feat: add fixed point comparison for history

README.md

@@ -359,12 +359,19 @@ Review commits on a per-commit basis:
 :CodeDiff history HEAD~10 --reverse
 :CodeDiff history origin/main..HEAD -r
 :CodeDiff history HEAD~20 % --reverse
+
+" Compare each commit against the current working tree
+:CodeDiff history --base WORKING
+
+" Compare each commit against HEAD
+:CodeDiff history --base HEAD
 ```
 
 The history panel shows a list of commits. Each commit can be expanded to show its changed files. Select a file to view the diff between the commit and its parent (`commit^` vs `commit`).
 
 **Options:**
 - `--reverse` or `-r`: Show commits in chronological order (oldest first) instead of reverse chronological. Useful for following development story from beginning to end, or reviewing PR changes in the order they were made.
+- `--base` or `-b`: Compare each commit against a fixed revision instead of its parent. Accepts any git revision (`HEAD`, branch name, commit hash) or `WORKING` for the current working tree.
 
 **History Keymaps:**
 - `i` - Toggle between list and tree view for files under commits

doc/codediff.txt

@@ -71,6 +71,30 @@ Directory comparison mode:
 Shows files as Added (A), Deleted (D), or Modified (M) based on file size and
 modification time. Select a file to view its diff.
 
+File history mode:                                    *codediff-history*
+>
+  :CodeDiff history
+  :CodeDiff history HEAD~10
+  :CodeDiff history origin/main..HEAD
+  :CodeDiff history HEAD~20 %
+  :CodeDiff history --reverse
+<
+
+Each commit is compared against its parent by default. Use --base to compare
+every commit against a fixed reference instead:
+>
+  :CodeDiff history --base WORKING
+  :CodeDiff history --base HEAD
+  :CodeDiff history --base main
+  :CodeDiff history HEAD~10 -b WORKING %
+<
+
+Options:
+  --reverse, -r   Show commits oldest first
+  --base, -b      Compare each commit against a fixed revision instead of
+                  its parent. Accepts any git revision (HEAD, branch name,
+                  commit hash) or WORKING for the current working tree.
+
 Git merge tool:
 >
   git config --global merge.tool codediff

docs/git-integration.md

@@ -170,6 +170,40 @@ The plugin uses standard git commands:
 - File content fetching is done asynchronously (non-blocking)
 - The diff computation happens after file content is retrieved
 
+## File History with Fixed Base (`--base`)
+
+By default, `:CodeDiff history` compares each commit against its parent.
+The `--base` flag changes this to compare every commit against a fixed reference:
+
+```vim
+" Compare each commit against the current working tree
+:CodeDiff history --base WORKING
+
+" Compare each commit against HEAD
+:CodeDiff history --base HEAD
+
+" Compare each commit against a branch
+:CodeDiff history --base main
+
+" Short form
+:CodeDiff history -b WORKING
+
+" Combine with range, file path, and other flags
+:CodeDiff history origin/main..HEAD --base WORKING %
+:CodeDiff history HEAD~10 -b HEAD --reverse
+```
+
+### Comparison Modes
+
+**Sequential (default):** Each commit compared to its parent.
+Useful for reviewing what changed in each individual commit.
+
+**Fixed base (`--base WORKING`):** Each commit compared to working tree.
+Useful for seeing how far each historical commit is from your current state.
+
+**Fixed base (`--base <revision>`):** Each commit compared to a specific ref.
+Useful for seeing how each commit differs from a branch tip or tag.
+
 ## Future Enhancements
 
 Potential improvements:

lua/codediff/commands.lua

@@ -232,6 +232,7 @@ local function handle_history(range, file_path, flags)
             commits = commits,
             range = range,
             file_path = history_opts.path,
+            base_revision = flags.base,
           },
         }
 
@@ -650,6 +651,7 @@ function M.vscode_diff(opts)
     -- Define flag spec for history command
     local flag_spec = {
       ["--reverse"] = { short = "-r", type = "boolean" },
+      ["--base"] = { short = "-b", type = "string" },
     }
 
     -- Parse args: separate positional from flags

lua/codediff/ui/history/render.lua

@@ -16,6 +16,7 @@ local keymaps_module = require("codediff.ui.history.keymaps")
 -- opts: { range, path, ... } original options
 function M.create(commits, git_root, tabpage, width, opts)
   opts = opts or {}
+  local base_revision = opts.base_revision
 
   -- Get history panel position and size from config (separate from explorer)
   local history_config = config.options.history or {}
@@ -94,6 +95,10 @@ function M.create(commits, git_root, tabpage, width, opts)
     title_text = "Commit History (" .. #commits .. ")"
   end
 
+  if base_revision then
+    title_text = title_text:gsub("%)$", ", base=" .. base_revision .. ")")
+  end
+
   -- Add title node
   tree_nodes[#tree_nodes + 1] = Tree.Node({
     id = "title",
@@ -267,8 +272,9 @@ function M.create(commits, git_root, tabpage, width, opts)
     end
 
     -- Check if already displaying same file
+    local target_hash = base_revision or (commit_hash .. "^")
     local session = lifecycle.get_session(tabpage)
-    if session and session.original_revision == commit_hash .. "^" and session.modified_revision == commit_hash then
+    if session and session.original_revision == target_hash and session.modified_revision == commit_hash then
       if session.modified_path == file_path or session.original_path == file_path then
         return
       end
@@ -279,9 +285,9 @@ function M.create(commits, git_root, tabpage, width, opts)
       local session_config = {
         mode = "history",
         git_root = git_root,
-        original_path = old_path or file_path,
+        original_path = base_revision and file_path or (old_path or file_path),
         modified_path = file_path,
-        original_revision = commit_hash .. "^",
+        original_revision = target_hash,
         modified_revision = commit_hash,
       }
       view.update(tabpage, session_config, true)

lua/codediff/ui/view/init.lua

@@ -422,6 +422,7 @@ function M.create(session_config, filetype, on_ready)
     local history_obj = history.create(commits, session_config.git_root, tabpage, nil, {
       range = session_config.history_data.range,
       file_path = session_config.history_data.file_path,
+      base_revision = session_config.history_data.base_revision,
     })
 
     -- Store history panel reference in lifecycle (reuse explorer slot)

plugin/codediff.lua

@@ -72,7 +72,7 @@ local function complete_codediff(arg_lead, cmd_line, _)
   if first_arg == "history" then
     -- If arg_lead starts with -, complete flags
     if arg_lead:match("^%-") then
-      local flag_candidates = { "--reverse", "-r" }
+      local flag_candidates = { "--reverse", "-r", "--base", "-b" }
       local filtered = {}
       for _, flag in ipairs(flag_candidates) do
         if flag:find(arg_lead, 1, true) == 1 then

tests/history_flags_spec.lua

@@ -65,3 +65,48 @@ describe("History Flag Parsing", function()
     assert.matches("requires a value", err)
   end)
 end)
+
+describe("History --base Flag Parsing", function()
+  local args_parser = require("codediff.core.args")
+  local flag_spec = {
+    ["--reverse"] = { short = "-r", type = "boolean" },
+    ["--base"] = { short = "-b", type = "string" },
+  }
+
+  it("parses --base with space syntax", function()
+    local positional, flags = args_parser.parse_args({ "--base", "WORKING" }, flag_spec)
+    assert.are.same({}, positional)
+    assert.are.same({ base = "WORKING" }, flags)
+  end)
+
+  it("parses -b short form", function()
+    local positional, flags = args_parser.parse_args({ "-b", "HEAD" }, flag_spec)
+    assert.are.same({}, positional)
+    assert.are.same({ base = "HEAD" }, flags)
+  end)
+
+  it("parses --base combined with --reverse", function()
+    local positional, flags = args_parser.parse_args({ "--base", "HEAD", "--reverse" }, flag_spec)
+    assert.are.same({}, positional)
+    assert.are.same({ base = "HEAD", reverse = true }, flags)
+  end)
+
+  it("parses --base with positional args", function()
+    local positional, flags = args_parser.parse_args({ "HEAD~10", "--base", "WORKING", "%" }, flag_spec)
+    assert.are.same({ "HEAD~10", "%" }, positional)
+    assert.are.same({ base = "WORKING" }, flags)
+  end)
+
+  it("returns error for --base without value", function()
+    local positional, flags, err = args_parser.parse_args({ "--base" }, flag_spec)
+    assert.is_nil(positional)
+    assert.is_nil(flags)
+    assert.matches("requires a value", err)
+  end)
+
+  it("parses --base with branch name", function()
+    local positional, flags = args_parser.parse_args({ "--base", "main" }, flag_spec)
+    assert.are.same({}, positional)
+    assert.are.same({ base = "main" }, flags)
+  end)
+end)