trinary: initial commit

cpp/trinary/trinary.cpp

@@ -0,0 +1,35 @@
+#include "trinary.h"
+
+namespace trinary {
+
+namespace {
+
+inline bool is_digit(char ch)
+{
+    return ch >= '0' && ch <= '2';
+}
+
+inline int digit_to_decimal(char ch)
+{
+    return ch - '0';
+}
+
+}
+
+int to_decimal(const std::string &input)
+{
+    int result = 0;
+
+    for (auto ch : input) {
+
+        if (!is_digit(ch))
+            return 0;
+
+        result *= 3;
+        result += digit_to_decimal(ch);
+    }
+
+    return result;
+}
+
+}

cpp/trinary/trinary.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#define EXERCISM_RUN_ALL_TESTS
+
+#include <string>
+
+namespace trinary {
+
+int to_decimal(const std::string &input);
+
+}