Add StringAt to ColumnDecimal

clickhouse/columns/decimal.cpp

@@ -1,6 +1,9 @@
 #include "decimal.h"
 #include "clickhouse/exceptions.h"
 
+#include <algorithm>
+#include <iterator>
+
 namespace {
 
 using clickhouse::ValidationError;
@@ -155,6 +158,42 @@ Int128 ColumnDecimal::At(size_t i) const {
     }
 }
 
+std::string ColumnDecimal::StringAt(size_t i) const {
+    auto scale = GetScale();
+
+    Int128 val = At(i);
+    std::string raw_str = Bignum::Int128ToString(val);
+    if (scale == 0) {
+        return raw_str;
+    }
+
+    std::string ret;
+    ret.reserve(GetPrecision() + 2); // extra space for '-' and '.';
+
+    auto it = raw_str.cbegin();
+    auto end = raw_str.cend();
+
+    if (it != end && *it == '-') {
+        ret.push_back(*it++);
+    }
+
+    int64_t str_len = std::distance(it, end);
+    int64_t integral_len = str_len - static_cast<int64_t>(scale);
+
+    if (integral_len > 0) {
+        std::copy(it, it + integral_len, std::back_inserter(ret));
+        it += integral_len;
+        ret.push_back('.');
+    }
+    else {
+        ret.append("0.");
+        ret.append(static_cast<size_t>(-integral_len), '0');
+    }
+    std::copy(it, end, std::back_inserter(ret));
+
+    return ret;
+}
+
 void ColumnDecimal::Reserve(size_t new_cap) {
     data_->Reserve(new_cap);
 }

clickhouse/columns/decimal.h

@@ -20,6 +20,9 @@ class ColumnDecimal : public Column {
     Int128 At(size_t i) const;
     inline auto operator[](size_t i) const { return At(i); }
 
+    // Returns string representation of the decimal value
+    std::string StringAt(size_t i) const;
+
 public:
     /// Increase the capacity of the column for large block insertion.
     void Reserve(size_t new_cap) override;

ut/columns_ut.cpp

@@ -124,6 +124,7 @@ TEST(ColumnsCase, DecimalStringValueMapping) {
     };
 
     std::vector<TestSample> samples = {
+        {18, 0, "0.0", Int128(0)},
         {18, 0, "0.123", Int128(0)},
         {18, 0, "123", Int128(123)},
         {18, 3, "0.123", Int128(123)},
@@ -144,6 +145,45 @@ TEST(ColumnsCase, DecimalStringValueMapping) {
 
 }
 
+TEST(ColumnsCase, DecimalStringAt) {
+    struct TestSample {
+        size_t precision;
+        size_t scale;
+        std::string str;
+        std::string expect;
+    };
+
+    std::vector<TestSample> samples = {
+        {18, 0, "0", "0"},
+        {18, 0, "123", "123"},
+        {18, 0, "-123", "-123"},
+        {18, 3, "0", "0.000"},
+        {18, 3, "1", "0.001"},
+        {18, 3, "12", "0.012"},
+        {18, 3, "123", "0.123"},
+        {18, 3, "-123", "-0.123"},
+        {18, 3, "0.123", "0.123"},
+        {18, 3, "1234", "1.234"},
+        {18, 3, "-1234", "-1.234"},
+        {18, 3, "-1.234", "-1.234"},
+        {18, 3, "123.0", "123.000"},
+        {18, 3, "123.", "123.000"},
+        {18, 3, "123000", "123.000"},
+        // Large values that do not fit in 64 bits.
+        {38, 4, "12345678901234567890.1234", "12345678901234567890.1234"},
+        {38, 4, "-12345678901234567890.1234", "-12345678901234567890.1234"},
+        {38, 0, "99999999999999999999999999999999999999", "99999999999999999999999999999999999999"},
+    };
+
+    for (auto & [precision, scale, str, expect] : samples) {
+        auto col = std::make_shared<ColumnDecimal>(precision, scale);
+        EXPECT_NO_THROW(col->Append(str)) << "exception for value \"" << str << "\"";
+        auto value = col->StringAt(0);
+        EXPECT_EQ(value, expect);
+    }
+
+}
+
 TEST(ColumnsCase, NumericInit) {
     auto col = std::make_shared<ColumnUInt32>(MakeNumbers());