refactor: pre calculate the empty hash to save cost

[cuiweixie]

Summary by CodeRabbit

• Refactor
  • Streamlined internal hashing by using a shared empty-hash value and removing a redundant helper, with references updated accordingly.
  • No changes to public APIs or visible behavior; hashing for leaf and inner nodes remains unchanged.
  • Maintains compatibility with existing data and workflows; no user-facing impact.

[cuiweixie]

#25358
@aljo242

[coderabbitai]

📝 Walkthrough

Walkthrough

Refactors empty hash handling in store/internal/tree/hash.go by replacing a helper function with a package-level byte slice initialized from a hex string, updating call sites accordingly, and adding the required import. Other hashing logic remains unchanged.

Changes

Cohort / File(s)	Summary
Hash empty value refactor store/internal/tree/hash.go	Added import encoding/hex; introduced package-level variable emptyHash []byte from a hex string; replaced calls to emptyHash() with emptyHash; removed the old emptyHash() function; left leaf/inner node hashing logic unchanged.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	The title succinctly and accurately summarizes the primary change: precomputing the empty hash to save runtime cost. This directly maps to the changes in store/internal/tree/hash.go where the emptyHash() function is replaced by a precomputed emptyHash variable and call sites are updated. The phrasing is concise, specific, and relevant to the changeset.

✨ Finishing touches

• 📝 Generate Docstrings

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 1

🧹 Nitpick comments (2)

store/internal/tree/hash.go (2)

5-5: Drop hex import after switching to array precompute (or handle the error).

If you adopt the array approach, encoding/hex is unused; remove it. If you keep hex decoding, don’t ignore the error—validate at init and panic on malformed constant.

Apply this diff when using the array:

 import (
   "crypto/sha256"
-  "encoding/hex"
   "hash"
   "math/bits"
 )

If keeping hex:

-var emptyHash, _ = hex.DecodeString("e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855")
+var emptyHash = func() []byte {
+  b, err = hex.DecodeString("e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855")
+  if err != nil { panic(err) }
+  return b
+}()

---

21-31: Trim the generator comment; keep a terse, source-of-truth note.

The multi-line “generate from this code” block is noisy. Replace with a concise doc explaining the precomputation.

Apply this diff:

-// generate from this code
-//
-// 
-//  func emptyHash() []byte {
-//    h := sha256.Sum256([]byte{})
-//    return h[:]
-//  }
-//
-// 
-//  func main() {
-//    println(hex.EncodeToString(emptyHash()))
-//  }
+// emptyHashArr is sha256.Sum256(nil) precomputed once to avoid per-call hashing.

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

Knowledge base: Disabled due to Reviews -> Disable Knowledge Base setting

📥 Commits

Reviewing files that changed from the base of the PR and between 9171179 and 75ee605.

📒 Files selected for processing (1)

• store/internal/tree/hash.go (2 hunks)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (1)

• GitHub Check: Summary

[aljo242]

Use a precomputed [32]byte and copy on return (32B), which is safe and still cheap.

why cant we just do this ^^

[cuiweixie]

Use a precomputed [32]byte and copy on return (32B), which is safe and still cheap.

why cant we just do this ^^

done

[aljo242]

I'd like to add a test to verify that this is the actual empty hash

[cuiweixie]

I'd like to add a test to verify that this is the actual empty hash

Done.