refactor(telemetry): use ranges algorithms in ValidationTracker, mark accessors nodiscard

Nine std::count_if calls over window1h_ and window7d_ took an iterator
pair; std::ranges::count_if takes the container. One hand-rolled
erase-while-iterating loop becomes a single std::erase_if: pending_ is a
hash_map, which is a std::unordered_map alias, so the C++20 overload applies.

The hard-trim loop below it is left as a loop on purpose. It re-tests
pending_.size() every step to stop as soon as the cap is met, which erase_if
cannot express.

The accessors return values a caller must use: 13 in ValidationTracker.h, the
two Unit.h mappers, the two HistogramBuckets.h helpers, getMeter() and
networkTypeFromId(). No caller in the chain discards any of them.

include/xrpl/beast/insight/Unit.h

@@ -53,7 +53,7 @@ enum class Unit : std::uint8_t {
  * @param unit The unit to translate.
  * @return A static, null-terminated UCUM code.
  */
-constexpr char const*
+[[nodiscard]] constexpr char const*
 otelUnitCode(Unit unit) noexcept
 {
     switch (unit)
@@ -78,7 +78,7 @@ otelUnitCode(Unit unit) noexcept
  * @param unit The unit to describe.
  * @return A static, null-terminated description.
  */
-constexpr char const*
+[[nodiscard]] constexpr char const*
 otelUnitDescription(Unit unit) noexcept
 {
     switch (unit)

include/xrpl/telemetry/HistogramBuckets.h

@@ -144,7 +144,7 @@ inline constexpr std::array kByteBuckets{
  * @return true when the ladder is non-empty, starts at or above zero, and
  *         every later edge is strictly greater than its predecessor.
  */
-constexpr bool
+[[nodiscard]] constexpr bool
 isAscendingNonNegative(std::span<double const> ladder) noexcept
 {
     if (ladder.empty() || ladder.front() < 0.0)
@@ -167,7 +167,7 @@ static_assert(isAscendingNonNegative(kByteBuckets));
  * @param ladder Bucket upper bounds.
  * @return A vector holding the same edges in the same order.
  */
-inline std::vector<double>
+[[nodiscard]] inline std::vector<double>
 toVector(std::span<double const> ladder)
 {
     return std::vector<double>(ladder.begin(), ladder.end());

include/xrpl/telemetry/Telemetry.h

@@ -393,7 +393,7 @@ public:
      * @param name  Meter name used to identify the instrumentation scope.
      * @return A shared pointer to the Meter.
      */
-    virtual opentelemetry::nostd::shared_ptr<opentelemetry::metrics::Meter>
+    [[nodiscard]] virtual opentelemetry::nostd::shared_ptr<opentelemetry::metrics::Meter>
     getMeter(std::string_view name = kMeterName) = 0;
 
     /**
@@ -486,7 +486,7 @@ makeTelemetrySetup(
  * @param networkId  The network identifier from [network_id] config.
  * @return "mainnet" (0), "testnet" (1), "devnet" (2), or "unknown".
  */
-std::string
+[[nodiscard]] std::string
 networkTypeFromId(std::uint32_t networkId);
 
 }  // namespace xrpl::telemetry

src/xrpld/telemetry/ValidationTracker.h

@@ -138,21 +138,21 @@ public:
      * Agreement percentage over the last 1 hour.
      * @return Percentage [0.0, 100.0], or 0.0 if no data.
      */
-    double
+    [[nodiscard]] double
     agreementPct1h() const;
 
     /**
      * Agreement percentage over the last 24 hours.
      * @return Percentage [0.0, 100.0], or 0.0 if no data.
      */
-    double
+    [[nodiscard]] double
     agreementPct24h() const;
 
     /**
      * Agreement percentage over the last 7 days.
      * @return Percentage [0.0, 100.0], or 0.0 if no data.
      */
-    double
+    [[nodiscard]] double
     agreementPct7d() const;
 
     /** @} */
@@ -165,37 +165,37 @@ public:
     /**
      * Number of agreements in the 1-hour window.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     agreements1h() const;
 
     /**
      * Number of misses in the 1-hour window.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     missed1h() const;
 
     /**
      * Number of agreements in the 24-hour window.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     agreements24h() const;
 
     /**
      * Number of misses in the 24-hour window.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     missed24h() const;
 
     /**
      * Number of agreements in the 7-day window.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     agreements7d() const;
 
     /**
      * Number of misses in the 7-day window.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     missed7d() const;
 
     /** @} */
@@ -208,25 +208,25 @@ public:
     /**
      * Total agreements since process start.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     totalAgreements() const;
 
     /**
      * Total misses since process start.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     totalMissed() const;
 
     /**
      * Total validations this node sent.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     totalValidationsSent() const;
 
     /**
      * Total network validations observed for comparison.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     totalValidationsChecked() const;
 
     /** @} */

src/xrpld/telemetry/detail/ValidationTracker.cpp

@@ -116,19 +116,11 @@ void
 ValidationTracker::evictOldPending(TimePoint now)
 {
     auto const cutoff = now - kLateRepairWindow;
-    for (auto it = pending_.begin(); it != pending_.end();)
-    {
-        if (it->second.reconciled && it->second.recordTime < cutoff)
-        {
-            it = pending_.erase(it);
-        }
-        else
-        {
-            ++it;
-        }
-    }
+    std::erase_if(pending_, [cutoff](auto const& entry) {
+        return entry.second.reconciled && entry.second.recordTime < cutoff;
+    });
 
-    // Hard trim if still over limit. The loop above already removed every
+    // Hard trim if still over limit. The pass above already removed every
     // reconciled entry older than the late-repair window, so here we drop
     // any remaining reconciled entry as a last resort.
     if (pending_.size() > kMaxPendingEvents)
@@ -155,7 +147,7 @@ ValidationTracker::agreementPct1h() const
     if (window1h_.empty())
         return 0.0;
     auto const agreed = static_cast<double>(
-        std::count_if(window1h_.begin(), window1h_.end(), [](auto const& e) { return e.agreed; }));
+        std::ranges::count_if(window1h_, [](auto const& e) { return e.agreed; }));
     return (agreed / static_cast<double>(window1h_.size())) * 100.0;
 }
 
@@ -165,8 +157,8 @@ ValidationTracker::agreementPct24h() const
     std::scoped_lock const lock(mutex_);
     if (window24h_.empty())
         return 0.0;
-    auto const agreed = static_cast<double>(std::count_if(
-        window24h_.begin(), window24h_.end(), [](auto const& e) { return e.agreed; }));
+    auto const agreed = static_cast<double>(
+        std::ranges::count_if(window24h_, [](auto const& e) { return e.agreed; }));
     return (agreed / static_cast<double>(window24h_.size())) * 100.0;
 }
 
@@ -175,7 +167,7 @@ ValidationTracker::agreements1h() const
 {
     std::scoped_lock const lock(mutex_);
     return static_cast<uint64_t>(
-        std::count_if(window1h_.begin(), window1h_.end(), [](auto const& e) { return e.agreed; }));
+        std::ranges::count_if(window1h_, [](auto const& e) { return e.agreed; }));
 }
 
 uint64_t
@@ -183,23 +175,23 @@ ValidationTracker::missed1h() const
 {
     std::scoped_lock const lock(mutex_);
     return static_cast<uint64_t>(
-        std::count_if(window1h_.begin(), window1h_.end(), [](auto const& e) { return !e.agreed; }));
+        std::ranges::count_if(window1h_, [](auto const& e) { return !e.agreed; }));
 }
 
 uint64_t
 ValidationTracker::agreements24h() const
 {
     std::scoped_lock const lock(mutex_);
-    return static_cast<uint64_t>(std::count_if(
-        window24h_.begin(), window24h_.end(), [](auto const& e) { return e.agreed; }));
+    return static_cast<uint64_t>(
+        std::ranges::count_if(window24h_, [](auto const& e) { return e.agreed; }));
 }
 
 uint64_t
 ValidationTracker::missed24h() const
 {
     std::scoped_lock const lock(mutex_);
-    return static_cast<uint64_t>(std::count_if(
-        window24h_.begin(), window24h_.end(), [](auto const& e) { return !e.agreed; }));
+    return static_cast<uint64_t>(
+        std::ranges::count_if(window24h_, [](auto const& e) { return !e.agreed; }));
 }
 
 double
@@ -209,7 +201,7 @@ ValidationTracker::agreementPct7d() const
     if (window7d_.empty())
         return 0.0;
     auto const agreed = static_cast<double>(
-        std::count_if(window7d_.begin(), window7d_.end(), [](auto const& e) { return e.agreed; }));
+        std::ranges::count_if(window7d_, [](auto const& e) { return e.agreed; }));
     return (agreed / static_cast<double>(window7d_.size())) * 100.0;
 }
 
@@ -218,7 +210,7 @@ ValidationTracker::agreements7d() const
 {
     std::scoped_lock const lock(mutex_);
     return static_cast<uint64_t>(
-        std::count_if(window7d_.begin(), window7d_.end(), [](auto const& e) { return e.agreed; }));
+        std::ranges::count_if(window7d_, [](auto const& e) { return e.agreed; }));
 }
 
 uint64_t
@@ -226,7 +218,7 @@ ValidationTracker::missed7d() const
 {
     std::scoped_lock const lock(mutex_);
     return static_cast<uint64_t>(
-        std::count_if(window7d_.begin(), window7d_.end(), [](auto const& e) { return !e.agreed; }));
+        std::ranges::count_if(window7d_, [](auto const& e) { return !e.agreed; }));
 }
 
 uint64_t