Commits & Branches

Every Git commit points to exactly one parent commit (except the very first commit, and merge commits, which have two) — the whole repository history is a graph built entirely from these parent pointers. A branch isn't a copy of anything; it's just a movable label pointing at one commit.

commits = {
    "c3": "c2",
    "c2": "c1",
    "c1": None,   # the root commit has no parent
}

def commit_history(commits, start):
    history = []
    current = start
    while current is not None:
        history.append(current)
        current = commits.get(current)
    return history

print(commit_history(commits, "c3"))   # ['c3', 'c2', 'c1']

Checking whether one commit is an ancestor of another (was it in that commit's history?) is exactly the same kind of graph-walk as the Debugging course's bisection module — follow parent pointers until you either find the target or run out of history:

def is_ancestor(commits, ancestor, descendant):
    current = descendant
    while current is not None:
        if current == ancestor:
            return True
        current = commits.get(current)
    return False

print(is_ancestor(commits, "c1", "c3"))   # True -- c1 is an ancestor of c3
print(is_ancestor(commits, "c3", "c1"))   # False -- c3 came AFTER c1, not before

This is exactly what git merge --ff-only checks before allowing a "fast-forward" merge — if the target branch is already an ancestor of what you're merging in, Git can just move the pointer forward with no actual merge commit needed at all.