[FEATURE] add lexicographic rank (#111)

Author: VascoSch92

CHANGELOG.md

@@ -22,6 +22,10 @@ The version is represented by three digits: a.b.c.
 
 ## Unreleased
 
+FEATURE:
+- `symmetria.Permutation`: add `lexicographic_rank` method
+- `symmetria.CycleDecomposition`: add `lexicographic_rank` method
+
 ## \[0.3.0\] - 2024-07-15
 
 FEATURE:

docs/source/pages/API_reference/elements/index.rst

@@ -115,6 +115,11 @@ Here, **P** denotes the class ``Permutation``, **C** the class ``Cycle``, and **
      - ✅
      - ❌
      - ✅
+   * - ``lexicographic_rank``
+     - Return the lexicographic rank of the permutation
+     - ✅
+     - ❌
+     - ✅
    * - ``map``
      - Return the map defining the permutation
      - ✅

symmetria/elements/cycle_decomposition.py

@@ -764,6 +764,27 @@ def is_regular(self) -> bool:
         """
         return all(len(cycle) == len(self[0]) for cycle in self)
 
+    def lexicographic_rank(self) -> int:
+        """Return the lexicographic rank of the cycle decomposition.
+
+        Recall that the lexicographic rank of a permutation refers to its position in the list of all
+        permutations of the same degree sorted in lexicographic order.
+
+        :return: the lexocographic rank of the cycle decomposition.
+        :rtype: int
+
+        :example:
+            >>> from symmetria import Cycle, CycleDecomposition
+            ...
+            >>> CycleDecomposition(Cycle(1)).lexicographic_rank()
+            1
+            >>> CycleDecomposition(Cycle(1, 3, 2)).lexicographic_rank()
+            5
+            >>> CycleDecomposition(Cycle(1, 3, 2), Cycle(4, 5)).lexicographic_rank()
+            50
+        """
+        return symmetria.elements.permutation.Permutation.from_cycle_decomposition(self).lexicographic_rank()
+
     @property
     def map(self) -> Dict[int, int]:
         """Return a dictionary representing the mapping of the cycle decomposition,

symmetria/elements/permutation.py

@@ -1,3 +1,4 @@
+from math import factorial
 from typing import Any, Set, Dict, List, Tuple, Union, Iterable
 from collections import OrderedDict
 
@@ -876,6 +877,39 @@ def is_regular(self) -> bool:
         cycle_decomposition = self.cycle_decomposition()
         return all(len(cycle) == len(cycle_decomposition[0]) for cycle in cycle_decomposition)
 
+    def lexicographic_rank(self) -> int:
+        """Return the lexicographic rank of the permutation.
+
+        Recall that the lexicographic rank of a permutation refers to its position in the list of all
+        permutations of the same degree sorted in lexicographic order.
+
+        :return: the lexocographic rank of the permutation.
+        :rtype: int
+
+        :example:
+            >>> from symmetria import Permutation
+            ...
+            >>> Permutation(1).lexicographic_rank()
+            1
+            >>> Permutation(1, 2, 3).lexicographic_rank()
+            1
+            >>> Permutation(1, 3, 2).lexicographic_rank()
+            2
+            >>> Permutation(3, 2, 1, 4).lexicographic_rank()
+            15
+        """
+        n = self.__len__()
+        rank = 1
+
+        for i in range(n):
+            right_smaller = 0
+            for j in range(i + 1, n):
+                if self[i + 1] > self[j + 1]:
+                    right_smaller += 1
+            rank += right_smaller * factorial(n - i - 1)
+
+        return rank
+
     @property
     def map(self) -> Dict[int, int]:
         """Return a dictionary representing the mapping of the permutation.

tests/tests_elements/tests_permutation/test_cases.py

@@ -225,6 +225,16 @@
     (Permutation(2, 1), True),
     (Permutation(2, 1, 3), False),
 ]
+TEST_LEXICOGRAPHIC_RANK = [
+    (Permutation(1), 1),
+    (Permutation(1, 2), 1),
+    (Permutation(2, 1), 2),
+    (Permutation(1, 2, 3), 1),
+    (Permutation(1, 3, 2), 2),
+    (Permutation(3, 2, 1), 6),
+    (Permutation(3, 2, 1, 4), 15),
+    (Permutation(1, 2, 5, 4, 3), 6),
+]
 TEST_MAP = [
     (Permutation(1), {1: 1}),
     (Permutation(2, 1), {1: 2, 2: 1}),

tests/tests_elements/tests_permutation/test_generic_methods.py

@@ -27,6 +27,7 @@
     TEST_IS_DERANGEMENT,
     TEST_ONE_LINE_NOTATION,
     TEST_IS_CONJUGATE_ERROR,
+    TEST_LEXICOGRAPHIC_RANK,
     TEST_CYCLE_DECOMPOSITION,
 )
 
@@ -256,6 +257,20 @@ def test_is_regular(permutation, expected_value) -> None:
     )
 
 
+@pytest.mark.parametrize(
+    argnames="permutation, expected_value",
+    argvalues=TEST_LEXICOGRAPHIC_RANK,
+    ids=[f"{p}.lexicographic_rank()={m}" for p, m in TEST_LEXICOGRAPHIC_RANK],
+)
+def test_lexicographic_rank(permutation, expected_value) -> None:
+    """Tests for the method `lexicographic_rank()`."""
+    _check_values(
+        expression=f"{permutation.rep()}.lexicographic_rank()",
+        evaluation=permutation.lexicographic_rank(),
+        expected=expected_value,
+    )
+
+
 @pytest.mark.parametrize(
     argnames="permutation, expected_value",
     argvalues=TEST_MAP,