Add more comment for khash in Index.

#include "temp_mapping.h"

#include "utils.h"

#define CHROMAP_VERSION "0.2.3-r424"

#define CHROMAP_VERSION "0.2.3-r426"

namespace chromap {

namespace chromap {

// Note that the max kmer size is 28 and it is saved in a unsigned 64-bit

// integer. So it can right shift by one bit is lossless and safe. The minimizer

// only occurs once when the lowest bit of the key value is set to 1.

#define KHashFunctionForIndex(a) ((a) >> 1)

#define KHashEqForIndex(a, b) ((a) >> 1 == (b) >> 1)

KHASH_INIT(k64, uint64_t, uint64_t, 1, KHashFunctionForIndex, KHashEqForIndex);

KHASH_INIT(/*name=*/k64, /*khkey_t=*/uint64_t, /*khval_t=*/uint64_t,

           /*kh_is_map=*/1, /*__hash_func=*/KHashFunctionForIndex,

           /*__hash_equal=*/KHashEqForIndex);

class Index {

 public: