fix decode_utf8 for codepoints >= U+010000

Fixes #1181.

Add unit test cases to cover UTF-8 decode/encode.

CMakeLists.txt

@@ -133,6 +133,6 @@ if (BUILD_TESTS)
     # target runs tests without building them.
     add_custom_target(run_tests COMMAND ${CMAKE_CTEST_COMMAND}
         DEPENDS libjsonnet_test libjsonnet_test_file libjsonnet_test_snippet
-                jsonnet parser_test lexer_test libjsonnet++_test libjsonnet_test_locale
+                jsonnet unicode_test parser_test lexer_test libjsonnet++_test libjsonnet_test_locale
     )
 endif()

core/BUILD

@@ -69,3 +69,12 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+cc_test(
+    name = "unicode_test",
+    srcs = ["unicode_test.cpp"],
+    deps = [
+        ":libjsonnet",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

core/CMakeLists.txt

@@ -87,6 +87,9 @@ function(add_test_executable test_name)
 endfunction()
 
 if (BUILD_TESTS)
+    add_test_executable(unicode_test)
+    add_test(unicode_test ${GLOBAL_OUTPUT_PATH}/unicode_test)
+
     add_test_executable(lexer_test)
     add_test(lexer_test ${GLOBAL_OUTPUT_PATH}/lexer_test)
 

core/unicode.h

@@ -114,7 +114,7 @@ static inline char32_t decode_utf8(const std::string &str, size_t &i)
         if ((c3 & 0xC0) != 0x80) {
             return JSONNET_CODEPOINT_ERROR;
         }
-        return ((c0 & 0x7) << 24ul) | ((c1 & 0x3F) << 12ul) | ((c2 & 0x3F) << 6) | (c3 & 0x3F);
+        return ((c0 & 0x7) << 18ul) | ((c1 & 0x3F) << 12ul) | ((c2 & 0x3F) << 6) | (c3 & 0x3F);
     } else {
         return JSONNET_CODEPOINT_ERROR;
     }

core/unicode_test.cpp

@@ -0,0 +1,114 @@
+/*
+Copyright 2025 Google Inc. All rights reserved.
+
+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 <array>
+#include <string>
+#include <sstream>
+#include <iostream>
+#include "unicode.h"
+#include "gtest/gtest.h"
+
+namespace jsonnet::internal {
+namespace {
+
+void testEncodeDecode(char32_t codepoint, const std::string &expect_utf8) {
+    std::string buffer;
+    size_t len = encode_utf8(codepoint, buffer);
+    EXPECT_EQ(len, expect_utf8.size());
+    EXPECT_EQ(buffer, expect_utf8);
+
+    size_t at = 0;
+    char32_t decoded = decode_utf8(expect_utf8, at);
+    EXPECT_EQ(decoded, codepoint);
+    EXPECT_EQ(at, expect_utf8.size() - 1);
+}
+
+TEST(Unicode, TestUTF8)
+{
+    // ASCII encodes as itself.
+    testEncodeDecode(0x00, std::string("\x00", 1));
+    testEncodeDecode(0x41, "A");
+    testEncodeDecode(0x7f, "\x7f");
+
+    testEncodeDecode(0x80, "\xc2\x80");
+    testEncodeDecode(0x100, "\xc4\x80");
+    testEncodeDecode(0x7ff, "\xdf\xbf");
+
+    testEncodeDecode(0x800, "\xe0\xa0\x80");
+    testEncodeDecode(0x1482, "\xe1\x92\x82");
+    testEncodeDecode(0xffff, "\xef\xbf\xbf");
+
+    testEncodeDecode(0x010000, "\xf0\x90\x80\x80");
+    testEncodeDecode(0x01f600, "\xf0\x9f\x98\x80"); // U+1F600 "Grinning Face"
+    testEncodeDecode(0x0f057e, "\xf3\xb0\x95\xbe"); // U+F057E Private use area character
+    testEncodeDecode(0x10ffff, "\xf4\x8f\xbf\xbf");
+}
+
+TEST(Unicode, TestUTF8RejectBad)
+{
+    const auto test_cases = std::array{
+        "\x80",  // Continuation byte without leading byte
+        "\xa0",  // Continuation byte without leading byte
+        "\xbf",  // Continuation byte without leading byte
+        "\xc0",  // Leading byte for 2-byte sequence (missing tail)
+        "\xe0",  // Leading byte for 3-byte sequence (missing tail)
+        "\xf0",  // Leading byte for 4-byte sequence (missing tail)
+        "\xf8\x83\x83\x83",  // Invalid leading byte
+        "\xe0\x80",  // Leading byte for 3-byte sequence (missing tail)
+        "\xf0\x80",  // Leading byte for 4-byte sequence (missing tail)
+        "\xf0\x80\x80",  // Leading byte for 4-byte sequence (missing tail)
+        "\xc0\xcf",  // Leading byte for 2-byte sequence (incorrect tail)
+        "\xe0\xcf",  // Leading byte for 3-byte sequence (incorrect tail)
+        "\xf0\xcf",  // Leading byte for 4-byte sequence (incorrect tail)
+        "\xe0\xcf\x80",  // Leading byte for 3-byte sequence (incorrect tail)
+        "\xf0\xcf\x80",  // Leading byte for 4-byte sequence (incorrect tail)
+        "\xe0\x80\xcf",  // Leading byte for 3-byte sequence (incorrect tail)
+        "\xf0\x80\xcf",  // Leading byte for 4-byte sequence (incorrect tail)
+        "\xf0\x80\x80\xcf",  // Leading byte for 4-byte sequence (incorrect tail)
+        "\xf0\x80\xcf\x80",  // Leading byte for 4-byte sequence (incorrect tail)
+        "\xf0\xcf\x80\x80",  // Leading byte for 4-byte sequence (incorrect tail)
+    };
+    for (size_t i = 0; i < test_cases.size(); ++i) {
+        const auto str = test_cases[i];
+        size_t at = 0;
+        char32_t c = decode_utf8(str, at);
+
+        EXPECT_EQ(c, JSONNET_CODEPOINT_ERROR) << "expect decode to reject. case " << i << std::endl;
+    }
+}
+
+TEST(Unicode, TestUTF8RoundTripExhaustive)
+{
+    // Encode every Unicode code-point as UTF-8 and verify that
+    // it decodes to the same value.
+    std::string buffer;
+    for (int x = 0; x < JSONNET_CODEPOINT_MAX; ++x) {
+        if (x == JSONNET_CODEPOINT_ERROR) {
+            continue;
+        }
+        buffer.clear();
+        encode_utf8(x, buffer);
+
+        size_t at = 0;
+        char32_t y = decode_utf8(buffer, at);
+        EXPECT_NE(y, JSONNET_CODEPOINT_ERROR) << "UTF-8 roundtrip failed for codepoint " << x << " decode rejects" << std::endl;
+        EXPECT_EQ(x, y) << "UTF-8 roundtrip failed for codepoint " << x << " converts to " << y << std::endl;
+        EXPECT_EQ(at, buffer.size() - 1) << "UTF-8 roundtrip failed for codepoint " << x << " decodes incorrect length" << std::endl;
+    }
+}
+
+}  // namespace
+}  // namespace jsonnet::internal