reducing the number of reads to do k-mer stats to 1 in 50 like FastQC

src/Module.cpp

@@ -1999,7 +1999,7 @@ ModuleKmerContent::summarize_module(FastqStats &stats) {
         stats.kmer_count[(i << Constants::bit_shift_kmer) | kmer];
 
       expected_count = pos_kmer_count[i] / dividend;
-      obs_exp_ratio = observed_count / expected_count;
+      obs_exp_ratio = (expected_count > 0) ? (observed_count / expected_count) : 0;
 
       if (i == 0 || obs_exp_ratio > obs_exp_max[kmer]) {
         obs_exp_max[kmer] = obs_exp_ratio;
@@ -2059,6 +2059,7 @@ ModuleKmerContent::make_html_data() {
   size_t xlim = 0;
   for (size_t i = 0; i < lim; ++i)
     xlim = max(xlim, where_obs_exp_is_max[kmers_to_report[i].first]);
+  xlim += kmer_size;
 
   for (size_t i = 0; i < lim; ++i) {
     const size_t kmer = kmers_to_report[i].first;

src/StreamReader.cpp

@@ -87,6 +87,7 @@ StreamReader::StreamReader(FalcoConfig &config,
 
   // keep track of which reads to do tile
   next_tile_read = 0;
+  next_kmer_read = 0;
   do_tile_read = true;
 
   // Subclasses will use this to deflate if necessary
@@ -256,7 +257,7 @@ StreamReader::process_sequence_base_from_buffer(FastqStats &stats) {
       cur_kmer = ((cur_kmer << Constants::bit_shift_base) | base_ind);
 
       // registers k-mer if seen at least k nucleotides since the last n
-      if (do_kmer && (num_bases_after_n >= Constants::kmer_size)) {
+      if (do_kmer && do_kmer_read && (num_bases_after_n >= Constants::kmer_size)) {
 
           stats.kmer_count[(read_pos << Constants::bit_shift_kmer)
                            | (cur_kmer & Constants::kmer_mask)]++;
@@ -354,6 +355,7 @@ StreamReader::read_sequence_line(FastqStats &stats) {
   num_bases_after_n = 1;
   still_in_buffer = true;
   next_truncation = 100;
+  do_kmer_read = (stats.num_reads == next_kmer_read);
 
   /*********************************************************/
   /********** THIS LOOP MUST BE ALWAYS OPTIMIZED ***********/
@@ -537,6 +539,7 @@ StreamReader::postprocess_fastq_record(FastqStats &stats) {
       next_tile_read += num_reads_for_tile;
     }
   }
+  next_kmer_read += do_kmer_read*num_reads_for_kmer;
 }
 
 /*******************************************************/

src/StreamReader.hpp

@@ -72,12 +72,14 @@ class StreamReader{
 
     // keep track of reads for which to do kmer and tile count
   static const size_t num_reads_for_tile = 10;
+  static const size_t num_reads_for_kmer = 50;
 
   bool continue_storing_sequences;
   bool do_kmer_read;
   bool do_tile_read;
 
   size_t next_tile_read;
+  size_t next_kmer_read;
 
   // Whether or not we have passed the buffer while reading and need to allocate
   // more space / use dynamically allocated space to process the base