diff --git a/src/pipdeptree/_models.py b/src/pipdeptree/_models.py
index 359a982e..3e747cef 100644
--- a/src/pipdeptree/_models.py
+++ b/src/pipdeptree/_models.py
@@ -256,7 +256,7 @@ def get_children(self, node_key: str) -> list[ReqPackage]:
         node = self.get_node_as_parent(node_key)
         return self._obj[node] if node else []
 
-    def filter_nodes(self, include: set[str] | None, exclude: set[str] | None) -> PackageDAG:  # noqa: C901, PLR0912
+    def filter_nodes(self, include: set[str] | None, exclude: set[str] | None) -> PackageDAG:  # noqa: C901
         """
         Filters nodes in a graph by given parameters.
 
@@ -294,23 +294,20 @@ def filter_nodes(self, include: set[str] | None, exclude: set[str] | None) -> Pa
                 continue
             if include is None or any(fnmatch(node.key, i) for i in include):
                 stack.append(node)
-            while True:
-                if len(stack) > 0:
-                    n = stack.pop()
-                    cldn = [c for c in self._obj[n] if not any(fnmatch(c.key, e) for e in exclude)]
-                    m[n] = cldn
-                    seen.add(n.key)
-                    for c in cldn:
-                        if c.key not in seen:
-                            cld_node = self.get_node_as_parent(c.key)
-                            if cld_node:
-                                stack.append(cld_node)
-                            else:
-                                # It means there's no root node corresponding to the child node i.e.
-                                # a dependency is missing
-                                continue
-                else:
-                    break
+            while stack:
+                n = stack.pop()
+                cldn = [c for c in self._obj[n] if not any(fnmatch(c.key, e) for e in exclude)]
+                m[n] = cldn
+                seen.add(n.key)
+                for c in cldn:
+                    if c.key not in seen:
+                        cld_node = self.get_node_as_parent(c.key)
+                        if cld_node:
+                            stack.append(cld_node)
+                        else:
+                            # It means there's no root node corresponding to the child node i.e.
+                            # a dependency is missing
+                            continue
 
         return self.__class__(m)
 
@@ -335,10 +332,7 @@ def reverse(self) -> ReversedPackageDAG:
                 # if v is already added to the dict, then ensure that
                 # we are using the same object. This check is required
                 # as we're using array mutation
-                try:
-                    node: ReqPackage = [p for p in m if p.key == v.key][0]
-                except IndexError:  # noqa: PERF203
-                    node = v
+                node: ReqPackage = next((p for p in m if p.key == v.key), v)
                 m[node].append(k.as_parent_of(v))
             if k.key not in child_keys:
                 m[k.as_requirement()] = []
@@ -383,10 +377,7 @@ def reverse(self) -> PackageDAG:  # type: ignore[override]
         child_keys = {r.key for r in chain.from_iterable(self._obj.values())}
         for k, vs in self._obj.items():
             for v in vs:
-                try:
-                    node = [p for p in m if p.key == v.key][0]
-                except IndexError:  # noqa: PERF203
-                    node = v.as_parent_of(None)
+                node = next((p for p in m if p.key == v.key), v.as_parent_of(None))
                 m[node].append(k)  # type: ignore[arg-type]
             if k.key not in child_keys:
                 m[k.dist] = []
diff --git a/src/pipdeptree/_render/mermaid.py b/src/pipdeptree/_render/mermaid.py
index f5b52c78..00ec6e33 100644
--- a/src/pipdeptree/_render/mermaid.py
+++ b/src/pipdeptree/_render/mermaid.py
@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from textwrap import dedent
+import itertools as it
 from typing import TYPE_CHECKING, Final
 
 from pipdeptree._models import ReversedPackageDAG
@@ -56,13 +56,12 @@ def mermaid_id(key: str) -> str:
             node_ids_map[key] = key
             return key
         # If the key is a reserved keyword, append a number to it.
-        number = 0
-        while True:
+        for number in it.count():
             new_id = f"{key}_{number}"
             if new_id not in node_ids_map:
                 node_ids_map[key] = new_id
                 return new_id
-            number += 1
+        raise NotImplementedError
 
     # Use a sets to avoid duplicate entries.
     nodes: set[str] = set()
@@ -95,20 +94,13 @@ def mermaid_id(key: str) -> str:
                     edges.add(f'{package_key} -- "{edge_label}" --> {dependency_key}')
 
     # Produce the Mermaid Markdown.
-    indent = " " * 4
-    output = dedent(
-        f"""\
-        flowchart TD
-        {indent}classDef missing stroke-dasharray: 5
-        """,
-    )
-    # Sort the nodes and edges to make the output deterministic.
-    output += indent
-    output += f"\n{indent}".join(node for node in sorted(nodes))
-    output += "\n" + indent
-    output += f"\n{indent}".join(edge for edge in sorted(edges))
-    output += "\n"
-    return output
+    lines = [
+        "flowchart TD",
+        "classDef missing stroke-dasharray: 5",
+        *sorted(nodes),
+        *sorted(edges),
+    ]
+    return "".join(f"{'    ' if i else ''}{line}\n" for i, line in enumerate(lines))
 
 
 __all__ = [
diff --git a/src/pipdeptree/_validate.py b/src/pipdeptree/_validate.py
index c1bf4bb4..f1e93551 100644
--- a/src/pipdeptree/_validate.py
+++ b/src/pipdeptree/_validate.py
@@ -76,7 +76,7 @@ def cyclic_deps(tree: PackageDAG) -> list[tuple[DistPackage, ReqPackage, ReqPack
                 if val is not None:
                     entry = tree.get(val)
                     if entry is not None:
-                        p_as_dep_of_r = [x for x in entry if x.key == p.key][0]
+                        p_as_dep_of_r = next(x for x in entry if x.key == p.key)
                         cyclic.append((p, r, p_as_dep_of_r))
     return cyclic
 
diff --git a/tests/conftest.py b/tests/conftest.py
index b38d4276..b47c9d2d 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -21,8 +21,7 @@ def func(simple_graph: MockGraph) -> Iterator[Mock]:
             as_req = Mock(key=nk, project_name=nk, specs=[("==", nv)])
             m.as_requirement = Mock(return_value=as_req)
             reqs = []
-            for child in children:
-                ck, cv = child
+            for ck, cv in children:
                 r = Mock(key=ck, project_name=ck, specs=cv)
                 reqs.append(r)
             m.requires = Mock(return_value=reqs)