Add helper function parseHashFormat[Opt] printHashFormat

Add hash format analogy of
parseHashTypeOpt, parseHashType, and printHashType.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libutil/hash.cc

@@ -386,6 +386,41 @@ Hash compressHash(const Hash & hash, unsigned int newSize)
 }
 
 
+std::optional<HashFormat> parseHashFormatOpt(std::string_view hashFormatName)
+{
+    if (hashFormatName == "base16") return HashFormat::Base16;
+    if (hashFormatName == "base32") return HashFormat::Base32;
+    if (hashFormatName == "base64") return HashFormat::Base64;
+    if (hashFormatName == "sri") return HashFormat::SRI;
+    return std::nullopt;
+}
+
+HashFormat parseHashFormat(std::string_view hashFormatName)
+{
+    auto opt_f = parseHashFormatOpt(hashFormatName);
+    if (opt_f)
+        return *opt_f;
+    throw UsageError("unknown hash format '%1%', expect 'base16', 'base32', 'base64', or 'sri'", hashFormatName);
+}
+
+std::string_view printHashFormat(HashFormat HashFormat)
+{
+    switch (HashFormat) {
+    case HashFormat::Base64:
+        return "base64";
+    case HashFormat::Base32:
+        return "base32";
+    case HashFormat::Base16:
+        return "base16";
+    case HashFormat::SRI:
+        return "sri";
+    default:
+        // illegal hash base enum value internally, as opposed to external input
+        // which should be validated with nice error message.
+        assert(false);
+    }
+}
+
 std::optional<HashType> parseHashTypeOpt(std::string_view s)
 {
     if (s == "md5") return htMD5;

src/libutil/hash.hh

@@ -181,6 +181,21 @@ HashResult hashPath(HashType ht, const Path & path,
  */
 Hash compressHash(const Hash & hash, unsigned int newSize);
 
+/**
+ * Parse a string representing a hash format.
+ */
+HashFormat parseHashFormat(std::string_view hashFormatName);
+
+/**
+ * std::optional version of parseHashFormat that doesn't throw error.
+ */
+std::optional<HashFormat> parseHashFormatOpt(std::string_view hashFormatName);
+
+/**
+ * The reverse of parseHashFormat.
+ */
+std::string_view printHashFormat(HashFormat hashFormat);
+
 /**
  * Parse a string representing a hash type.
  */

src/libutil/tests/hash.cc

@@ -79,6 +79,21 @@ namespace nix {
                 "7299aeadb6889018501d289e4900f7e4331b99dec4b5433a"
                 "c7d329eeb6dd26545e96e55b874be909");
     }
+
+    /* ----------------------------------------------------------------------------
+     * parseHashFormat, parseHashFormatOpt, printHashFormat
+     * --------------------------------------------------------------------------*/
+
+    TEST(hashFormat, testRoundTripPrintParse) {
+        for (const HashFormat hashFormat: { HashFormat::Base64, HashFormat::Base32, HashFormat::Base16, HashFormat::SRI}) {
+            ASSERT_EQ(parseHashFormat(printHashFormat(hashFormat)), hashFormat);
+            ASSERT_EQ(*parseHashFormatOpt(printHashFormat(hashFormat)), hashFormat);
+        }
+    }
+
+    TEST(hashFormat, testParseHashFormatOptException) {
+        ASSERT_EQ(parseHashFormatOpt("sha0042"), std::nullopt);
+    }
 }
 
 namespace rc {