From 5850f0b742430a1b1b5ae7d2491dce6ca10e20d3 Mon Sep 17 00:00:00 2001
From: oberien <jaro.fietz@gmx.de>
Date: Fri, 19 Jan 2018 14:55:20 +0100
Subject: [PATCH] Fix off-by-ones

---
 src/libcore/iter/mod.rs | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index 443b1567e1b4..33c00989fd28 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -698,13 +698,16 @@ impl<I> Iterator for StepBy<I> where I: Iterator {
     #[inline]
     fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
         if self.first_take {
+            self.first_take = false;
+            let first = self.iter.next();
             if n == 0 {
-                self.first_take = false;
-                return self.iter.next()
+                return first;
             }
             n -= 1;
         }
-        self.iter.nth(n * self.step)
+        // n and self.step are indices, thus we need to add 1 before multiplying.
+        // After that we need to subtract 1 from the result to convert it back to an index.
+        self.iter.nth((n + 1) * (self.step + 1) - 1)
     }
 }