From aae1edda0dca57b1be244ac4092f477e68f2641f Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Fri, 27 Mar 2026 15:48:06 -1000
Subject: [PATCH 1/3] [sensor] Replace truncation guard with
 ESPHOME_DEBUG_ASSERT

---
 esphome/components/sensor/filter.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/esphome/components/sensor/filter.h b/esphome/components/sensor/filter.h
index 0dc7fc85dd..c6609bd1fb 100644
--- a/esphome/components/sensor/filter.h
+++ b/esphome/components/sensor/filter.h
@@ -508,10 +508,9 @@ template<size_t N> class CalibratePolynomialFilter : public Filter {
   explicit CalibratePolynomialFilter(std::initializer_list<float> coefficients) {
     size_t i = 0;
     for (float c : coefficients) {
-      if (i >= N)
-        break;
       this->coefficients_[i++] = c;
     }
+    ESPHOME_DEBUG_ASSERT(i == N);
   }
   optional<float> new_value(float value) override {
     return calibrate_polynomial_compute(this->coefficients_.data(), N, value);

From 55963a6ea05373c0a28caa7752bdc3780968b6b0 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Fri, 27 Mar 2026 15:53:18 -1000
Subject: [PATCH 2/3] [sensor] Assert size before writing, not after

---
 esphome/components/sensor/filter.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/esphome/components/sensor/filter.h b/esphome/components/sensor/filter.h
index c6609bd1fb..a3bd7b6b51 100644
--- a/esphome/components/sensor/filter.h
+++ b/esphome/components/sensor/filter.h
@@ -506,11 +506,11 @@ optional<float> calibrate_polynomial_compute(const float *coefficients, size_t c
 template<size_t N> class CalibratePolynomialFilter : public Filter {
  public:
   explicit CalibratePolynomialFilter(std::initializer_list<float> coefficients) {
+    ESPHOME_DEBUG_ASSERT(coefficients.size() == N);
     size_t i = 0;
     for (float c : coefficients) {
       this->coefficients_[i++] = c;
     }
-    ESPHOME_DEBUG_ASSERT(i == N);
   }
   optional<float> new_value(float value) override {
     return calibrate_polynomial_compute(this->coefficients_.data(), N, value);

From ae2a82edd64cf3f923c372060c3929a7684999da Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Fri, 27 Mar 2026 16:14:51 -1000
Subject: [PATCH 3/3] Use init_array_from helper for optimal codegen

Uses memcpy for trivially copyable types, element-wise copy otherwise.
ESPHOME_DEBUG_ASSERT catches size mismatches in integration tests.
---
 esphome/components/sensor/filter.h |  6 +-----
 esphome/core/helpers.h             | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/esphome/components/sensor/filter.h b/esphome/components/sensor/filter.h
index a3bd7b6b51..be63f99fae 100644
--- a/esphome/components/sensor/filter.h
+++ b/esphome/components/sensor/filter.h
@@ -506,11 +506,7 @@ optional<float> calibrate_polynomial_compute(const float *coefficients, size_t c
 template<size_t N> class CalibratePolynomialFilter : public Filter {
  public:
   explicit CalibratePolynomialFilter(std::initializer_list<float> coefficients) {
-    ESPHOME_DEBUG_ASSERT(coefficients.size() == N);
-    size_t i = 0;
-    for (float c : coefficients) {
-      this->coefficients_[i++] = c;
-    }
+    init_array_from(this->coefficients_, coefficients);
   }
   optional<float> new_value(float value) override {
     return calibrate_polynomial_compute(this->coefficients_.data(), N, value);
diff --git a/esphome/core/helpers.h b/esphome/core/helpers.h
index 82c6b3833c..51feaa57c5 100644
--- a/esphome/core/helpers.h
+++ b/esphome/core/helpers.h
@@ -497,6 +497,21 @@ template<typename T, size_t MAX_CAPACITY = std::numeric_limits<uint16_t>::max()>
   index_type capacity_{0};
 };
 
+/// Initialize a std::array from an initializer_list. Uses memcpy for trivially copyable types (optimal codegen),
+/// falls back to element-wise copy for non-trivially copyable types (e.g. TemplatableValue).
+/// N is set by code generation; ESPHOME_DEBUG_ASSERT catches mismatches in debug/integration tests.
+template<typename T, size_t N> inline void init_array_from(std::array<T, N> &dest, std::initializer_list<T> src) {
+  ESPHOME_DEBUG_ASSERT(src.size() == N);
+  if constexpr (std::is_trivially_copyable_v<T>) {
+    __builtin_memcpy(dest.data(), src.begin(), N * sizeof(T));
+  } else {
+    size_t i = 0;
+    for (const auto &v : src) {
+      dest[i++] = v;
+    }
+  }
+}
+
 /// Fixed-capacity vector - allocates once at runtime, never reallocates
 /// This avoids std::vector template overhead (_M_realloc_insert, _M_default_append)
 /// when size is known at initialization but not at compile time