1 files changed, 62 insertions, 0 deletions
diff --git a/src/branches.rs b/src/branches.rs
new file mode 100644
index 0000000..0e1e04f
--- /dev/null
+++ b/src/branches.rs
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later WITH GPL-3.0-linking-exception
+// SPDX-FileCopyrightText: 2021 Alyssa Ross <hi@alyssa.is>
+
+use std::borrow::Cow;
+use std::collections::BTreeMap;
+
+use once_cell::sync::Lazy;
+use regex::{Regex, RegexSet};
+
+const NEXT_BRANCH_TABLE: [(&str, &str); 8] = [
+    (r"\Astaging(-[\d.]+)?\z", "staging-next$1"),
+    (r"\Astaging-next\z", "master"),
+    (r"\Amaster\z", "nixpkgs-unstable"),
+    (r"\Amaster\z", "nixos-unstable-small"),
+    (r"\Anixos-(.*)-small\z", "nixos-$1"),
+    (r"\Arelease-([\d.]+)\z", "nixpkgs-$1-darwin"),
+    (r"\Arelease-([\d.]+)\z", "nixos-$1-small"),
+    (r"\Astaging-next-([\d.]*)\z", "release-$1"),
+];
+
+static BRANCH_NEXTS: Lazy<BTreeMap<&str, Vec<&str>>> = Lazy::new(|| {
+    NEXT_BRANCH_TABLE
+        .iter()
+        .fold(BTreeMap::new(), |mut map, (pattern, next)| {
+            map.entry(pattern).or_insert_with(Vec::new).push(next);
+            map
+        })
+});
+
+static BRANCH_NEXTS_BY_INDEX: Lazy<Vec<&Vec<&str>>> = Lazy::new(|| BRANCH_NEXTS.values().collect());
+
+static BRANCH_PATTERNS: Lazy<Vec<Regex>> = Lazy::new(|| {
+    BRANCH_NEXTS
+        .keys()
+        .copied()
+        .map(Regex::new)
+        .map(Result::unwrap)
+        .collect()
+});
+
+static BRANCH_REGEXES: Lazy<RegexSet> = Lazy::new(|| RegexSet::new(BRANCH_NEXTS.keys()).unwrap());
+
+pub fn next_branches(branch: &str) -> Vec<Cow<str>> {
+    BRANCH_REGEXES
+        .matches(branch)
+        .iter()
+        .flat_map(|index| {
+            let regex = BRANCH_PATTERNS.get(index).unwrap();
+            BRANCH_NEXTS_BY_INDEX
+                .get(index)
+                .unwrap()
+                .iter()
+                .map(move |next| regex.replace(branch, *next))
+        })
+        .collect()
+}
+
+#[test]
+fn test_next_branches() {
+    let res = next_branches("release-20.09");
+    assert_eq!(res, vec!["nixpkgs-20.09-darwin", "nixos-20.09-small"])
+}