perf: replace O(n²) hex-encoding loops with std::to_chars

[jiuker]

perf: replace O(n²) hex-encoding loops with std::to_chars
fix #219

Summary by CodeRabbit

• Refactor
  • Optimized internal cryptographic routines for signature and hash generation by improving hex-encoding performance and reducing temporary allocations. These changes preserve existing outputs and public interfaces while improving speed and reliability of signing and hashing operations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 102a2521-9652-46b9-8309-f6d3b1210d0e

📥 Commits

Reviewing files that changed from the base of the PR and between 1e53dbd and 38a2a3f.

📒 Files selected for processing (1)

• src/signer.cc

🚧 Files skipped from review as they are similar to previous changes (1)

• src/signer.cc

---

📝 Walkthrough

Walkthrough

This PR replaces O(n²) hex-encoding loops in two hot signing paths (GetSignature and Sha256Hash) with O(n) implementations using std::to_chars, eliminating repeated string reallocations by pre-allocating output buffers once and writing hex digits directly.

Changes

Hex encoding optimization

Layer / File(s)	Summary
GetSignature hex encoding src/signer.cc	Adds <charconv> and refactors the signature hex formatting loop from snprintf("%02x") with incremental += to a pre-sized string buffer with std::to_chars direct writes and manual zero-padding when needed.
Sha256Hash hex encoding src/utils.cc	Adds <charconv> and refactors the hash hex string construction from per-byte snprintf appends to direct buffer writes via std::to_chars, with zero-padding normalization to handle single-character outputs.

🎯 2 (Simple) | ⏱️ ~10 minutes

A rabbit hops through the code so fast,
No more allocations lurking in the past,
Two strings now write in one swift pass,
Performance gains—so snappy, so crass! 🐰✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately describes the main change: replacing O(n²) hex-encoding loops with std::to_chars in hot signing paths.
Linked Issues check	✅ Passed	The PR implementation matches all coding requirements: both hot paths (GetSignature in src/signer.cc and Sha256Hash in src/utils.cc) now use std::to_chars with pre-sized buffers instead of snprintf+concatenation loops.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to the linked issue #219: modifications target only the two identified hex-encoding paths with no unrelated alterations.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 Infer (1.2.0)

src/signer.cc

src/signer.cc:18:10: fatal error: 'miniocpp/signer.h' file not found
18 | #include "miniocpp/signer.h"
| ^~~~~~~~~~~~~~~~~~~
1 error generated.
Error: the following clang command did not run successfully:
/opt/infer-linux-x86_64-v1.2.0/lib/infer/facebook-clang-plugins/clang/install/bin/clang-18
@/tmp/coderabbit-infer/38a2a3f3458888b53174d8cc1d48ba0b4552a9c5-e7aff5a48cec001f/tmp/clang_command_.tmp.d56d74.txt
++Contents of '/tmp/coderabbit-infer/38a2a3f3458888b53174d8cc1d48ba0b4552a9c5-e7aff5a48cec001f/tmp/clang_command_.tmp.d56d74.txt':
"-cc1" "-load"
"/opt/infer-linux-x86_64-v1.2.0/lib/infer/infer/bin/../../facebook-clang-plugins/libtooling/build/FacebookClangPlugin.dylib"
"-add-plugin" "BiniouASTExporter" "-plugin-arg-BiniouASTExporter" "-"
"-plugin-arg-BiniouASTExporter" "PREPEND_CURRENT_DIR=1"
"-plugin-arg-BiniouASTExporter" "MAX_STRING_SIZE=65535" "-cc1" "-triple"
"x86_64-unknown-linux-gnu" "-emit-obj" "-mrelax-all" "-disable-free

... [truncated 1045 characters] ...

plugins/clang/install/lib/clang/18/include"
"-internal-isystem" "/usr/local/include" "-internal-isystem"
"/usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include"
"-internal-externc-isystem" "/usr/include/x86_64-linux-gnu"
"-internal-externc-isystem" "/include" "-internal-externc-isystem"
"/usr/include" "-Wno-ignored-optimization-argument" "-Wno-everything"
"-fdeprecated-macro" "-ferror-limit" "19" "-fgnuc-version=4.2.1"
"-fskip-odr-check-in-gmf" "-fcxx-exceptions" "-fexceptions"
"-D__GCC_HAVE_DWARF2_CFI_ASM=1" "-o"
"/tmp/coderabbit-infer/e7aff5a48cec001f/file.o" "-x" "c++"
"src/signer.cc" "-O0" "-fno-builtin" "-include"
"/opt/infer-linux-x86_64-v1.2.0/lib/infer/infer/bin/../lib/clang_wrappers/global_defines.h"
"-Wno-everything"

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/signer.cc`:
- Around line 93-97: The loop converting HMAC bytes to hex uses
std::to_chars(..., hash[i], 16) which can treat char as signed and produce
negative values; fix by casting each byte to an unsigned type before conversion
(e.g., convert hash[i] to unsigned char or uint8_t and then to an integer type)
so std::to_chars sees 0..255 values. Update the call site around std::to_chars
and the hash[i] argument in signer.cc (the to_chars invocation and the
surrounding byte-to-hex logic) to use the unsigned-cast value, leaving the rest
of the hex-padding logic unchanged.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 02f84aef-c08c-41ea-b889-6d901a8a5b20

📥 Commits

Reviewing files that changed from the base of the PR and between f35873b and 1e53dbd.

📒 Files selected for processing (2)

• src/signer.cc
• src/utils.cc