Merge pull request #554 from sir-gon/feature/ctci-recursive-staircase

Feature/ctci recursive staircase

Author: sir-gon

docs/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci-recursive-staircase-solution-notes.md

@@ -36,3 +36,35 @@ so that repeated cases are not recalculated.
 
 The trade-off is that the algorithm now requires
 more memory to run in less time.
+
+## Generalized solution
+
+In order to comply with some clean code best practices,
+I noticed that the step limit in the algorithm is a hard-coded number,
+so to comply with the "no magic numbers" rule,
+I was forced to find a more generalized solution.
+
+Then I found the following pattern:
+
+- First cases are:
+
+$$ \begin{matrix}
+    \text{stepPerms(0)} = 0 \\
+    \text{stepPerms(1)} = 1 \\
+    \text{stepPerms(2)} = 2 \\
+  \end{matrix}
+$$
+
+- Next step combinations above 2 and less than the step limit are:
+
+$$ \text{stepPerms(number of steps)} = 2^\text{number of steps} + 1 $$
+
+- When `number of steps` are above the limit, the pattern is
+the sum of latest `number of steps` previous calls of
+`stepPerms(x)` results as follows:
+
+$$ \displaystyle\sum_{
+  i=\text{number of steps} - \text{limit}}
+  ^\text{number of steps}
+  stepPerms(\text{number of steps} - i)
+$$

src/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci_recursive_staircase.py

@@ -1,26 +1,39 @@
 # pylint: disable=line-too-long
 # @link Problem definition [[docs/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci-recursive-staircase.md]] # noqa
+# @see Solution Notes: [[docs/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci-recursive-staircase-solution-notes.md]] # noqa
 # pylint: enable=line-too-long
 
 from typing import Dict
 
+TOP_LIMIT = 10 ** 10 + 7
+STEPS_LIMIT = 3
 
-def step_perms_comput_with_cache(n_steps: int, cache: Dict[int, int]) -> int:
+
+def step_perms_comput_with_cache(
+        n_steps: int,
+        cache: Dict[int, int],
+        steps_limit: int) -> int:
+
+    # Base cases
     if 0 <= n_steps <= 2:
         return n_steps
-    if n_steps == 3:
-        return 4
 
     result = 0
-    for i in range(1, 4):
-        if n_steps - i not in cache:
-            cache[n_steps - i] = step_perms_comput_with_cache(n_steps - i, cache)
+    for i in range(1, min(steps_limit, n_steps) + 1):
+        search_key: int = n_steps - i
+
+        if search_key not in cache:
+            cache[search_key] = step_perms_comput_with_cache(
+                search_key,
+                cache,
+                steps_limit
+            )
 
-        result += cache[n_steps - i]
+        result += cache[search_key]
 
-    return result
+    return (result + 1) if n_steps <= steps_limit else result
 
 
 def step_perms(n_steps: int) -> int:
     initial_cache: Dict[int, int] = {}
-    return step_perms_comput_with_cache(n_steps, initial_cache) % (10 ** 10 + 7)
+    return step_perms_comput_with_cache(n_steps, initial_cache, STEPS_LIMIT) % TOP_LIMIT

src/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci_recursive_staircase.testcases.json

@@ -0,0 +1,49 @@
+[
+    {
+        "title": "Sample Test case 0",
+        "tests": [
+            {
+                "input": 1,
+                "expected": 1
+            },
+            {
+                "input": 3,
+                "expected": 4
+            },
+            {
+                "input": 7,
+                "expected": 44
+            }
+        ]
+    },
+    {
+        "title": "Sample Test case 9",
+        "tests": [
+            {
+                "input": 5,
+                "expected": 13
+            },
+            {
+                "input": 8,
+                "expected": 81
+            }
+        ]
+    },
+    {
+        "title": "Sample Test case 10",
+        "tests": [
+            {
+                "input": 15,
+                "expected": 5768
+            },
+            {
+                "input": 20,
+                "expected": 121415
+            },
+            {
+                "input": 27,
+                "expected": 8646064
+            }
+        ]
+    }
+]

src/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci_recursive_staircase_generalized.testcases.json

@@ -0,0 +1,32 @@
+[
+  {
+    "title": "Own sample 1",
+    "tests": [
+      {
+        "input": 4,
+        "limit": 3,
+        "expected": 7
+      }
+    ]
+  },
+  {
+    "title": "Own sample 2",
+    "tests": [
+      {
+        "input": 5,
+        "limit": 4,
+        "expected": 15
+      }
+    ]
+  },
+  {
+    "title": "Own sample 3",
+    "tests": [
+      {
+        "input": 6,
+        "limit": 2,
+        "expected": 13
+      }
+    ]
+  }
+]

src/hackerrank/interview_preparation_kit/recursion_and_backtracking/ctci_recursive_staircase_test.py

@@ -1,56 +1,26 @@
 import unittest
-from .ctci_recursive_staircase import step_perms
+import json
+from pathlib import Path
+from typing import Dict
+
+from .ctci_recursive_staircase import step_perms, step_perms_comput_with_cache
 from .ctci_recursive_staircase_alternative import step_perms as step_perms_alt
 
-TEST_CASES = [
-    {
-        'title': 'Sample Test case 0',
-        'tests': [
-            {
-                'input': 1,
-                'answer': 1
-            },
-            {
-                'input': 3,
-                'answer': 4
-            },
-            {
-                'input': 7,
-                'answer': 44
-            }
-        ]
-    },
-    {
-        'title': 'Sample Test case 9',
-        'tests': [
-            {
-                'input': 5,
-                'answer': 13
-            },
-            {
-                'input': 8,
-                'answer': 81
-            }
-        ]
-    },
-    {
-        'title': 'Sample Test case 10',
-        'tests': [
-            {
-                'input': 15,
-                'answer': 5768
-            },
-            {
-                'input': 20,
-                'answer': 121415
-            },
-            {
-                'input': 27,
-                'answer': 8646064
-            }
-        ]
-    }
-]
+FILE_PATH = str(Path(__file__).resolve().parent)
+
+with open(
+        FILE_PATH +
+        '/ctci_recursive_staircase.testcases.json',
+        encoding="utf-8"
+        ) as file1:
+    TEST_CASES = json.load(file1)
+
+with open(
+        FILE_PATH +
+        '/ctci_recursive_staircase_generalized.testcases.json',
+        encoding="utf-8"
+        ) as file2:
+    TEST_CASES_GENERALIZED = json.load(file2)
 
 
 class TestRecursionFibonacciNumbers(unittest.TestCase):
@@ -73,9 +43,26 @@ def test_step_perms(self):
             for _, _tt in enumerate(testset['tests']):
 
                 self.assertEqual(
-                    step_perms(_tt['input']), _tt['answer'],
+                    step_perms(_tt['input']), _tt['expected'],
                     f"{_} | step_perms({_tt['input']}) must be "
-                    f"=> {_tt['answer']} in {testset['title']}")
+                    f"=> {_tt['expected']} in {testset['title']}")
+
+    def test_step_perms_comput_with_cache(self):
+
+        for _, testset in enumerate(TEST_CASES_GENERALIZED):
+
+            for _, _tt in enumerate(testset['tests']):
+
+                initial_cache: Dict[int, int] = {}
+
+                self.assertEqual(
+                    step_perms_comput_with_cache(
+                        _tt['input'], initial_cache,
+                        _tt['limit']),
+                    _tt['expected'],
+                    f"{_} | step_perms_comput_with_cache("
+                    f"{_tt['input']}, {initial_cache}, {_tt['limit']}) must be "
+                    f"=> {_tt['expected']} in {testset['title']}")
 
     def test_step_perms_alt(self):
 
@@ -84,6 +71,6 @@ def test_step_perms_alt(self):
             for _, _tt in enumerate(testset['tests']):
 
                 self.assertEqual(
-                    step_perms_alt(_tt['input']), _tt['answer'],
+                    step_perms_alt(_tt['input']), _tt['expected'],
                     f"{_} | step_perms_alt({_tt['input']}) must be "
-                    f"=> {_tt['answer']} in {testset['title']}")
+                    f"=> {_tt['expected']} in {testset['title']}")