Add :linear filter

docs/src/reference/filters.md

@@ -74,6 +74,14 @@ All subdirectories matching the pattern in arbitarily deep subdirectories of the
 ### Match nested files **`::**/X`**
 All files matching the pattern in arbitarily deep subdirectories of the input
 
+## History filters
+
+These filter do not modify git trees, but instead only operate on the commit graph.
+
+### Linearize history **:linear**
+Produce a filtered history that does not contain any merge commits. This is done by
+simply dropping all parents except the first on every commit.
+
 Filter order matters
 --------------------
 

src/filter/mod.rs

@@ -61,8 +61,9 @@ enum Op {
     Nop,
     Empty,
     Fold,
-    Squash,
     Paths,
+    Squash,
+    Linear,
 
     #[cfg(feature = "search")]
     Index,
@@ -185,6 +186,7 @@ fn spec2(op: &Op) -> String {
         Op::Index => ":INDEX".to_string(),
         Op::Fold => ":FOLD".to_string(),
         Op::Squash => ":SQUASH".to_string(),
+        Op::Linear => ":linear".to_string(),
         Op::Subdir(path) => format!(":/{}", path.to_string_lossy()),
         Op::File(path) => format!("::{}", path.to_string_lossy()),
         Op::Prefix(path) => format!(":prefix={}", path.to_string_lossy()),
@@ -274,6 +276,24 @@ fn apply_to_commit2(
         Op::Squash => {
             return Some(history::rewrite_commit(repo, commit, &[], &commit.tree()?)).transpose()
         }
+        Op::Linear => {
+            let p: Vec<_> = commit.parents().collect();
+            if p.len() == 0 {
+                return Ok(Some(commit.id()));
+            }
+            let parent = some_or!(apply_to_commit2(op, &p[0], transaction)?, {
+                return Ok(None);
+            });
+
+            let parent_commit = repo.find_commit(parent)?;
+            return Some(history::rewrite_commit(
+                repo,
+                commit,
+                &[&parent_commit],
+                &commit.tree()?,
+            ))
+            .transpose();
+        }
         _ => {
             if let Some(oid) = transaction.get(filter, commit.id()) {
                 return Ok(Some(oid));
@@ -457,6 +477,7 @@ fn apply2<'a>(
         Op::Empty => return Ok(tree::empty(repo)),
         Op::Fold => Ok(tree),
         Op::Squash => Ok(tree),
+        Op::Linear => Ok(tree),
 
         Op::Glob(pattern) => {
             let pattern = glob::Pattern::new(pattern)?;
@@ -555,6 +576,7 @@ fn unapply2<'a>(
 ) -> JoshResult<git2::Tree<'a>> {
     return match op {
         Op::Nop => Ok(tree),
+        Op::Linear => Ok(tree),
         Op::Empty => Ok(parent_tree),
 
         Op::Chain(a, b) => {

src/filter/parse.rs

@@ -30,6 +30,7 @@ fn make_op(args: &[&str]) -> JoshResult<Op> {
             "#
         ))),
         ["SQUASH"] => Ok(Op::Squash),
+        ["linear"] => Ok(Op::Linear),
         ["PATHS"] => Ok(Op::Paths),
         #[cfg(feature = "search")]
         ["INDEX"] => Ok(Op::Index),

tests/filter/linear.t

@@ -0,0 +1,73 @@
+  $ git init -q 1> /dev/null
+
+  $ echo contents1 > file1
+  $ git add .
+  $ git commit -m "add file1" 1> /dev/null
+
+  $ git log --graph --pretty=%s
+  * add file1
+
+  $ git checkout -b branch2
+  Switched to a new branch 'branch2'
+
+  $ echo contents2 > file1
+  $ git add .
+  $ git commit -m "mod file1" 1> /dev/null
+
+  $ git checkout master
+  Switched to branch 'master'
+
+  $ echo contents3 > file2
+  $ git add .
+  $ git commit -m "add file2" 1> /dev/null
+
+  $ git merge -q branch2 --no-ff
+
+  $ git log --graph --pretty=%s
+  *   Merge branch 'branch2'
+  |\  
+  | * mod file1
+  * | add file2
+  |/  
+  * add file1
+
+  $ josh-filter -s :linear refs/heads/master --update refs/heads/filtered
+
+  $ git log --graph --pretty=%s refs/heads/filtered
+  * Merge branch 'branch2'
+  * add file2
+  * add file1
+
+  $ git ls-tree --name-only -r refs/heads/filtered
+  file1
+  file2
+
+  $ git checkout filtered
+  Switched to branch 'filtered'
+
+  $ echo contents4 > file2
+  $ git add .
+  $ git commit -m "mod file2" 1> /dev/null
+
+  $ git log --graph --pretty=%s refs/heads/filtered
+  * mod file2
+  * Merge branch 'branch2'
+  * add file2
+  * add file1
+
+  $ josh-filter -s :linear refs/heads/master --update refs/heads/filtered --reverse
+
+  $ git log --graph --pretty=%s refs/heads/master
+  * mod file2
+  *   Merge branch 'branch2'
+  |\  
+  | * mod file1
+  * | add file2
+  |/  
+  * add file1
+
+  $ git log --graph --pretty=%s refs/heads/filtered
+  * mod file2
+  * Merge branch 'branch2'
+  * add file2
+  * add file1