nw_aligner: store cigar in Result as char const * instead of std::string&

Avoids the cppcoreguidelines-avoid-const-or-ref-data-members warning
on Result::cigar_string. Both callers only used .data(); semantics
(pointer into NwAligner's internal buffer, invalidated by the next
align() call) are unchanged.

Co-Authored-By: Florian Filloux <ffillouxdev@users.noreply.github.com>

Author: frederic-mahe

src/algo.cc

@@ -474,7 +474,7 @@ namespace {
 
       std::fprintf(parameters.uclustfile.get(), "H\t%u\t%u\t%.1f\t+\t0\t0\t%s\t",
                    swarmid - 1, hit_seq.length, result.percent_id,
-                   result.differences > 0 ? result.cigar_string.data() : "=");
+                   result.differences > 0 ? result.cigar_string : "=");
 
       data.fprint_id(parameters.uclustfile.get(), hit, parameters.opt_usearch_abundance, parameters.opt_append_abundance);
       std::fprintf(parameters.uclustfile.get(), "\t");

src/algod1.cc

@@ -993,7 +993,7 @@ namespace {
                        cluster_no,
                        amp_seq.length,
                        result.percent_id,
-                       result.differences > 0 ? result.cigar_string.data() : "=");
+                       result.differences > 0 ? result.cigar_string : "=");
 
           data.fprint_id(parameters.uclustfile.get(), amp_id, parameters.opt_usearch_abundance, parameters.opt_append_abundance);
           std::fprintf(parameters.uclustfile.get(), "\t");

src/utils/nw_aligner.cc

@@ -274,5 +274,5 @@ auto NwAligner::align(Sequence const & dseq, Sequence const & qseq) -> NwAligner
   auto const percent_id =
     one_hundred * static_cast<double>(length - nwdiff) / static_cast<double>(length);
 
-  return Result{cigar_string_, nwdiff, length, percent_id};
+  return Result{cigar_string_.data(), nwdiff, length, percent_id};
 }

src/utils/nw_aligner.h

@@ -44,7 +44,7 @@ struct Sequence;  // defined in db.h
 
   align() returns a small Result aggregating the four values consumers
   need: cigar string, number of differences, alignment length, and
-  percent identity. The cigar_string reference lives in this NwAligner
+  percent identity. The cigar_string pointer lives in this NwAligner
   object's internal buffer and is only valid until the next align()
   call on the same object.
 */
@@ -59,10 +59,10 @@ class NwAligner {
   };
 
   struct Result {
-    std::string const & cigar_string;
-    uint64_t            differences;
-    uint64_t            length;       // == nwalignmentlength
-    double              percent_id;
+    char const * cigar_string;
+    uint64_t     differences;
+    uint64_t     length;       // == nwalignmentlength
+    double       percent_id;
   };
 
   NwAligner(uint64_t longest_sequence,