From 52dd79691b82e32fd9e61ade165c551109de7af8 Mon Sep 17 00:00:00 2001
From: Maurice Makaay <account+github@makaay.nl>
Date: Fri, 24 Sep 2021 14:15:22 +0200
Subject: [PATCH] Read unencrypted DSMR telegrams in chunks (#2382)

Co-authored-by: Maurice Makaay <mmakaay1@xs4all.net>
---
 esphome/components/dsmr/dsmr.cpp | 31 +++++++++++++++++++------------
 esphome/components/dsmr/dsmr.h   |  1 +
 2 files changed, 20 insertions(+), 12 deletions(-)

diff --git a/esphome/components/dsmr/dsmr.cpp b/esphome/components/dsmr/dsmr.cpp
index 54c4343cfe..b798fe5d44 100644
--- a/esphome/components/dsmr/dsmr.cpp
+++ b/esphome/components/dsmr/dsmr.cpp
@@ -20,19 +20,22 @@ void Dsmr::loop() {
 }
 
 void Dsmr::receive_telegram_() {
-  while (available()) {
+  int count = MAX_BYTES_PER_LOOP;
+  while (available() && count-- > 0) {
     const char c = read();
 
-    if (c == '/') {  // header: forward slash
+    // Find a new telegram header, i.e. forward slash.
+    if (c == '/') {
       ESP_LOGV(TAG, "Header found");
       header_found_ = true;
       footer_found_ = false;
       telegram_len_ = 0;
     }
-
     if (!header_found_)
       continue;
-    if (telegram_len_ >= MAX_TELEGRAM_LENGTH) {  // Buffer overflow
+
+    // Check for buffer overflow.
+    if (telegram_len_ >= MAX_TELEGRAM_LENGTH) {
       header_found_ = false;
       footer_found_ = false;
       ESP_LOGE(TAG, "Error: Message larger than buffer");
@@ -45,18 +48,22 @@ void Dsmr::receive_telegram_() {
     while (c == '(' && (telegram_[telegram_len_ - 1] == '\n' || telegram_[telegram_len_ - 1] == '\r'))
       telegram_len_--;
 
+    // Store the byte in the buffer.
     telegram_[telegram_len_] = c;
     telegram_len_++;
-    if (c == '!') {  // footer: exclamation mark
+
+    // Check for a footer, i.e. exlamation mark, followed by a hex checksum.
+    if (c == '!') {
       ESP_LOGV(TAG, "Footer found");
       footer_found_ = true;
-    } else {
-      if (footer_found_ && c == 10) {  // last \n after footer
-        header_found_ = false;
-        // Parse message
-        if (parse_telegram())
-          return;
-      }
+      continue;
+    }
+    // Check for the end of the hex checksum, i.e. a newline.
+    if (footer_found_ && c == '\n') {
+      header_found_ = false;
+      // Parse the telegram and publish sensor values.
+      if (parse_telegram())
+        return;
     }
   }
 }
diff --git a/esphome/components/dsmr/dsmr.h b/esphome/components/dsmr/dsmr.h
index dfee3b338a..4f9a66b3d0 100644
--- a/esphome/components/dsmr/dsmr.h
+++ b/esphome/components/dsmr/dsmr.h
@@ -17,6 +17,7 @@ namespace esphome {
 namespace dsmr {
 
 static constexpr uint32_t MAX_TELEGRAM_LENGTH = 1500;
+static constexpr uint32_t MAX_BYTES_PER_LOOP = 50;
 static constexpr uint32_t POLL_TIMEOUT = 1000;
 
 using namespace ::dsmr::fields;