[core] Add StringRef::starts_with (#18142)

esphome/core/string_ref.h

@@ -76,6 +76,13 @@ class StringRef {
   constexpr bool empty() const { return len_ == 0; }
   constexpr const_reference operator[](size_type pos) const { return *(base_ + pos); }
 
+  /// True if the view begins with the given prefix (std::string::starts_with-like)
+  bool starts_with(const StringRef &prefix) const {
+    return len_ >= prefix.len_ && std::memcmp(base_, prefix.base_, prefix.len_) == 0;
+  }
+  bool starts_with(const char *prefix) const { return this->starts_with(StringRef(prefix)); }
+  bool starts_with(const std::string &prefix) const { return this->starts_with(StringRef(prefix)); }
+
   /// Copy characters to destination buffer (std::string::copy-like, but returns 0 instead of throwing on out-of-range)
   size_type copy(char *dest, size_type count, size_type pos = 0) const {
     if (pos >= len_)

tests/components/core/test_string_ref.cpp

@@ -0,0 +1,62 @@
+#include <gtest/gtest.h>
+
+#include "esphome/core/string_ref.h"
+
+namespace esphome::core::testing {
+
+TEST(StringRefStartsWith, ProperPrefixMatches) {
+  StringRef ref("FR:R20:12345", 12);
+  EXPECT_TRUE(ref.starts_with("FR:"));
+}
+
+TEST(StringRefStartsWith, WholeStringIsAPrefixOfItself) {
+  StringRef ref("TP96", 4);
+  EXPECT_TRUE(ref.starts_with("TP96"));
+}
+
+TEST(StringRefStartsWith, PrefixLongerThanViewFails) {
+  StringRef ref("TP", 2);
+  EXPECT_FALSE(ref.starts_with("TP96"));
+}
+
+TEST(StringRefStartsWith, DifferentContentFails) {
+  StringRef ref("TP96", 4);
+  EXPECT_FALSE(ref.starts_with("FR:"));
+}
+
+TEST(StringRefStartsWith, EmptyPrefixAlwaysMatches) {
+  StringRef ref("abc", 3);
+  EXPECT_TRUE(ref.starts_with(""));
+  StringRef empty;
+  EXPECT_TRUE(empty.starts_with(""));
+}
+
+TEST(StringRefStartsWith, EmptyViewOnlyMatchesEmptyPrefix) {
+  StringRef empty;
+  EXPECT_FALSE(empty.starts_with("a"));
+}
+
+TEST(StringRefStartsWith, WorksOnANonTerminatedBuffer) {
+  // The reason the helper exists: a bounded view over a buffer with no
+  // terminator anywhere near the viewed bytes.
+  const char raw[] = {'R', 'a', 'd', 'o', 'n', 'X'};
+  StringRef ref(raw, 5);
+  EXPECT_TRUE(ref.starts_with("Radon"));
+  EXPECT_FALSE(ref.starts_with("RadonEye"));
+  EXPECT_FALSE(ref.starts_with("adon"));
+}
+
+TEST(StringRefStartsWith, StdStringOverload) {
+  StringRef ref("TP96", 4);
+  EXPECT_TRUE(ref.starts_with(std::string("TP")));
+  EXPECT_FALSE(ref.starts_with(std::string("96")));
+}
+
+TEST(StringRefStartsWith, RefOverloadComparesOnlyTheViewedLength) {
+  // The prefix is a bounded view: bytes past its length must not be compared.
+  StringRef ref("FR:123", 6);
+  StringRef prefix("FR:xyz", 3);
+  EXPECT_TRUE(ref.starts_with(prefix));
+}
+
+}  // namespace esphome::core::testing