Make sure returned number of bins does not exceed nbins (#62)

Due to floating point accuracy count_cycles might return more bins that what is specified by argument nbins.

Solves #60

src/rainflow.py

@@ -157,7 +157,16 @@ def count_cycles(series, ndigits=None, nbins=None, binsize=None):
     if binsize is not None:
         nmax = 0
         for rng, count in cycles:
-            n = int(math.ceil(rng / binsize))  # using int for Python 2 compatibility
+            quotient = rng / binsize
+            n = int(math.ceil(quotient))  # using int for Python 2 compatibility
+
+            if nbins and n > nbins:
+                # Due to floating point accuracy we may get n > nbins,
+                # in which case we move rng to the preceeding bin.
+                if (quotient % 1) > 1e-6:
+                    raise Exception("Unexpected error")
+                n = n - 1
+
             counts[n * binsize] += count
             nmax = max(n, nmax)
 

tests/test_rainflow.py

@@ -301,3 +301,18 @@ def test_reversals_yield_value(series, cycles, counts, approx):
 )
 def test_reversals_small_series(series, reversals):
     assert list(rainflow.reversals(series)) == reversals
+
+
+def test_num_bins():
+    # This test checks for a bug reported in issue #60 where the
+    # returned number of bins was different than the nbins argument
+    # due to floating point accuracy.
+    series = [
+        0,
+        3517.860166127188,
+        -3093.4966492094213,
+        0,
+    ]
+    nbins = 100
+    result = rainflow.count_cycles(series, nbins=nbins)
+    assert len(result) == nbins