Restore k-fold cross validation support

1 files changed, 91 insertions, 8 deletions

diff --git a/lib/dfatool.py b/lib/dfatool.py
index 63639d3..0da8cc9 100644
--- a/lib/dfatool.py
+++ b/lib/dfatool.py
@@ -194,10 +194,19 @@ class KeysightCSV:
         return timestamps, currents
 
 
-def _xv_partitions_kfold(length, num_slices):
+def _xv_partitions_kfold(length, k=10):
+    """
+    Return k pairs of training and validation sets for k-fold cross-validation on `length` items.
+
+    In k-fold cross-validation, every k-th item is used for validation and the remainder is used for training.
+    As there are k ways to do this (items 0, k, 2k, ... vs. items 1, k+1, 2k+1, ... etc), this function returns k pairs of training and validation set.
+
+    Note that this function operates on indices, not data.
+    """
     pairs = []
+    num_slices = k
     indexes = np.arange(length)
-    for i in range(0, num_slices):
+    for i in range(num_slices):
         training = np.delete(indexes, slice(i, None, num_slices))
         validation = indexes[i::num_slices]
         pairs.append((training, validation))
@@ -205,6 +214,15 @@ def _xv_partitions_kfold(length, num_slices):
 
 
 def _xv_partition_montecarlo(length):
+    """
+    Return training and validation set for Monte Carlo cross-validation on `length` items.
+
+    This function operates on indices, not data. It randomly partitions range(length) into a list of training indices and a list of validation indices.
+
+    The training set contains 2/3 of all indices; the validation set consits of the remaining 1/3.
+
+    Example: 9 items -> training = [7, 3, 8, 0, 4, 2], validation = [ 1, 6, 5]
+    """
     shuffled = np.random.permutation(np.arange(length))
     border = int(length * float(2) / 3)
     training = shuffled[:border]
@@ -233,7 +251,7 @@ class CrossValidator:
         model_class -- model class/type used for model synthesis,
             e.g. PTAModel or AnalyticModel. model_class must have a
             constructor accepting (by_name, parameters, arg_count, verbose = False)
-            and provide an assess method.
+            and provide an `assess` method.
         by_name -- measurements aggregated by state/transition/function/... name.
             Layout: by_name[name][attribute] = list of data. Additionally,
             by_name[name]['attributes'] must be set to the list of attributes,
@@ -245,6 +263,53 @@ class CrossValidator:
         self.parameters = sorted(parameters)
         self.arg_count = arg_count
 
+    def kfold(self, model_getter, k=10):
+        """
+        Perform k-fold cross-validation and return average model quality.
+
+        The by_name data is divided into 1-1/k training and 1/k validation in a deterministic manner.
+        After creating a model for the training set, the
+        model type returned by model_getter is evaluated on the validation set.
+        This is repeated k times; the average of all measures is returned to the user.
+
+        arguments:
+        model_getter -- function with signature (model_object) -> model,
+            e.g. lambda m: m.get_fitted()[0] to evaluate the parameter-aware
+            model with automatic parameter detection.
+        k -- step size for k-fold cross-validation. The validation set contains 100/k % of data.
+
+        return value:
+        dict of model quality measures.
+        {
+            'by_name' : {
+                for each name: {
+                    for each attribute: {
+                        'mae' : mean of all mean absolute errors
+                        'mae_list' : list of the individual MAE values encountered during cross-validation
+                        'smape' : mean of all symmetric mean absolute percentage errors
+                        'smape_list' : list of the individual SMAPE values encountered during cross-validation
+                    }
+                }
+            }
+        }
+        """
+
+        # training / validation subsets for each state and transition
+        subsets_by_name = dict()
+        training_and_validation_sets = list()
+
+        for name in self.names:
+            sample_count = len(self.by_name[name]["param"])
+            subsets_by_name[name] = list()
+            subsets_by_name[name] = _xv_partitions_kfold(sample_count, k)
+
+        for i in range(k):
+            training_and_validation_sets.append(dict())
+            for name in self.names:
+                training_and_validation_sets[i][name] = subsets_by_name[name][i]
+
+        return self._generic_xv(model_getter, training_and_validation_sets)
+
     def montecarlo(self, model_getter, count=200):
         """
         Perform Monte Carlo cross-validation and return average model quality.
@@ -276,6 +341,25 @@ class CrossValidator:
             }
         }
         """
+
+        # training / validation subsets for each state and transition
+        subsets_by_name = dict()
+        training_and_validation_sets = list()
+
+        for name in self.names:
+            sample_count = len(self.by_name[name]["param"])
+            subsets_by_name[name] = list()
+            for _ in range(count):
+                subsets_by_name[name].append(_xv_partition_montecarlo(sample_count))
+
+        for i in range(count):
+            training_and_validation_sets.append(dict())
+            for name in self.names:
+                training_and_validation_sets[i][name] = subsets_by_name[name][i]
+
+        return self._generic_xv(model_getter, training_and_validation_sets)
+
+    def _generic_xv(self, model_getter, training_and_validation_sets):
         ret = {"by_name": dict()}
 
         for name in self.names:
@@ -286,8 +370,8 @@ class CrossValidator:
                     "smape_list": list(),
                 }
 
-        for _ in range(count):
-            res = self._single_montecarlo(model_getter)
+        for training_and_validation_by_name in training_and_validation_sets:
+            res = self._single_xv(model_getter, training_and_validation_by_name)
             for name in self.names:
                 for attribute in self.by_name[name]["attributes"]:
                     ret["by_name"][name][attribute]["mae_list"].append(
@@ -308,7 +392,7 @@ class CrossValidator:
 
         return ret
 
-    def _single_montecarlo(self, model_getter):
+    def _single_xv(self, model_getter, tv_set_dict):
         training = dict()
         validation = dict()
         for name in self.names:
@@ -319,8 +403,7 @@ class CrossValidator:
                 training[name]["isa"] = self.by_name[name]["isa"]
                 validation[name]["isa"] = self.by_name[name]["isa"]
 
-            data_count = len(self.by_name[name]["param"])
-            training_subset, validation_subset = _xv_partition_montecarlo(data_count)
+            training_subset, validation_subset = tv_set_dict[name]
 
             for attribute in self.by_name[name]["attributes"]:
                 self.by_name[name][attribute] = np.array(self.by_name[name][attribute])