git-commit: introduce git-commit crate

Porting the commit code that is found in heartwood[0]. It represents a
parsed git commit object -- sans the id for the commit itself.

[0]: https://github.com/radicle-dev/heartwood/blob/16b22d9178fa6a1aaf2ecb01ec97e067399c5286/radicle/src/git.rs#L392-L575

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

git-commit/src/headers.rs

@@ -0,0 +1,71 @@
+use std::borrow::Cow;
+
+const BEGIN_SSH: &str = "-----BEGIN SSH SIGNATURE-----\n";
+const BEGIN_PGP: &str = "-----BEGIN PGP SIGNATURE-----\n";
+
+/// A collection of headers stored in a [`super::Commit`].
+///
+/// Note: these do not include `tree`, `parent`, `author`, and `committer`.
+#[derive(Clone, Debug, Default)]
+pub struct Headers(pub(super) Vec<(String, String)>);
+
+/// A `gpgsig` signature stored in a [`super::Commit`].
+pub enum Signature<'a> {
+    /// A PGP signature, i.e. starts with `-----BEGIN PGP SIGNATURE-----`.
+    Pgp(Cow<'a, str>),
+    /// A SSH signature, i.e. starts with `-----BEGIN SSH SIGNATURE-----`.
+    Ssh(Cow<'a, str>),
+}
+
+impl<'a> Signature<'a> {
+    fn from_str(s: &'a str) -> Result<Self, UnknownScheme> {
+        if s.starts_with(BEGIN_SSH) {
+            Ok(Signature::Ssh(Cow::Borrowed(s)))
+        } else if s.starts_with(BEGIN_PGP) {
+            Ok(Signature::Pgp(Cow::Borrowed(s)))
+        } else {
+            Err(UnknownScheme)
+        }
+    }
+}
+
+pub struct UnknownScheme;
+
+impl<'a> ToString for Signature<'a> {
+    fn to_string(&self) -> String {
+        match self {
+            Signature::Pgp(pgp) => pgp.to_string(),
+            Signature::Ssh(ssh) => ssh.to_string(),
+        }
+    }
+}
+
+impl Headers {
+    pub fn new() -> Self {
+        Headers(Vec::new())
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = (&str, &str)> {
+        self.0.iter().map(|(k, v)| (k.as_str(), v.as_str()))
+    }
+
+    pub fn values<'a>(&'a self, name: &'a str) -> impl Iterator<Item = &'a str> + '_ {
+        self.iter()
+            .filter_map(move |(k, v)| (k == name).then_some(v))
+    }
+
+    pub fn signatures(&self) -> impl Iterator<Item = Signature> + '_ {
+        self.0.iter().filter_map(|(k, v)| {
+            if k == "gpgsig" {
+                Signature::from_str(v).ok()
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Push a header to the end of the headers section.
+    pub fn push(&mut self, name: &str, value: &str) {
+        self.0.push((name.to_owned(), value.trim().to_owned()));
+    }
+}