ICU-22781 Fix &Improve MeasureUnit identifier generation for constant denominators (C++)

See #3364

icu4c/source/i18n/measunit_extra.cpp

@@ -31,7 +31,7 @@
 #include "unicode/ustringtrie.h"
 #include "uresimp.h"
 #include "util.h"
-#include <climits>
+#include <limits.h>
 #include <cstdlib>
 U_NAMESPACE_BEGIN
 
@@ -1269,6 +1269,51 @@ MeasureUnitImpl::extractIndividualUnitsWithIndices(UErrorCode &status) const {
     return result;
 }
 
+int32_t countCharacter(const CharString &str, char c) {
+    int32_t count = 0;
+    for (int32_t i = 0, n = str.length(); i < n; i++) {
+        if (str[i] == c) {
+            count++;
+        }
+    }
+    return count;
+}
+
+/**
+ * Internal function that returns a string of the constants in the correct
+ * format.
+ *
+ * Example:
+ * 1000 --> "-per-1000"
+ * 1000000 --> "-per-1e6"
+ *
+ * NOTE: this function is only used when the constant denominator is greater
+ * than 0.
+ */
+CharString getConstantsString(uint64_t constantDenominator, UErrorCode &status) {
+    U_ASSERT(constantDenominator > 0 && constantDenominator <= LLONG_MAX);
+
+    CharString result;
+    result.appendNumber(constantDenominator, status);
+    if (U_FAILURE(status)) {
+        return result;
+    }
+
+    if (constantDenominator <= 1000) {
+        return result;
+    }
+
+    // Check if the constant is a power of 10.
+    int32_t zeros = countCharacter(result, '0');
+    if (zeros == result.length() - 1 && result[0] == '1') {
+        result.clear();
+        result.append(StringPiece("1e"), status);
+        result.appendNumber(zeros, status);
+    }
+
+    return result;
+}
+
 /**
  * Normalize a MeasureUnitImpl and generate the identifier string in place.
  */
@@ -1319,8 +1364,8 @@ void MeasureUnitImpl::serialize(UErrorCode &status) {
                     result.append(StringPiece("-per-"), status);
                 }
 
-                if (this->constantDenominator != 0) {
-                    result.appendNumber(this->constantDenominator, status);
+                if (this->constantDenominator > 0) {
+                    result.append(getConstantsString(this->constantDenominator, status), status);
                     result.append(StringPiece("-"), status);
                     constantDenominatorAppended = true;
                 }
@@ -1333,9 +1378,9 @@ void MeasureUnitImpl::serialize(UErrorCode &status) {
         this->singleUnits[i]->appendNeutralIdentifier(result, status);
     }
 
-    if (!constantDenominatorAppended && this->constantDenominator != 0) {
+    if (!constantDenominatorAppended && this->constantDenominator > 0) {
         result.append(StringPiece("-per-"), status);
-        result.appendNumber(this->constantDenominator, status);
+        result.append(getConstantsString(this->constantDenominator, status), status);
     }
 
     if (U_FAILURE(status)) {

icu4c/source/test/intltest/units_test.cpp

@@ -53,6 +53,7 @@ class UnitsTest : public IntlTest {
     void testUnitPreferencesWithCLDRTests();
     void testUnitsConstantsDenomenator();
     void testMeasureUnit_withConstantDenominator();
+    void testUnitsConstantsDenomenator_getIdentifier();
     void testConverter();
 };
 
@@ -72,6 +73,7 @@ void UnitsTest::runIndexedTest(int32_t index, UBool exec, const char *&name, cha
     TESTCASE_AUTO(testUnitPreferencesWithCLDRTests);
     TESTCASE_AUTO(testUnitsConstantsDenomenator);
     TESTCASE_AUTO(testMeasureUnit_withConstantDenominator);
+    TESTCASE_AUTO(testUnitsConstantsDenomenator_getIdentifier);
     TESTCASE_AUTO(testConverter);
     TESTCASE_AUTO_END;
 }
@@ -1342,4 +1344,39 @@ void UnitsTest::testMeasureUnit_withConstantDenominator() {
     status.reset();
 }
 
+void UnitsTest::testUnitsConstantsDenomenator_getIdentifier() {
+    IcuTestErrorCode status(*this, "UnitTests::testUnitsConstantsDenomenator_getIdentifier");
+
+    // Test Cases
+    struct TestCase {
+        const char *source;
+        const char *expectedIdentifier;
+    } testCases[]{
+        {"meter-per-1000", "meter-per-1000"},
+        {"meter-per-1000-kilometer", "meter-per-1000-kilometer"},
+        {"meter-per-1000000", "meter-per-1e6"},
+        {"meter-per-1000000-kilometer", "meter-per-1e6-kilometer"},
+        {"meter-per-1000000000", "meter-per-1e9"},
+        {"meter-per-1000000000-kilometer", "meter-per-1e9-kilometer"},
+        {"meter-per-1000000000000", "meter-per-1e12"},
+        {"meter-per-1000000000000-kilometer", "meter-per-1e12-kilometer"},
+        {"meter-per-1000000000000000", "meter-per-1e15"},
+        {"meter-per-1e15-kilometer", "meter-per-1e15-kilometer"},
+        {"meter-per-1000000000000000000", "meter-per-1e18"},
+        {"meter-per-1e18-kilometer", "meter-per-1e18-kilometer"},
+        {"meter-per-1000000000000001", "meter-per-1000000000000001"},
+        {"meter-per-1000000000000001-kilometer", "meter-per-1000000000000001-kilometer"},
+    };
+
+    for (const auto &testCase : testCases) {
+        MeasureUnit unit = MeasureUnit::forIdentifier(testCase.source, status);
+        if (status.errIfFailureAndReset("forIdentifier(\"%s\")", testCase.source)) {
+            continue;
+        }
+
+        auto actualIdentifier = unit.getIdentifier();
+        assertEquals(" getIdentifier(\"%s\")", testCase.expectedIdentifier, actualIdentifier);
+    }
+}
+
 #endif /* #if !UCONFIG_NO_FORMATTING */