fix: prevent hashCode recomputation when computed value is 0 in SegmentCodec by guizmaii · Pull Request #3985 · zio/zio-http

guizmaii · 2026-03-02T06:59:04Z

Summary

Applies the if (h == 0) 1 else h cached hashCode sentinel pattern to SegmentCodec.hashCode
Prevents recomputation (including Tuple2 allocation) on every hashCode call when the computed value happens to be 0

Background: the cached hashCode sentinel pattern

SegmentCodec.hashCode uses the lazily-initialized cached hashCode pattern described in Joshua Bloch's Effective Java (Item 11, 3rd edition):

// Effective Java, Item 11 — Lazily initialized, cached hashCode
private volatile int hashCode;

@Override public int hashCode() {
    int result = hashCode;
    if (result == 0) {
        result = /* compute hash */;
        hashCode = result;
    }
    return result;
}

The canonical real-world example is java.lang.String.hashCode() in OpenJDK, which uses hash == 0 as the sentinel for "not yet computed."

The one weakness of this pattern is that when the computed hash genuinely equals 0 (~1 in 2^32 probability), it will be recomputed on every call. The if (h == 0) 1 else h refinement remaps hash-0 to hash-1, eliminating even that edge case. The collision of mapping one extra value to 1 is negligible.

Test plan

Existing SegmentCodecSpec tests pass (7 tests)
New test verifies hashCode is stable across multiple calls and non-zero
New test verifies all standard segment codec types produce non-zero hashCode

Use the standard if-zero-then-one sentinel pattern to avoid recomputing the hashCode on every call when the computed value happens to be 0. Co-Authored-By: Jules Ivanic <jules.ivanic@gmail.com>

netlify · 2026-03-02T06:59:08Z

✅ Deploy Preview for zio-http ready!

Name	Link
🔨 Latest commit	`5b8390b`
🔍 Latest deploy log	https://app.netlify.com/projects/zio-http/deploys/69a5353b74aae300088bc3d6
😎 Deploy Preview	https://deploy-preview-3985--zio-http.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

To edit notification comments on pull requests, go to your Netlify project configuration.

Copilot

Pull request overview

Updates SegmentCodec.hashCode caching to avoid the “computed hash is 0” sentinel collision, and adds tests around hashCode stability / non-zero behavior.

Changes:

Remap computed hash 0 to 1 when populating the cached hash code.
Add spec coverage asserting hashCode stability and that standard codecs produce non-zero hash codes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
zio-http/shared/src/main/scala/zio/http/codec/SegmentCodec.scala	Adjusts cached hash computation to remap `0` to `1` before caching.
zio-http/shared/src/test/scala/zio/http/codec/SegmentCodecSpec.scala	Adds tests asserting stable and non-zero `hashCode` behavior for segment codecs.

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

Copilot · 2026-03-24T18:51:36Z

+      test("hashCode is stable across multiple calls") {
+        val codec = SegmentCodec.literal("test")
+        val hash1 = codec.hashCode
+        val hash2 = codec.hashCode
+        assertTrue(hash1 == hash2, hash1 != 0)
+      },
+      test("hashCode is non-zero for all standard segment codecs") {
+        val codecs: List[SegmentCodec[_]] = List(
+          SegmentCodec.empty,
+          SegmentCodec.literal("users"),
+          SegmentCodec.int("id"),
+          SegmentCodec.long("id"),
+          SegmentCodec.string("name"),
+          SegmentCodec.uuid("uuid"),
+          SegmentCodec.trailing,
+        )
+        assertTrue(codecs.forall(_.hashCode != 0))
+      },


These tests don’t deterministically cover the specific regression this PR fixes (the computed hash being exactly 0 is astronomically unlikely, so the tests will almost always pass even without the remapping). To actually validate the behavior, consider making the raw hash computation injectable/overridable (e.g., a protected[codec] def computeHash: Int) so a test-only codec can force computeHash == 0 and assert the cached value becomes 1 and remains stable.

Copilot · 2026-03-24T18:51:36Z

-    if (_hashCode == 0) _hashCode = (this.getClass.getName(), render).hashCode
+    if (_hashCode == 0) {
+      val h = (this.getClass.getName(), render).hashCode
+      _hashCode = if (h == 0) 1 else h


The remap-to-1 behavior is a subtle sentinel workaround, but it’s not self-explanatory in code. Please add a short inline comment (or a named constant) explaining that 0 is reserved as the 'not computed' sentinel and is therefore remapped to avoid recomputation.

Suggested change

_hashCode = if (h == 0) 1 else h

_hashCode = if (h == 0) 1 else h // 0 is reserved as "not computed", so remap real hash 0 to 1 to avoid recomputation

Copilot · 2026-03-24T18:51:36Z

@@ -45,7 +45,10 @@ sealed trait SegmentCodec[A] { self =>
  def format(value: A): Path

  override val hashCode: Int = {


The PR description discusses a lazily-initialized cached hashCode that can be recomputed on every call when the computed value is 0. In the diff, hashCode is overridden as a val, which is computed once at initialization time and won’t be recomputed on subsequent calls regardless of the sentinel value. Either adjust the implementation to match the described lazy-cached pattern (i.e., def hashCode with a cached var) or update the PR description/tests to match the eager val behavior.

fix: prevent hashCode recomputation when computed value is 0

5b8390b

Use the standard if-zero-then-one sentinel pattern to avoid recomputing the hashCode on every call when the computed value happens to be 0. Co-Authored-By: Jules Ivanic <jules.ivanic@gmail.com>

guizmaii self-assigned this Mar 2, 2026

987Nabil marked this pull request as ready for review March 24, 2026 18:44

987Nabil requested review from 987Nabil, jdegoes, kyri-petrou and vigoo as code owners March 24, 2026 18:44

Copilot AI review requested due to automatic review settings March 24, 2026 18:44

987Nabil merged commit 86b24ff into main Mar 24, 2026
84 checks passed

987Nabil deleted the hot-path/segmentcodec-hashcode-sentinel branch March 24, 2026 18:44

Copilot AI reviewed Mar 24, 2026

View reviewed changes

Copilot started reviewing on behalf of 987Nabil March 24, 2026 19:30 View session

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

fix: prevent hashCode recomputation when computed value is 0 in SegmentCodec#3985

fix: prevent hashCode recomputation when computed value is 0 in SegmentCodec#3985
987Nabil merged 1 commit intomainfrom
hot-path/segmentcodec-hashcode-sentinel

guizmaii commented Mar 2, 2026 •

edited

Loading

Uh oh!

netlify Bot commented Mar 2, 2026 •

edited

Loading

Uh oh!

Uh oh!

Copilot AI left a comment

Uh oh!

Copilot AI Mar 24, 2026

Uh oh!

Copilot AI Mar 24, 2026

Uh oh!

Copilot AI Mar 24, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

	_hashCode = if (h == 0) 1 else h
	_hashCode = if (h == 0) 1 else h // 0 is reserved as "not computed", so remap real hash 0 to 1 to avoid recomputation

		@@ -45,7 +45,10 @@ sealed trait SegmentCodec[A] { self =>
		def format(value: A): Path

		override val hashCode: Int = {

Uh oh!

Conversation

guizmaii commented Mar 2, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Summary

Background: the cached hashCode sentinel pattern

Test plan

Uh oh!

netlify Bot commented Mar 2, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

✅ Deploy Preview for zio-http ready!

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Copilot AI Mar 24, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Mar 24, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Mar 24, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

guizmaii commented Mar 2, 2026 •

edited

Loading

netlify Bot commented Mar 2, 2026 •

edited

Loading