An example of a switch on strings using a compile time hash

switch_on_strings_with_compile_time_hash.cc

// This is a demonstration of using a COMPILE-TIME hash to do a
// switch statement with a string to answer this question.
//
// https://stackoverflow.com/questions/4165131/c-c-switch-for-non-integers
//
// It is based on the StackOverflow question:
// https://stackoverflow.com/questions/2111667/compile-time-string-hashing
//
// And the solution
// https://stackoverflow.com/questions/2111667/compile-time-string-hashing/23683218#23683218
//

#include <iostream>
#include <string>
#include <vector>
namespace detail {

// CRC32 Table (zlib polynomial)
static constexpr uint32_t crc_table[256] =
{
    0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f,
    0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
    0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2,
    0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
    0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
    0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
    0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c,
    0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
    0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423,
    0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
    0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106,
    0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
    0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d,
    0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
    0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
    0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
    0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7,
    0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
    0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa,
    0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
    0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81,
    0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
    0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84,
    0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
    0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
    0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
    0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e,
    0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
    0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55,
    0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
    0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28,
    0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
    0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f,
    0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
    0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
    0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
    0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69,
    0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
    0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc,
    0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
    0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693,
    0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
    0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
};

//constexpr combine
template<size_t idx>
constexpr uint32_t combine_crc32(const char * str, uint32_t part) {
  return (part >> 8) ^ crc_table[(part ^ str[idx]) & 0x000000FF];
}

//constexpr driver
template<size_t idx>
constexpr uint32_t crc32(const char * str) {
  return combine_crc32<idx>(str, crc32<idx - 1>(str));
}

//constexpr recursion stopper
template<>
constexpr uint32_t crc32<size_t(-1)>(const char * str) {
  return 0xFFFFFFFF;
}

//runtime combine
uint32_t combine_crc32_s(size_t idx, const char * str, uint32_t part) {
  return (part >> 8) ^ crc_table[(part ^ str[idx]) & 0x000000FF];
}

//runtime driver
uint32_t crc32_s(size_t idx, const char * str) {
  if( idx==static_cast<size_t>(-1) )return 0xFFFFFFFF;
  return combine_crc32_s(idx, str, crc32_s(idx-1,str));
}

} //namespace detail

//constexpr that returns unsigned int
template <size_t len>
constexpr uint32_t uint_crc32(const char (&str)[len]) {
  return detail::crc32<len - 2>(str) ^ 0xFFFFFFFF;
}

//constexpr that returns signed int
template <size_t len>
constexpr int int_crc32(const char (&str)[len]) {
  return static_cast<int>( uint_crc32(str) );
}

//runtime that returns unsigned int
uint32_t uint_crc32_s( const char* str ) {
  return detail::crc32_s(strlen(str)-1,str) ^ 0xFFFFFFFF;
}

//runtime that returns signed int
int int_crc32_s( const char* str) {
  return static_cast<int>( uint_crc32_s(str) );
}

//convenient macros
#define S_SWITCH( X ) const char* SWITCH_KEY(X.c_str()); switch( int_crc32_s(X.c_str()) )
#define S_CASE( X ) case int_crc32(X): if( strcmp(SWITCH_KEY,X) ){ goto S_DEFAULT_LABEL;}
#define S_DEFAULT S_DEFAULT_LABEL: default:

int main()
{
    std::string str;
    std::cin >> str;

    //two strings that yield same crc32
    const char* collision1="DeferredAmbient_6_1_18-1of2_5";
    const char* collision2="PostEffect_Lighting_18_6-0of1_8_14_13-1of2_19";

    //without macros (you need to check for collision)
    switch( int_crc32_s(str.c_str()) )
    {
        case int_crc32("foo"): if( str=="foo"){std::cout << "foo you\n"; break;}
        case int_crc32("bar"): if( str=="bar"){std::cout << "bar you\n"; break;}
        case int_crc32("baz"): if( str=="baz"){std::cout << "baz you\n"; break;}
        case int_crc32("PostEffect_Lighting_18_6-0of1_8_14_13-1of2_19"):
            if( str=="PostEffect_Lighting_18_6-0of1_8_14_13-1of2_19"){
                std::cout << "jackpot!\n"; break;
            }
        default: std::cout << "just you\n";
    }

    //with macros
    S_SWITCH( str )
    {
        S_CASE("foo"){ std::cout << "foo you\n"; break; }
        S_CASE("bar"){ std::cout << "bar you\n"; break; }
        S_CASE("baz"){ std::cout << "baz you\n"; break; }
        S_CASE("PostEffect_Lighting_18_6-0of1_8_14_13-1of2_19"){ std::cout << "jackpot!\n"; break; }
        S_DEFAULT{ std::cout << "just you\n"; }
    }
}