1 files changed, 87 insertions, 0 deletions
diff --git a/src/buildtool/crypto/hash_function.hpp b/src/buildtool/crypto/hash_function.hpp
new file mode 100644
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+++ b/src/buildtool/crypto/hash_function.hpp
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+#ifndef INCLUDED_SRC_BUILDTOOL_CRYPTO_HASH_FUNCTION_HPP
+#define INCLUDED_SRC_BUILDTOOL_CRYPTO_HASH_FUNCTION_HPP
+
+#include <cstdint>
+#include <functional>
+#include <optional>
+#include <string>
+
+#include "src/buildtool/crypto/hash_generator.hpp"
+
+/// \brief Hash function used for the entire buildtool.
+class HashFunction {
+  public:
+    enum class JustHash : std::uint8_t {
+        Native,     ///< SHA1 for plain hashes, and Git for blobs and trees.
+        Compatible  ///< SHA256 for all hashes.
+    };
+
+    /// \brief Set globally used hash type.
+    static void SetHashType(JustHash type) {
+        [[maybe_unused]] auto _ = HashType(type);
+    }
+
+    /// \brief Compute a plain hash.
+    [[nodiscard]] static auto ComputeHash(std::string const& data) noexcept
+        -> HashGenerator::HashDigest {
+        return ComputeTaggedHash(data);
+    }
+
+    /// \brief Compute a blob hash.
+    [[nodiscard]] static auto ComputeBlobHash(std::string const& data) noexcept
+        -> HashGenerator::HashDigest {
+        static auto const kBlobTagCreator =
+            [](std::string const& data) -> std::string {
+            return {"blob " + std::to_string(data.size()) + '\0'};
+        };
+        return ComputeTaggedHash(data, kBlobTagCreator);
+    }
+
+    /// \brief Compute a tree hash.
+    [[nodiscard]] static auto ComputeTreeHash(std::string const& data) noexcept
+        -> HashGenerator::HashDigest {
+        static auto const kTreeTagCreator =
+            [](std::string const& data) -> std::string {
+            return {"tree " + std::to_string(data.size()) + '\0'};
+        };
+        return ComputeTaggedHash(data, kTreeTagCreator);
+    }
+
+    /// \brief Obtain incremental hasher for computing plain hashes.
+    [[nodiscard]] static auto Hasher() noexcept -> HashGenerator::Hasher {
+        switch (HashType()) {
+            case JustHash::Native:
+                return HashGenerator{HashGenerator::HashType::SHA1}
+                    .IncrementalHasher();
+            case JustHash::Compatible:
+                return HashGenerator{HashGenerator::HashType::SHA256}
+                    .IncrementalHasher();
+        }
+    }
+
+  private:
+    static constexpr auto kDefaultType = JustHash::Native;
+
+    [[nodiscard]] static auto HashType(
+        std::optional<JustHash> type = std::nullopt) -> JustHash {
+        static JustHash type_{kDefaultType};
+        if (type) {
+            type_ = *type;
+        }
+        return type_;
+    }
+
+    [[nodiscard]] static auto ComputeTaggedHash(
+        std::string const& data,
+        std::function<std::string(std::string const&)> const& tag_creator =
+            {}) noexcept -> HashGenerator::HashDigest {
+        auto hasher = Hasher();
+        if (tag_creator and HashType() == JustHash::Native) {
+            hasher.Update(tag_creator(data));
+        }
+        hasher.Update(data);
+        return *std::move(hasher).Finalize();
+    }
+};
+
+#endif  // INCLUDED_SRC_BUILDTOOL_CRYPTO_HASH_FUNCTION_HPP