From 11cf89ca37cfd9e8bfb213b262be4b166ea85a1f Mon Sep 17 00:00:00 2001
From: Christoph Otter <chris@confio.gmbh>
Date: Mon, 28 Aug 2023 16:52:32 +0200
Subject: [PATCH 1/2] Impl nth for External(Partial)Iterator

---
 packages/std/src/imports.rs | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/packages/std/src/imports.rs b/packages/std/src/imports.rs
index 8f99e6941e..7df3f22c95 100644
--- a/packages/std/src/imports.rs
+++ b/packages/std/src/imports.rs
@@ -200,6 +200,15 @@ struct ExternalPartialIterator {
 impl Iterator for ExternalPartialIterator {
     type Item = Vec<u8>;
 
+    /// The default implementation calls `next` repeatedly,
+    /// which we can do a little more efficiently by using `db_next_key` instead.
+    /// It is used by `skip`, so it allows cheaper skipping.
+    #[cfg(feature = "cosmwasm_1_4")]
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        skip_iter(self.iterator_id, n);
+        self.next()
+    }
+
     fn next(&mut self) -> Option<Self::Item> {
         // here we differentiate between the two types
         let next_result = match self.partial_type {
@@ -229,6 +238,15 @@ struct ExternalIterator {
 impl Iterator for ExternalIterator {
     type Item = Record;
 
+    /// The default implementation calls `next` repeatedly,
+    /// which we can do a little more efficiently by using `db_next_key` instead.
+    /// It is used by `skip`, so it allows cheaper skipping.
+    #[cfg(feature = "cosmwasm_1_4")]
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        skip_iter(self.iterator_id, n);
+        self.next()
+    }
+
     fn next(&mut self) -> Option<Self::Item> {
         let next_result = unsafe { db_next(self.iterator_id) };
         let kv_region_ptr = next_result as *mut Region;
@@ -242,6 +260,20 @@ impl Iterator for ExternalIterator {
     }
 }
 
+/// Helper function to skip `count` elements of an iterator.
+#[cfg(all(feature = "iterator", feature = "cosmwasm_1_4"))]
+fn skip_iter(iter_id: u32, count: usize) {
+    for _ in 0..count {
+        let region = unsafe { db_next_key(iter_id) };
+        if region == 0 {
+            // early return
+            return;
+        }
+        // just deallocate the region
+        unsafe { consume_region(region as *mut Region) };
+    }
+}
+
 /// A stateless convenience wrapper around imports provided by the VM
 #[derive(Copy, Clone)]
 pub struct ExternalApi {}

From 8e4907ae2d5cb16ff228220df48907f9172819d3 Mon Sep 17 00:00:00 2001
From: Christoph Otter <chris@confio.gmbh>
Date: Mon, 28 Aug 2023 17:03:08 +0200
Subject: [PATCH 2/2] Add changelog entry

---
 CHANGELOG.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2a859fa4c0..8724ef54eb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -38,6 +38,9 @@ and this project adheres to
 - cosmwasm-vm: Use `wasmparser` for initial validation instead of `parity-wasm`
   ([#1786])
 - cosmwasm-std: Make constructors `Decimal{,256}::{percent,permille,bps}` const
+- cosmwasm-std: Use new `db_next_key` import to make `skip` and `nth`
+  implementation of `range` iterators more efficient. This requires the
+  `cosmwasm_1_4` feature to be enabled. ([#1838])
 
 [#1667]: https://github.com/CosmWasm/cosmwasm/pull/1667
 [#1674]: https://github.com/CosmWasm/cosmwasm/pull/1674
@@ -46,6 +49,7 @@ and this project adheres to
 [#1701]: https://github.com/CosmWasm/cosmwasm/pull/1701
 [#1786]: https://github.com/CosmWasm/cosmwasm/pull/1786
 [#1793]: https://github.com/CosmWasm/cosmwasm/pull/1793
+[#1838]: https://github.com/CosmWasm/cosmwasm/pull/1838
 
 ## [1.3.3] - 2023-08-22