[libc][FixedVector] Add more helper methods (#94278)

This adds:
- A ctor accepting a start and end iterator
- A ctor accepting a count and const T&
- size()
- subscript operators
- begin() and end() iterators

Author: PiJoules

libc/src/__support/fixedvector.h

@@ -24,6 +24,17 @@ template <typename T, size_t CAPACITY> class FixedVector {
 public:
   constexpr FixedVector() = default;
 
+  using iterator = typename cpp::array<T, CAPACITY>::iterator;
+  constexpr FixedVector(iterator begin, iterator end) {
+    for (; begin != end; ++begin)
+      push_back(*begin);
+  }
+
+  constexpr FixedVector(size_t count, const T &value) {
+    for (size_t i = 0; i < count; ++i)
+      push_back(value);
+  }
+
   bool push_back(const T &obj) {
     if (item_count == CAPACITY)
       return false;
@@ -43,8 +54,14 @@ template <typename T, size_t CAPACITY> class FixedVector {
     return true;
   }
 
+  T &operator[](size_t idx) { return store[idx]; }
+
+  const T &operator[](size_t idx) const { return store[idx]; }
+
   bool empty() const { return item_count == 0; }
 
+  size_t size() const { return item_count; }
+
   // Empties the store for all practical purposes.
   void reset() { item_count = 0; }
 
@@ -64,7 +81,6 @@ template <typename T, size_t CAPACITY> class FixedVector {
   }
   LIBC_INLINE constexpr reverse_iterator rend() { return store.rend(); }
 
-  using iterator = typename cpp::array<T, CAPACITY>::iterator;
   LIBC_INLINE constexpr iterator begin() { return store.begin(); }
   LIBC_INLINE constexpr iterator end() { return iterator{&store[item_count]}; }
 };

libc/test/src/__support/CMakeLists.txt

@@ -132,6 +132,7 @@ add_libc_test(
   SRCS
     fixedvector_test.cpp
   DEPENDS
+    libc.src.__support.CPP.array
     libc.src.__support.fixedvector
 )
 

libc/test/src/__support/fixedvector_test.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/__support/CPP/array.h"
 #include "src/__support/fixedvector.h"
 #include "test/UnitTest/Test.h"
 
@@ -69,3 +70,29 @@ TEST(LlvmLibcFixedVectorTest, Iteration) {
   for (int &x : v)
     ASSERT_GE(x, 0);
 }
+
+TEST(LlvmLibcFixedVectorTest, ConstructionFromIterators) {
+  LIBC_NAMESPACE::cpp::array<int, 4> arr{1, 2, 3, 4};
+  LIBC_NAMESPACE::FixedVector<int, 5> vec(arr.begin(), arr.end());
+  ASSERT_EQ(vec.size(), arr.size());
+  for (size_t i = 0; i < arr.size(); ++i)
+    ASSERT_EQ(vec[i], arr[i]);
+}
+
+TEST(LlvmLibcFixedVectorTest, ConstructionFromCountAndValue) {
+  constexpr int kVal = 10;
+  LIBC_NAMESPACE::FixedVector<int, 5> vec(4, kVal);
+  ASSERT_EQ(vec.size(), size_t(4));
+  for (size_t i = 0; i < vec.size(); ++i)
+    ASSERT_EQ(vec[i], kVal);
+}
+
+TEST(LlvmLibcFixedVectorTest, ForwardIteration) {
+  LIBC_NAMESPACE::cpp::array<int, 4> arr{1, 2, 3, 4};
+  LIBC_NAMESPACE::FixedVector<int, 5> vec(arr.begin(), arr.end());
+  ASSERT_EQ(vec.size(), arr.size());
+  for (auto it = vec.begin(); it != vec.end(); ++it) {
+    auto idx = it - vec.begin();
+    ASSERT_EQ(*it, arr[idx]);
+  }
+}