From bac53951b827ac71abfe6a213be9d1e1a89d50aa Mon Sep 17 00:00:00 2001
From: Vladislav Shchapov <vladislav@shchapov.ru>
Date: Sun, 4 Sep 2022 23:41:16 +0500
Subject: [PATCH] Add starts_with to basic_string_view. (#3080)

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
---
 include/fmt/core.h | 12 ++++++++++++
 test/core-test.cc  | 10 ++++++++++
 2 files changed, 22 insertions(+)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index a4b200d7..bc87b2f5 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -486,6 +486,18 @@ template <typename Char> class basic_string_view {
     size_ -= n;
   }
 
+  FMT_CONSTEXPR_CHAR_TRAITS bool starts_with(
+      basic_string_view<Char> sv) const noexcept {
+    return size_ >= sv.size_ &&
+           std::char_traits<Char>::compare(data_, sv.data_, sv.size_) == 0;
+  }
+  FMT_CONSTEXPR_CHAR_TRAITS bool starts_with(Char c) const noexcept {
+    return size_ >= 1 && std::char_traits<Char>::eq(*data_, c);
+  }
+  FMT_CONSTEXPR_CHAR_TRAITS bool starts_with(const Char* s) const {
+    return starts_with(basic_string_view<Char>(s));
+  }
+
   // Lexicographically compare this string reference to other.
   FMT_CONSTEXPR_CHAR_TRAITS auto compare(basic_string_view other) const -> int {
     size_t str_size = size_ < other.size_ ? size_ : other.size_;
diff --git a/test/core-test.cc b/test/core-test.cc
index 96cde1ed..6d1309fe 100644
--- a/test/core-test.cc
+++ b/test/core-test.cc
@@ -70,6 +70,16 @@ TEST(string_view_test, compare) {
   EXPECT_LT(string_view("foo").compare(string_view("fop")), 0);
   EXPECT_GT(string_view("foo").compare(string_view("fo")), 0);
   EXPECT_LT(string_view("fo").compare(string_view("foo")), 0);
+
+  EXPECT_TRUE(string_view("foo").starts_with('f'));
+  EXPECT_FALSE(string_view("foo").starts_with('o'));
+  EXPECT_FALSE(string_view().starts_with('o'));
+
+  EXPECT_TRUE(string_view("foo").starts_with("fo"));
+  EXPECT_TRUE(string_view("foo").starts_with("foo"));
+  EXPECT_FALSE(string_view("foo").starts_with("fooo"));
+  EXPECT_FALSE(string_view().starts_with("fooo"));
+
   check_op<std::equal_to>();
   check_op<std::not_equal_to>();
   check_op<std::less>();