fix: hunk nav and insertion anchoring

lua/jumpy/diff.lua

@@ -77,7 +77,7 @@ local function myers_diff(old_lines, new_lines)
 
     if d > 1 then
       if x == prev_x then
-        table.insert(edits, 1, { op = "insert", new_idx = y })
+        table.insert(edits, 1, { op = "insert", old_idx = x + 1, new_idx = y })
         y = y - 1
       else
         table.insert(edits, 1, { op = "delete", old_idx = x })
@@ -120,6 +120,9 @@ function M.compute(old_lines, new_lines)
         current_hunk.old_count = current_hunk.old_count + 1
         table.insert(current_hunk.removed_lines, old_lines[edit.old_idx])
       elseif edit.op == "insert" then
+        if not current_hunk.old_start then
+          current_hunk.old_start = edit.old_idx
+        end
         if not current_hunk.new_start then
           current_hunk.new_start = edit.new_idx
         end

lua/jumpy/navigate.lua

@@ -5,6 +5,23 @@ local render = require("jumpy.render")
 local MSG_NO_HUNKS = "jumpy: no hunks"
 local MSG_NO_HUNK_UNDER_CURSOR = "jumpy: no hunk under cursor"
 
+local function center_cursor()
+  vim.cmd("normal! zz")
+end
+
+local function set_cursor_centered(line)
+  local line_count = vim.api.nvim_buf_line_count(0)
+  local target_line = math.min(math.max(line, 1), line_count)
+  vim.api.nvim_win_set_cursor(0, { target_line, 0 })
+  center_cursor()
+end
+
+local function current_hunk_line(bufnr, idx, hunk)
+  local line = hunk.old_start + M._get_offset(bufnr, idx)
+  local line_count = vim.api.nvim_buf_line_count(bufnr)
+  return math.min(math.max(line, 1), line_count)
+end
+
 local function get_active_hunks(bufnr)
   local state = render.get_state(bufnr)
   if not state then
@@ -32,8 +49,8 @@ function M.hunk_at_cursor()
 
   for _, entry in ipairs(active) do
     local hunk = entry.hunk
-    local hunk_start = hunk.old_start
-    local hunk_end = hunk.old_start + math.max(hunk.old_count, 1) - 1
+    local hunk_start = current_hunk_line(bufnr, entry.idx, hunk)
+    local hunk_end = hunk_start + math.max(hunk.old_count, 1) - 1
 
     if cursor_line >= hunk_start and cursor_line <= hunk_end then
       return entry.idx
@@ -54,13 +71,14 @@ function M.next_hunk()
   end
 
   for _, entry in ipairs(active) do
-    if entry.hunk.old_start > cursor_line then
-      vim.api.nvim_win_set_cursor(0, { entry.hunk.old_start, 0 })
+    local hunk_line = current_hunk_line(bufnr, entry.idx, entry.hunk)
+    if hunk_line > cursor_line then
+      set_cursor_centered(hunk_line)
       return
     end
   end
 
-  vim.api.nvim_win_set_cursor(0, { active[1].hunk.old_start, 0 })
+  set_cursor_centered(current_hunk_line(bufnr, active[1].idx, active[1].hunk))
 end
 
 function M.prev_hunk()
@@ -74,13 +92,14 @@ function M.prev_hunk()
   end
 
   for i = #active, 1, -1 do
-    if active[i].hunk.old_start < cursor_line then
-      vim.api.nvim_win_set_cursor(0, { active[i].hunk.old_start, 0 })
+    local hunk_line = current_hunk_line(bufnr, active[i].idx, active[i].hunk)
+    if hunk_line < cursor_line then
+      set_cursor_centered(hunk_line)
       return
     end
   end
 
-  vim.api.nvim_win_set_cursor(0, { active[#active].hunk.old_start, 0 })
+  set_cursor_centered(current_hunk_line(bufnr, active[#active].idx, active[#active].hunk))
 end
 
 function M.accept()
@@ -267,7 +286,7 @@ local function jump_to(bufnr, line)
   end
   vim.cmd("normal! m'")
   vim.api.nvim_win_set_buf(0, bufnr)
-  vim.api.nvim_win_set_cursor(0, { line, 0 })
+  set_cursor_centered(line)
 end
 
 function M.first_hunk_any_buf()
@@ -285,12 +304,13 @@ function M._advance_to_next(bufnr)
   if #active > 0 then
     local cursor_line = vim.api.nvim_win_get_cursor(0)[1]
     for _, entry in ipairs(active) do
-      if entry.hunk.old_start >= cursor_line then
-        vim.api.nvim_win_set_cursor(0, { entry.hunk.old_start, 0 })
+      local hunk_line = current_hunk_line(bufnr, entry.idx, entry.hunk)
+      if hunk_line >= cursor_line then
+        set_cursor_centered(hunk_line)
         return
       end
     end
-    vim.api.nvim_win_set_cursor(0, { active[1].hunk.old_start, 0 })
+    set_cursor_centered(current_hunk_line(bufnr, active[1].idx, active[1].hunk))
   else
     offset_table[bufnr] = nil
     local all = get_all_active_hunks()

lua/jumpy/render.lua

@@ -10,6 +10,13 @@ local function build_virt_lines(added_lines)
   return virt_lines
 end
 
+local function insertion_anchor(bufnr, old_start)
+  local line_count = vim.api.nvim_buf_line_count(bufnr)
+  local is_eof = old_start > line_count
+  local anchor_line = is_eof and line_count - 1 or old_start - 1
+  return math.max(0, anchor_line), not is_eof
+end
+
 local buf_states = {}
 
 function M.get_state(bufnr)
@@ -89,12 +96,11 @@ function M.show(bufnr, hunks, original_lines, proposed_lines)
 
       local virt_lines = build_virt_lines(hunk.added_lines)
 
-      local anchor_line = math.max(0, hunk.old_start - 1)
-      anchor_line = math.min(anchor_line, vim.api.nvim_buf_line_count(bufnr) - 1)
+      local anchor_line, virt_lines_above = insertion_anchor(bufnr, hunk.old_start)
 
       local id = vim.api.nvim_buf_set_extmark(bufnr, ns, anchor_line, 0, {
         virt_lines = virt_lines,
-        virt_lines_above = anchor_line ~= 0,
+        virt_lines_above = virt_lines_above,
         sign_text = "+",
         sign_hl_group = "JumpyAddedSign",
         priority = 100,
@@ -179,17 +185,17 @@ function M.update_hunk_lines(bufnr, hunk_idx, new_added_lines)
     local virt_lines = build_virt_lines(new_added_lines)
 
     local anchor_line
+    local virt_lines_above = false
     if hunk.old_count > 0 then
       anchor_line = math.min(hunk.old_start - 1 + hunk.old_count - 1, vim.api.nvim_buf_line_count(bufnr) - 1)
       anchor_line = math.max(0, anchor_line)
     else
-      anchor_line = math.max(0, hunk.old_start - 1)
-      anchor_line = math.min(anchor_line, vim.api.nvim_buf_line_count(bufnr) - 1)
+      anchor_line, virt_lines_above = insertion_anchor(bufnr, hunk.old_start)
     end
 
     local id = vim.api.nvim_buf_set_extmark(bufnr, ns, anchor_line, 0, {
       virt_lines = virt_lines,
-      virt_lines_above = hunk.old_count == 0 and anchor_line ~= 0,
+      virt_lines_above = virt_lines_above,
       priority = 100,
     })
     table.insert(hunk.extmarks, id)

tests/diff_spec.lua

@@ -89,6 +89,7 @@ describe("diff.compute", function()
     local hunks = diff.compute(old, new)
 
     assert.are.equal(1, #hunks)
+    assert.are.equal(1, hunks[1].old_start)
     assert.are.same({ "a" }, hunks[1].added_lines)
   end)
 
@@ -98,9 +99,45 @@ describe("diff.compute", function()
     local hunks = diff.compute(old, new)
 
     assert.are.equal(1, #hunks)
+    assert.are.equal(3, hunks[1].old_start)
     assert.are.same({ "c" }, hunks[1].added_lines)
   end)
 
+  it("keeps repeated insertion hunks anchored to old-file lines", function()
+    local old = {
+      "local function one()",
+      "end",
+      "",
+      "local function two()",
+      "end",
+      "",
+      "local function three()",
+      "end",
+    }
+    local new = {
+      "--- One.",
+      "local function one()",
+      "end",
+      "",
+      "--- Two.",
+      "local function two()",
+      "end",
+      "",
+      "--- Three.",
+      "local function three()",
+      "end",
+    }
+    local hunks = diff.compute(old, new)
+
+    assert.are.equal(3, #hunks)
+    assert.are.equal(1, hunks[1].old_start)
+    assert.are.equal(4, hunks[2].old_start)
+    assert.are.equal(7, hunks[3].old_start)
+    assert.are.same({ "--- One." }, hunks[1].added_lines)
+    assert.are.same({ "--- Two." }, hunks[2].added_lines)
+    assert.are.same({ "--- Three." }, hunks[3].added_lines)
+  end)
+
   it("handles complete replacement", function()
     local old = { "a", "b" }
     local new = { "x", "y", "z" }