1 files changed, 139 insertions, 0 deletions
diff --git a/src/utils/cpp/incremental_reader.cpp b/src/utils/cpp/incremental_reader.cpp
new file mode 100644
index 00000000..c1b5bc3a
--- /dev/null
+++ b/src/utils/cpp/incremental_reader.cpp
@@ -0,0 +1,139 @@
+// Copyright 2025 Huawei Cloud Computing Technology Co., Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/utils/cpp/incremental_reader.hpp"
+
+#include <cstdint>
+#include <exception>
+#include <string_view>
+
+#include "fmt/core.h"
+#include "src/utils/cpp/in_place_visitor.hpp"
+
+namespace {
+void DisposeFile(gsl::owner<std::FILE*> file) noexcept {
+    if (file == nullptr) {
+        return;
+    }
+    std::fclose(file);
+}
+}  // namespace
+
+auto IncrementalReader::FromFile(std::size_t chunk_size,
+                                 std::filesystem::path const& path) noexcept
+    -> expected<IncrementalReader, std::string> {
+    if (chunk_size == 0) {
+        return unexpected<std::string>{
+            "IncrementalReader: the chunk size cannot be 0"};
+    }
+
+    try {
+        // Ensure this is a file:
+        if (not std::filesystem::is_regular_file(path)) {
+            return unexpected{fmt::format(
+                "IncrementalReader: not a file :\n {} ", path.string())};
+        }
+
+        // Open file for reading:
+        static constexpr std::string_view kReadBinary = "rb";
+        auto file = std::shared_ptr<std::FILE>{
+            std::fopen(path.c_str(), kReadBinary.data()), ::DisposeFile};
+        if (file == nullptr) {
+            return unexpected{
+                fmt::format("IncrementalReader: failed to open the file:\n{}",
+                            path.string())};
+        }
+
+        std::size_t const content_size = std::filesystem::file_size(path);
+        return IncrementalReader{chunk_size,
+                                 content_size,
+                                 std::move(file),
+                                 /*buffer=*/std::string(chunk_size, '\0')};
+    } catch (std::exception const& e) {
+        return unexpected{fmt::format(
+            "IncrementalReader: While processing {}\ngot an exception: {}",
+            path.string(),
+            e.what())};
+    } catch (...) {
+        return unexpected{fmt::format(
+            "IncrementalReader: While processing {}\ngot an unknown exception",
+            path.string())};
+    }
+}
+
+auto IncrementalReader::ReadChunk(std::size_t offset) const noexcept
+    -> expected<std::string_view, std::string> {
+    using Result = expected<std::string_view, std::string>;
+    InPlaceVisitor const visitor{
+        [this, offset](FileSource const& file) -> Result {
+            return ReadFromFile(file, offset);
+        },
+    };
+
+    try {
+        return std::visit(visitor, content_);
+    } catch (std::exception const& e) {
+        return unexpected{fmt::format(
+            "IncrementalReader: ReadChunk got an exception:\n{}", e.what())};
+    } catch (...) {
+        return unexpected<std::string>{
+            "IncrementalReader: ReadChunk got an unknown exception"};
+    }
+}
+
+auto IncrementalReader::ReadFromFile(FileSource const& file, std::size_t offset)
+    const -> expected<std::string_view, std::string> {
+    if (file == nullptr) {
+        return unexpected<std::string>{
+            "IncrementalReader: ReadFromFile: got corrupted file"};
+    }
+
+    if (std::fseek(file.get(), gsl::narrow<std::int64_t>(offset), SEEK_SET) !=
+        0) {
+        return unexpected<std::string>{
+            "IncrementalReader: ReadFromFile: failed to set offset"};
+    }
+
+    std::size_t read = 0;
+    while (std::feof(file.get()) == 0 and std::ferror(file.get()) == 0 and
+           read < buffer_.size()) {
+        read += std::fread(
+            &buffer_[read], sizeof(char), buffer_.size() - read, file.get());
+    }
+    if (std::ferror(file.get()) != 0) {
+        return unexpected{
+            fmt::format("IncrementalReader: ReadFromFile: ferror {}",
+                        std::ferror(file.get()))};
+    }
+    return std::string_view{buffer_.data(), read};
+}
+
+IncrementalReader::Iterator::Iterator(
+    gsl::not_null<IncrementalReader const*> const& owner,
+    std::size_t offset) noexcept
+    : owner_{owner}, offset_{offset} {}
+
+auto IncrementalReader::Iterator::operator*() const noexcept
+    -> expected<std::string_view, std::string> {
+    return owner_->ReadChunk(offset_);
+}
+
+auto IncrementalReader::Iterator::operator++() noexcept
+    -> IncrementalReader::Iterator& {
+    offset_ += owner_->chunk_size_;
+    if (offset_ >= owner_->content_size_) {
+        offset_ = owner_->GetEndOffset();
+    }
+    return *this;
+}