1 files changed, 89 insertions, 0 deletions
diff --git a/patchtree/header.py b/patchtree/header.py
new file mode 100644
index 0000000..d797ff0
--- /dev/null
+++ b/patchtree/header.py
@@ -0,0 +1,89 @@
+from __future__ import annotations
+from typing import TYPE_CHECKING
+
+from importlib import metadata
+import shlex
+
+if TYPE_CHECKING:
+    from .context import Context
+    from .config import Config
+
+
+class Header:
+    """
+    Patch output header generator.
+
+    The header is formatted as
+
+    #. shebang
+    #. patchtree version info
+    #. extra version info (empty by default)
+    #. license (empty by default)
+    """
+
+    config: Config
+    context: Context
+
+    name = "patchtree"
+    """Program name shown in version info."""
+
+    license = None
+    """License text (optional)."""
+
+    def __init__(self, config: Config, context: Context):
+        self.config = config
+        self.context = context
+
+    def write(self) -> str:
+        return "".join(
+            (
+                self.write_shebang(),
+                self.write_version(),
+                self.write_version_extra(),
+                self.write_license(),
+            )
+        )
+
+    def write_shebang(self) -> str:
+        """
+        Write a shebang line to apply the output patch unless the ``--no-shebang`` option was passed.
+        """
+
+        if self.config.no_shebang:
+            return ""
+
+        # NOTE: the GIT_DIR environment variable is set in order to allow users to apply the
+        # .patch file to a target tree already tracked using git (which may be under a
+        # different directory relative to the repository root, causing the patch to be skipped
+        # silently). This effectively makes git-apply always behave as if it is outside a git
+        # tree, while still applying changes described using the extended git diff format.
+        cmd = ["/usr/bin/env", "-S", "GIT_DIR="]
+        cmd.append(shlex.join(self.context.get_apply_cmd()))
+        cmdline = " ".join(cmd)
+        return f"#!{cmdline}\n"
+
+    def write_version(self) -> str:
+        """
+        Write the patchtree name and version number.
+        """
+
+        version = metadata.version("patchtree")
+        return f"{self.name} output (version {version})\n"
+
+    def write_version_extra(self) -> str:
+        """
+        Write extra version information (empty).
+
+        This method is meant to be implemented by subclasses of Header defined in the :ref:`configuration file <ptconfig>`.
+        """
+
+        return ""
+
+    def write_license(self) -> str:
+        """
+        Write a license if it is defined.
+        """
+
+        if self.license is None:
+            return ""
+        return f"{self.license}\n"