Add main implementation with a placeholder for should_include_hunk

bin/mechanicaldiff.py

@@ -3,9 +3,60 @@
 import sys
 
 
-def main():
-    data = sys.stdin.read()
-    sys.stdout.write(data)
+def should_include_hunk(hunk_text: str) -> bool:
+    return True
+
+
+def main() -> None:
+    lines = sys.stdin.read().splitlines(keepends=True)
+    preamble_lines = []
+    sections = []
+    current = None
+
+    for line in lines:
+        if line.startswith("diff --git "):
+            if current is not None:
+                sections.append(current)
+            current = {"header": [line], "hunks": []}
+            continue
+
+        if current is None:
+            preamble_lines.append(line)
+            continue
+
+        if line.startswith("@@ "):
+            current["hunks"].append([line])
+        else:
+            if current["hunks"]:
+                current["hunks"][-1].append(line)
+            else:
+                current["header"].append(line)
+
+    if current is not None:
+        sections.append(current)
+
+    output_lines = []
+    output_lines.extend(preamble_lines)
+
+    for section in sections:
+        if not section["hunks"]:
+            output_lines.extend(section["header"])
+            continue
+
+        kept_hunks = []
+        for hunk_lines in section["hunks"]:
+            hunk_text = "".join(hunk_lines)
+            if should_include_hunk(hunk_text):
+                kept_hunks.append(hunk_lines)
+
+        if not kept_hunks:
+            continue
+
+        output_lines.extend(section["header"])
+        for hunk_lines in kept_hunks:
+            output_lines.extend(hunk_lines)
+
+    sys.stdout.write("".join(output_lines))
 
 
 if __name__ == "__main__":