kata1.cpp

// Always include C headers before C++ headers
#include <cctype>

#include <iostream>
#include <string>

// Use this with caution
using namespace std;

string compress(const string& sIn)
{
  // We will use c_str() to ensure a NUL sentinel at the end
  auto pszIn = sIn.c_str();

  string sRet;
  while(*pszIn)
  {
    if(!isalpha(*pszIn)) 
    {
      throw runtime_error("Invalid String");
    }
    
    // There is always at least 1 count
    int c = 1;
    
    // Keep skipping forward as long as the current and next match, since we have a NUL as a sentinel, we dont bother about EOS
    while(*pszIn == pszIn[1])
    {
      ++c;
      ++pszIn;
    }

    // Append the character and count;
    sRet += *pszIn;
    sRet += to_string(c);

    // Jump to next group
    ++pszIn;
  }
  
  if(sRet.size() > sIn.size())
  {
    throw runtime_error("Cannot compress");
  }
  
  return sRet;
}

const string g_sTest[] = 
{
  "aaabccdefgggg",
  "abcdef",
  "aaaaaabbbbacde",
  "azzzxaaacccddddeef"
};


int main()
{
  for(const auto &s: g_sTest)
  {
    cout << s << ": " << compress(s) << endl;
  }
}