[api] Optimize varint encoding to eliminate uint64_t widening

Replace ProtoVarInt temporaries with direct uint32_t encoding functions
to avoid unnecessary 64-bit widening on 32-bit architectures (ESP32 Xtensa).

- Add encode_varint_to_buffer() free function for direct buffer encoding
- Replace encode_varint_raw(ProtoVarInt) with uint32_t-native implementation
- Add encode_varint_raw_64() for the few call sites needing 64-bit (BLE)
- Remove unused ProtoVarInt::encode() and encode_to_buffer_unchecked()
- Remove dead null checks from ProtoVarInt::parse()

Net savings: -152 bytes flash on ESP32 IDF.

esphome/components/api/api_frame_helper_plaintext.cpp

@@ -295,9 +295,8 @@ APIError APIPlaintextFrameHelper::write_protobuf_messages(ProtoWriteBuffer buffe
     buf_start[header_offset] = 0x00;  // indicator
 
     // Encode varints directly into buffer
-    ProtoVarInt(msg.payload_size).encode_to_buffer_unchecked(buf_start + header_offset + 1, size_varint_len);
-    ProtoVarInt(msg.message_type)
-        .encode_to_buffer_unchecked(buf_start + header_offset + 1 + size_varint_len, type_varint_len);
+    encode_varint_to_buffer(msg.payload_size, buf_start + header_offset + 1);
+    encode_varint_to_buffer(msg.message_type, buf_start + header_offset + 1 + size_varint_len);
 
     // Add iovec for this message (header + payload)
     size_t msg_len = static_cast<size_t>(total_header_len + msg.payload_size);

esphome/components/api/proto.h

@@ -57,6 +57,16 @@ inline uint16_t count_packed_varints(const uint8_t *data, size_t len) {
   return count;
 }
 
+/// Encode a varint directly into a pre-allocated buffer.
+/// Caller must ensure buffer has space (use ProtoSize::varint() to calculate).
+inline void encode_varint_to_buffer(uint32_t val, uint8_t *buffer) {
+  while (val > 0x7F) {
+    *buffer++ = static_cast<uint8_t>(val | 0x80);
+    val >>= 7;
+  }
+  *buffer = static_cast<uint8_t>(val);
+}
+
 /*
  * StringRef Ownership Model for API Protocol Messages
  * ===================================================
@@ -93,17 +103,14 @@ class ProtoVarInt {
   ProtoVarInt() : value_(0) {}
   explicit ProtoVarInt(uint64_t value) : value_(value) {}
 
+  /// Parse a varint from buffer. consumed must be a valid pointer (not null).
   static optional<ProtoVarInt> parse(const uint8_t *buffer, uint32_t len, uint32_t *consumed) {
-    if (len == 0) {
-      if (consumed != nullptr)
-        *consumed = 0;
+    if (len == 0)
       return {};
-    }
 
     // Most common case: single-byte varint (values 0-127)
     if ((buffer[0] & 0x80) == 0) {
-      if (consumed != nullptr)
-        *consumed = 1;
+      *consumed = 1;
       return ProtoVarInt(buffer[0]);
     }
 
@@ -122,14 +129,11 @@ class ProtoVarInt {
       result |= uint64_t(val & 0x7F) << uint64_t(bitpos);
       bitpos += 7;
       if ((val & 0x80) == 0) {
-        if (consumed != nullptr)
-          *consumed = i + 1;
+        *consumed = i + 1;
         return ProtoVarInt(result);
       }
     }
 
-    if (consumed != nullptr)
-      *consumed = 0;
     return {};  // Incomplete or invalid varint
   }
 
@@ -153,50 +157,6 @@ class ProtoVarInt {
     // with ZigZag encoding
     return decode_zigzag64(this->value_);
   }
-  /**
-   * Encode the varint value to a pre-allocated buffer without bounds checking.
-   *
-   * @param buffer The pre-allocated buffer to write the encoded varint to
-   * @param len The size of the buffer in bytes
-   *
-   * @note The caller is responsible for ensuring the buffer is large enough
-   *       to hold the encoded value. Use ProtoSize::varint() to calculate
-   *       the exact size needed before calling this method.
-   * @note No bounds checking is performed for performance reasons.
-   */
-  void encode_to_buffer_unchecked(uint8_t *buffer, size_t len) {
-    uint64_t val = this->value_;
-    if (val <= 0x7F) {
-      buffer[0] = val;
-      return;
-    }
-    size_t i = 0;
-    while (val && i < len) {
-      uint8_t temp = val & 0x7F;
-      val >>= 7;
-      if (val) {
-        buffer[i++] = temp | 0x80;
-      } else {
-        buffer[i++] = temp;
-      }
-    }
-  }
-  void encode(std::vector<uint8_t> &out) {
-    uint64_t val = this->value_;
-    if (val <= 0x7F) {
-      out.push_back(val);
-      return;
-    }
-    while (val) {
-      uint8_t temp = val & 0x7F;
-      val >>= 7;
-      if (val) {
-        out.push_back(temp | 0x80);
-      } else {
-        out.push_back(temp);
-      }
-    }
-  }
 
  protected:
   uint64_t value_;
@@ -256,8 +216,24 @@ class ProtoWriteBuffer {
  public:
   ProtoWriteBuffer(std::vector<uint8_t> *buffer) : buffer_(buffer) {}
   void write(uint8_t value) { this->buffer_->push_back(value); }
-  void encode_varint_raw(ProtoVarInt value) { value.encode(*this->buffer_); }
-  void encode_varint_raw(uint32_t value) { this->encode_varint_raw(ProtoVarInt(value)); }
+  void encode_varint_raw(uint32_t value) {
+    if (value <= 0x7F) {
+      this->buffer_->push_back(static_cast<uint8_t>(value));
+      return;
+    }
+    while (value) {
+      uint8_t temp = value & 0x7F;
+      value >>= 7;
+      this->buffer_->push_back(value ? (temp | 0x80) : temp);
+    }
+  }
+  void encode_varint_raw_64(uint64_t value) {
+    while (value > 0x7F) {
+      this->buffer_->push_back(static_cast<uint8_t>(value | 0x80));
+      value >>= 7;
+    }
+    this->buffer_->push_back(static_cast<uint8_t>(value));
+  }
   /**
    * Encode a field key (tag/wire type combination).
    *
@@ -307,7 +283,7 @@ class ProtoWriteBuffer {
     if (value == 0 && !force)
       return;
     this->encode_field_raw(field_id, 0);  // type 0: Varint - uint64
-    this->encode_varint_raw(ProtoVarInt(value));
+    this->encode_varint_raw_64(value);
   }
   void encode_bool(uint32_t field_id, bool value, bool force = false) {
     if (!value && !force)
@@ -938,7 +914,7 @@ inline void ProtoWriteBuffer::encode_message(uint32_t field_id, const ProtoMessa
   this->buffer_->resize(this->buffer_->size() + varint_length_bytes);
 
   // Write the length varint directly
-  ProtoVarInt(msg_length_bytes).encode_to_buffer_unchecked(this->buffer_->data() + begin, varint_length_bytes);
+  encode_varint_to_buffer(msg_length_bytes, this->buffer_->data() + begin);
 
   // Now encode the message content - it will append to the buffer
   value.encode(*this);