New racetrack filtering module

Author: Gunnstein

.gitignore

@@ -3,6 +3,7 @@
 !setup.cfg
 !example.py
 !fatpack/
+!fatpack/*
 !*.png
 !README.rst
 !LICENSE

fatpack/racetrack.py

@@ -0,0 +1,93 @@
+# -*- coding: utf-8 -*-
+"""
+Implementation of the racetrack amplitude filter. The implementation is
+based on the following resources:
+
+    `H. O. Fuchs et al. Shortcuts in cumulative damage analysis.
+    SAE Automobile engineering meeting paper 730565. (1973)`
+
+    `H. Wu et. al. Validation of the multiaxial racetrack amplitude filter.
+    International Journal of Fatigue, 87 (2016) 167–179`
+
+"""
+from __future__ import (division, print_function, absolute_import,
+                        unicode_literals)
+import numpy as np
+from math import fabs
+from .rainflow import find_reversals
+
+__all__ = ["racetrack_filter", "find_reversals_racetrack_filtered"]
+
+
+def racetrack_filter(reversals, h):
+    """Racetrack filter for accelerated fatigue testing.
+
+    The racetrack amplitude filter removes low amplitude cycles from the
+    reversals without altering the sequence of the remaining cycles. The
+    racetrack filter therefore allows to accelerate variable amplitude
+    fatigue testing by removing low amplitude cycles which does not
+    significantly affect the overall fatigue damage and at the same time
+    preserves sequence effects inherent in the original sequence.
+
+    Arguments
+    ---------
+    reversals : ndarray
+        An 1D-array of reversals.
+    h : float
+        Racetrack width, cycles with range lower than width are filtered out.
+
+    Returns
+    -------
+    signal : ndarray
+        Signal after applying racetrack filter.
+    indices : ndarray
+        Indices of racetrack filtered signal.
+    """
+    y = reversals
+    yprev = None
+    ix = []
+    for n, yn in enumerate(y):
+        if (n == 0) or (n == y.size-1):
+            yprev = yn
+            ix.append(n)
+            continue
+        dy = yn - yprev
+        if fabs(dy) > h / 2.:
+            yprev = yn - dy/fabs(dy) * h/2.
+            ix.append(n)
+    ix = np.array(ix, dtype=np.int)
+    return y[ix], ix
+
+
+def find_reversals_racetrack_filtered(y, h, k=64):
+    """Return reversals (peaks and valleys) and indices of reversals in `y`.
+
+    The data points in the dataseries `y` are classified into `k` constant
+    sized intervals and then peak-valley filtered to yield the successive
+    extremas of the dataseries `y`. The reversals are then filtered with the
+    racetrack amplitude filter and then peak-valley filtered again to find
+    the racetrack filtered reversals.
+
+    Arguments
+    ---------
+    y : ndarray
+        Dataseries containing the signal to find the reversals for.
+    h : float
+        Racetrack width, cycles with range lower than width are filtered out.
+    k : int
+        The number of intervals to divide the min-max range of the dataseries
+        into.
+
+    Returns
+    -------
+    reversals : ndarray
+        Reversals of the initial data series `y` after racetrack filtering.
+    indices : ndarray
+        The indices of the initial data series `y` which corresponds to the
+        reversals.
+    """
+    _, ix = find_reversals(y, k=k)
+    z, ixz = racetrack_filter(y[ix], h)
+    ix = ix[ixz]
+    rev, ixr = find_reversals(z, k=k)
+    return y[ix[ixr]], ix[ixr]