From 98d82c145abc5c1e794526a36eb0196c93cef302 Mon Sep 17 00:00:00 2001
From: perro <hi@perrotuerto.blog>
Date: Mon, 22 May 2023 10:49:10 -0700
Subject: [PATCH] Ejercicio 15a

---
 exercises/ex15/stack.py      | 70 ++++++++++++++++++++++++++++++++++++
 exercises/ex15/test_stack.py | 32 +++++++++++++++++
 2 files changed, 102 insertions(+)
 create mode 100644 exercises/ex15/stack.py
 create mode 100644 exercises/ex15/test_stack.py

diff --git a/exercises/ex15/stack.py b/exercises/ex15/stack.py
new file mode 100644
index 0000000..80e3df5
--- /dev/null
+++ b/exercises/ex15/stack.py
@@ -0,0 +1,70 @@
+class StackNode(object):
+    def __init__(self, value, nxt):
+        self.value = value
+        self.next = nxt
+
+    def __repr__(self):
+        nval = self.next and self.next.value or None
+        return f"[{self.value}:{repr(nval)}]"
+
+
+class Stack(object):
+    def __init__(self):
+        self.topmost = None
+
+    def push(self, obj):
+        """
+        Pushes a new value to the top of the stack.
+        Añade un nuevo valor en el tope del stack.
+        """
+        if not self.topmost:
+            self.topmost = StackNode(obj, None)
+        else:
+            self.topmost = StackNode(obj, self.topmost)
+
+    def pop(self):
+        """
+        Pops the value that is currently on the top of the stack.
+        Remueve el valor que está en el tope del stack.
+        """
+        if self.topmost:
+            node = self.topmost
+            if self.topmost.next:
+                self.topmost = self.topmost.next
+            else:
+                self.topmost = None
+            return node.value
+
+    def top(self):
+        """
+        Returns the reference to the first item, does not remove.
+        Regresa la referencia al primer ítem sin removerlo.
+        """
+        if self.topmost:
+            return self.topmost.value
+
+    def count(self):
+        """
+        Returns number of items.
+        Regresa la cantidad de ítems.
+        """
+        count = 0
+        if self.topmost:
+            node = self.topmost
+            while node:
+                count += 1
+                node = node.next
+        return count
+
+    def dump(self, mark="----"):
+        """
+        Debugging function that dumps the contents of the stack.
+        Función de depuración que arroja los contenidos del stack.
+        """
+        if self.topmost:
+            node = self.topmost
+            print(node.value)
+            while node.next:
+                node = node.next
+                print(mark)
+                print(node.value)
diff --git a/exercises/ex15/test_stack.py b/exercises/ex15/test_stack.py
new file mode 100644
index 0000000..2defd41
--- /dev/null
+++ b/exercises/ex15/test_stack.py
@@ -0,0 +1,32 @@
+from stack import Stack
+
+
+def test_push():
+    colors = Stack()
+    colors.push("Pthalo Blue")
+    assert colors.count() == 1
+    colors.push("Ultramarine Blue")
+    assert colors.count() == 2
+
+
+def test_pop():
+    colors = Stack()
+    colors.push("Magenta")
+    colors.push("Alizarin")
+    assert colors.pop() == "Alizarin"
+    assert colors.count() == 1
+    assert colors.pop() == "Magenta"
+    assert colors.count() == 0
+    assert colors.pop() is None
+
+
+def test_top():
+    colors = Stack()
+    colors.push("Cadmium Red Light")
+    assert colors.top() == "Cadmium Red Light"
+    colors.push("Hansa Yellow")
+    assert colors.top() == "Hansa Yellow"
+    colors.pop()
+    assert colors.top() == "Cadmium Red Light"
+    colors.pop()
+    assert colors.top() is None