clarified coboundary enumeration code

1 files changed, 7 insertions, 6 deletions

diff --git a/ripser.cpp b/ripser.cpp
index 6241eb3..6502f63 100644
--- a/ripser.cpp
+++ b/ripser.cpp
@@ -240,17 +240,18 @@ public:
 
 class simplex_coboundary_enumerator {
 private:
-	index_t idx, modified_idx, v, k;
+	index_t idx_below, idx_above, v, k;
 	const binomial_coeff_table& binomial_coeff;
 
 public:
 	simplex_coboundary_enumerator(index_t _idx, index_t _dim, index_t _n, const binomial_coeff_table& _binomial_coeff)
-	    : idx(_idx), modified_idx(_idx), v(_n - 1), k(_dim + 1), binomial_coeff(_binomial_coeff) {}
+	    : idx_below(_idx), idx_above(0), v(_n - 1), k(_dim + 1), binomial_coeff(_binomial_coeff) {}
 
 	bool has_next() {
-		while ((v != -1) && (binomial_coeff(v, k) <= idx)) {
-			idx -= binomial_coeff(v, k);
-			modified_idx += binomial_coeff(v, k + 1) - binomial_coeff(v, k);
+		while ((v != -1) && (binomial_coeff(v, k) <= idx_below)) {
+			idx_below -= binomial_coeff(v, k);
+			idx_above += binomial_coeff(v, k + 1);
+
 			--v;
 			--k;
 			assert(k != -1);
@@ -259,7 +260,7 @@ public:
 	}
 
 	std::pair<entry_t, index_t> next() {
-		auto result = std::make_pair(make_entry(modified_idx + binomial_coeff(v, k + 1), k & 1 ? -1 : 1), v);
+		auto result = std::make_pair(make_entry(idx_above + binomial_coeff(v, k + 1) + idx_below, k & 1 ? -1 : 1), v);
 		--v;
 		return result;
 	}