // 2 4 6
 ifilter(i3, (x) => x % 4 == 0); // 0 -4 -8 -12 ...
+imapfilter(i1, (x) => x != 4 ? x * 2 : null); // 2 6
 ipartition(i3, (x) => x % 4 == 0); // [0 -4 -8 ..., -1 -2 -3 -5 ...]
 iunique(iarray([1, 4, 2, 4, 3, 1, 5])); // 1 4 2 3 5
 
diff --git a/mod.test.ts b/mod.test.ts
index dcd0903..f1facf5 100644
--- a/mod.test.ts
+++ b/mod.test.ts
@@ -17,6 +17,7 @@ import {
   ilength,
   iloop,
   imap,
+  imapfilter,
   imaterialize,
   imax,
   imin,
@@ -241,10 +242,20 @@ describe("Iterators", () => {
   });
 
   it("filters an iterator with a predicate", () => {
-    checkit(imap(IEMPTY, (i) => i % 3 == 0), []);
+    checkit(ifilter(IEMPTY, (i) => i % 3 == 0), []);
     checkit(ifilter(irange(12), (i) => i % 3 == 0), [0, 3, 6, 9]);
   });
 
+  it("maps and filters in a single pass", () => {
+    checkit(imapfilter(IEMPTY, (i) => i % 2 == 0 ? -i : null), []);
+    checkit(imapfilter(irange(8), (i) => i % 2 == 0 ? -i : null), [
+      -0,
+      -2,
+      -4,
+      -6,
+    ]);
+  });
+
   it("filters an iterator with a type guard", () => {
     let result: Iterable<number> = ifiltertype(
       <(number | string)[]> [1, "a", 2, "b"],
diff --git a/mod.ts b/mod.ts
index cfc5494..18faa9f 100644
--- a/mod.ts
+++ b/mod.ts
@@ -338,6 +338,26 @@ export function ifilter<T>(
   };
 }
 
+/**
+ * Apply map, and filter invalid results in the same pass.
+ */
+export function imapfilter<T1, T2, E>(
+  iterable: Iterable<T1>,
+  mapfunc: (_: T1) => Exclude<T2, E> | typeof exclusion,
+  exclusion: E = null as unknown as E,
+): Iterable<T2> {
+  return {
+    [Symbol.iterator]: function* () {
+      for (let value of iterable) {
+        const mapped = mapfunc(value);
+        if (mapped !== exclusion) {
+          yield mapped as T2;
+        }
+      }
+    },
+  };
+}
+
 /**
  * Type filter, to return a list of instances of a given type
  */