Add command to find base commit for creating a fixup

23 files changed, 420 insertions, 0 deletions

diff --git a/docs/Config.md b/docs/Config.md
index 741a2c2b9..8b37c6aaa 100644
--- a/docs/Config.md
+++ b/docs/Config.md
@@ -209,6 +209,7 @@ keybinding:
     commitChangesWithoutHook: 'w' # commit changes without pre-commit hook
     amendLastCommit: 'A'
     commitChangesWithEditor: 'C'
+    findBaseCommitForFixup: '<c-f>'
     confirmDiscard: 'x'
     ignoreFile: 'i'
     refreshFiles: 'r'
diff --git a/docs/Fixup_Commits.md b/docs/Fixup_Commits.md
new file mode 100644
index 000000000..1d148c546
--- /dev/null
+++ b/docs/Fixup_Commits.md
@@ -0,0 +1,64 @@
+# Fixup Commits
+
+## Background
+
+There's this common scenario that you have a PR in review, the reviewer is
+requesting some changes, and you make those changes and would normally simply
+squash them into the original commit that they came from. If you do that,
+however, there's no way for the reviewer to see what you changed. You could just
+make a separate commit with those changes at the end of the branch, but this is
+not ideal because it results in a git history that is not very clean.
+
+To help with this, git has a concept of fixup commits: you do make a separate
+commit, but the subject of this commit is the string "fixup! " followed by the
+original commit subject. This both tells the reviewer what's going on (you are
+making a change that you later will squash into the designated commit), and it
+provides an easy way to actually perform this squash operation when you are
+ready to do that (before merging).
+
+## Creating fixup commits
+
+You could of course create fixup commits manually by typing in the commit
+message with the prefix yourself. But lazygit has an easier way to do that:
+in the Commits view, select the commit that you want to create a fixup for, and
+press shift-F (for "Create fixup commit for this commit"). This automatically
+creates a commit with the appropriate subject line.
+
+Don't confuse this with the lowercase "f" command ("Fixup commit"); that one
+squashes the selected commit into its parent, this is not what we want here.
+
+## Squashing fixup commits
+
+When you're ready to merge the branch and want to squash all these fixup commits
+that you created, that's very easy to do: select the first commit of your branch
+and hit shift-S (for "Squash all 'fixup!' commits above selected commit
+(autosquash)"). Boom, done.
+
+## Finding the commit to create a fixup for
+
+When you are making changes to code that you changed earlier in a long branch,
+it can be tedious to find the commit to squash it into. Lazygit has a command to
+help you with this, too: in the Files view, press ctrl-f to select the right
+base commit in the Commits view automatically. From there, you can either press
+shift-F to create a fixup commit for it, or shift-A to amend your changes into
+the commit if you haven't published your branch yet.
+
+This command works in many cases, and when it does it almost feels like magic,
+but it's important to understand its limitations because it doesn't always work.
+The way it works is that it looks at the deleted lines of your current
+modifications, blames them to find out which commit those lines come from, and
+if they all come from the same commit, it selects it. So here are cases where it
+doesn't work:
+
+- Your current diff has only added lines, but no deleted lines. In this case
+  there's no way for lazygit to know which commit you want to add them to.
+- The deleted lines belong to multiple different commits. In this case you can
+  help lazygit by staging a set of files or hunks that all belong to the same
+  commit; if some changes are staged, the ctrl-f command works only on those.
+- The found commit is already on master; in this case, lazygit refuses to select
+  it, because it doesn't make sense to create fixups for it, let alone amend to
+  it.
+
+To sum it up: the command works great if you are changing code again that you
+changed or added earlier in the same branch. This is a common enough case to
+make the command useful.
diff --git a/docs/keybindings/Keybindings_en.md b/docs/keybindings/Keybindings_en.md
index f7aaa46c8..ef6299613 100644
--- a/docs/keybindings/Keybindings_en.md
+++ b/docs/keybindings/Keybindings_en.md
@@ -123,6 +123,7 @@ _Legend: `<c-b>` means ctrl+b, `<a-b>` means alt+b, `B` means shift+b_
   <kbd>w</kbd>: Commit changes without pre-commit hook
   <kbd>A</kbd>: Amend last commit
   <kbd>C</kbd>: Commit changes using git editor
+  <kbd>&lt;c-f&gt;</kbd>: Find base commit for fixup
   <kbd>e</kbd>: Edit file
   <kbd>o</kbd>: Open file
   <kbd>i</kbd>: Ignore or exclude file
diff --git a/docs/keybindings/Keybindings_ja.md b/docs/keybindings/Keybindings_ja.md
index a5a54b872..36389ecc1 100644
--- a/docs/keybindings/Keybindings_ja.md
+++ b/docs/keybindings/Keybindings_ja.md
@@ -196,6 +196,7 @@ _Legend: `<c-b>` means ctrl+b, `<a-b>` means alt+b, `B` means shift+b_
   <kbd>w</kbd>: pre-commitフックを実行せずに変更をコミット
   <kbd>A</kbd>: 最新のコミットにamend
   <kbd>C</kbd>: gitエディタを使用して変更をコミット
+  <kbd>&lt;c-f&gt;</kbd>: Find base commit for fixup
   <kbd>e</kbd>: ファイルを編集
   <kbd>o</kbd>: ファイルを開く
   <kbd>i</kbd>: ファイルをignore
diff --git a/docs/keybindings/Keybindings_ko.md b/docs/keybindings/Keybindings_ko.md
index fc7291c4b..d96283192 100644
--- a/docs/keybindings/Keybindings_ko.md
+++ b/docs/keybindings/Keybindings_ko.md
@@ -336,6 +336,7 @@ _Legend: `<c-b>` means ctrl+b, `<a-b>` means alt+b, `B` means shift+b_
   <kbd>w</kbd>: Commit changes without pre-commit hook
   <kbd>A</kbd>: 마지맛 커밋 수정
   <kbd>C</kbd>: Git 편집기를 사용하여 변경 내용을 커밋합니다.
+  <kbd>&lt;c-f&gt;</kbd>: Find base commit for fixup
   <kbd>e</kbd>: 파일 편집
   <kbd>o</kbd>: 파일 닫기
   <kbd>i</kbd>: Ignore file
diff --git a/docs/keybindings/Keybindings_nl.md b/docs/keybindings/Keybindings_nl.md
index c2d81a06d..23bf0a477 100644
--- a/docs/keybindings/Keybindings_nl.md
+++ b/docs/keybindings/Keybindings_nl.md
@@ -56,6 +56,7 @@ _Legend: `<c-b>` means ctrl+b, `<a-b>` means alt+b, `B` means shift+b_
   <kbd>w</kbd>: Commit veranderingen zonder pre-commit hook
   <kbd>A</kbd>: Wijzig laatste commit
   <kbd>C</kbd>: Commit veranderingen met de git editor
+  <kbd>&lt;c-f&gt;</kbd>: Find base commit for fixup
   <kbd>e</kbd>: Verander bestand
   <kbd>o</kbd>: Open bestand
   <kbd>i</kbd>: Ignore or exclude file
diff --git a/docs/keybindings/Keybindings_pl.md b/docs/keybindings/Keybindings_pl.md
index 2136a5cdf..601ec72f5 100644
--- a/docs/keybindings/Keybindings_pl.md
+++ b/docs/keybindings/Keybindings_pl.md
@@ -157,6 +157,7 @@ _Legend: `<c-b>` means ctrl+b, `<a-b>` means alt+b, `B` means shift+b_
   <kbd>w</kbd>: Zatwierdź zmiany bez skryptu pre-commit
   <kbd>A</kbd>: Zmień ostatni commit
   <kbd>C</kbd>: Zatwierdź zmiany używając edytora
+  <kbd>&lt;c-f&gt;</kbd>: Find base commit for fixup
   <kbd>e</kbd>: Edytuj plik
   <kbd>o</kbd>: Otwórz plik
   <kbd>i</kbd>: Ignore or exclude file
diff --git a/docs/keybindings/Keybindings_ru.md b/docs/keybindings/Keybindings_ru.md
index ab82515ad..6277f3db8 100644
--- a/docs/keybindings/Keybindings_ru.md
+++ b/docs/keybindings/Keybindings_ru.md
@@ -330,6 +330,7 @@ _Связки клавиш_
   <kbd>w</kbd>: Закоммитить изменения без предварительного хука коммита
   <kbd>A</kbd>: Правка последнего коммита
   <kbd>C</kbd>: Сохранить изменения с помощью редактора git
+  <kbd>&lt;c-f&gt;</kbd>: Find base commit for fixup
   <kbd>e</kbd>: Редактировать файл
   <kbd>o</kbd>: Открыть файл
   <kbd>i</kbd>: Игнорировать или исключить файл
diff --git a/docs/keybindings/Keybindings_zh-CN.md b/docs/keybindings/Keybindings_zh-CN.md
index 49bce5ece..4b4a8e028 100644
--- a/docs/keybindings/Keybindings_zh-CN.md
+++ b/docs/keybindings/Keybindings_zh-CN.md
@@ -204,6 +204,7 @@ _Legend: `<c-b>` means ctrl+b, `<a-b>` means alt+b, `B` means shift+b_
   <kbd>w</kbd>: 提交更改而无需预先提交钩子
   <kbd>A</kbd>: 修补最后一次提交
   <kbd>C</kbd>: 提交更改（使用编辑器编辑提交信息）
+  <kbd>&lt;c-f&gt;</kbd>: Find base commit for fixup
   <kbd>e</kbd>: 编辑文件
   <kbd>o</kbd>: 打开文件
   <kbd>i</kbd>: 忽略文件
diff --git a/docs/keybindings/Keybindings_zh-TW.md b/docs/keybindings/Keybindings_zh-TW.md
index 201f79555..8fd6a274b 100644
--- a/docs/keybindings/Keybindings_zh-TW.md
+++ b/docs/keybindings/Keybindings_zh-TW.md
@@ -299,6 +299,7 @@ _說明：`<c-b>` 表示 Ctrl+B、`<a-b>` 表示 Alt+B，`B`表示 Shift+B_
   <kbd>w</kbd>: 沒有預提交 hook 就提交更改
   <kbd>A</kbd>: 修正上次提交
   <kbd>C</kbd>: 使用 git 編輯器提交變更
+  <kbd>&lt;c-f&gt;</kbd>: Find base commit for fixup
   <kbd>e</kbd>: 編輯檔案
   <kbd>o</kbd>: 開啟檔案
   <kbd>i</kbd>: 忽略或排除檔案
diff --git a/pkg/commands/git.go b/pkg/commands/git.go
index 510661034..b1b04a72f 100644
--- a/pkg/commands/git.go
+++ b/pkg/commands/git.go
@@ -21,6 +21,7 @@ import (
 
 // GitCommand is our main git interface
 type GitCommand struct {
+	Blame       *git_commands.BlameCommands
 	Branch      *git_commands.BranchCommands
 	Commit      *git_commands.CommitCommands
 	Config      *git_commands.ConfigCommands
@@ -160,6 +161,7 @@ func NewGitCommandAux(
 	patchCommands := git_commands.NewPatchCommands(gitCommon, rebaseCommands, commitCommands, statusCommands, stashCommands, patchBuilder)
 	bisectCommands := git_commands.NewBisectCommands(gitCommon)
 	worktreeCommands := git_commands.NewWorktreeCommands(gitCommon)
+	blameCommands := git_commands.NewBlameCommands(gitCommon)
 
 	branchLoader := git_commands.NewBranchLoader(cmn, cmd, branchCommands.CurrentBranchInfo, configCommands)
 	commitFileLoader := git_commands.NewCommitFileLoader(cmn, cmd)
@@ -171,6 +173,7 @@ func NewGitCommandAux(
 	tagLoader := git_commands.NewTagLoader(cmn, cmd)
 
 	return &GitCommand{
+		Blame:       blameCommands,
 		Branch:      branchCommands,
 		Commit:      commitCommands,
 		Config:      configCommands,
diff --git a/pkg/commands/git_commands/blame.go b/pkg/commands/git_commands/blame.go
new file mode 100644
index 000000000..aba1c63fe
--- /dev/null
+++ b/pkg/commands/git_commands/blame.go
@@ -0,0 +1,33 @@
+package git_commands
+
+import (
+	"fmt"
+)
+
+type BlameCommands struct {
+	*GitCommon
+}
+
+func NewBlameCommands(gitCommon *GitCommon) *BlameCommands {
+	return &BlameCommands{
+		GitCommon: gitCommon,
+	}
+}
+
+// Blame a range of lines. For each line, output the hash of the commit where
+// the line last changed, then a space, then a description of the commit (author
+// and date), another space, and then the line. For example:
+//
+//	ac90ebac688fe8bc2ffd922157a9d2c54681d2aa (Stefan Haller 2023-08-01 14:54:56 +0200 11) func NewBlameCommands(gitCommon *GitCommon) *BlameCommands {
+//	ac90ebac688fe8bc2ffd922157a9d2c54681d2aa (Stefan Haller 2023-08-01 14:54:56 +0200 12) 	return &BlameCommands{
+//	ac90ebac688fe8bc2ffd922157a9d2c54681d2aa (Stefan Haller 2023-08-01 14:54:56 +0200 13) 		GitCommon: gitCommon,
+func (self *BlameCommands) BlameLineRange(filename string, commit string, firstLine int, numLines int) (string, error) {
+	cmdArgs := NewGitCmd("blame").
+		Arg("-l").
+		Arg(fmt.Sprintf("-L%d,+%d", firstLine, numLines)).
+		Arg(commit).
+		Arg("--").
+		Arg(filename)
+
+	return self.cmd.New(cmdArgs.ToArgv()).RunWithOutput()
+}
diff --git a/pkg/commands/git_commands/commit.go b/pkg/commands/git_commands/commit.go
index 443289f6e..e0b5b8a9a 100644
--- a/pkg/commands/git_commands/commit.go
+++ b/pkg/commands/git_commands/commit.go
@@ -198,6 +198,20 @@ func (self *CommitCommands) GetCommitMessagesFirstLine(shas []string) (string, e
 	return self.cmd.New(cmdArgs).DontLog().RunWithOutput()
 }
 
+// Example output:
+//
+//	cd50c79ae Preserve the commit message correctly even if the description has blank lines
+//	3ebba5f32 Add test demonstrating a bug with preserving the commit message
+//	9a423c388 Remove unused function
+func (self *CommitCommands) GetShasAndCommitMessagesFirstLine(shas []string) (string, error) {
+	cmdArgs := NewGitCmd("show").
+		Arg("--no-patch", "--pretty=format:%h %s").
+		Arg(shas...).
+		ToArgv()
+
+	return self.cmd.New(cmdArgs).DontLog().RunWithOutput()
+}
+
 func (self *CommitCommands) GetCommitsOneline(shas []string) (string, error) {
 	cmdArgs := NewGitCmd("show").
 		Arg("--no-patch", "--oneline").
diff --git a/pkg/commands/git_commands/diff.go b/pkg/commands/git_commands/diff.go
index 41e71e941..372939024 100644
--- a/pkg/commands/git_commands/diff.go
+++ b/pkg/commands/git_commands/diff.go
@@ -78,3 +78,11 @@ func (self *DiffCommands) OpenDiffToolCmdObj(opts DiffToolCmdOptions) oscommands
 		Arg("--", opts.Filepath).
 		ToArgv())
 }
+
+func (self *DiffCommands) DiffIndexCmdObj(diffArgs ...string) oscommands.ICmdObj {
+	return self.cmd.New(
+		NewGitCmd("diff-index").
+			Arg("--submodule", "--no-ext-diff", "--no-color", "--patch").
+			Arg(diffArgs...).ToArgv(),
+	)
+}
diff --git a/pkg/config/user_config.go b/pkg/config/user_config.go
index ffc27eb88..d0d518455 100644
--- a/pkg/config/user_config.go
+++ b/pkg/config/user_config.go
@@ -366,6 +366,7 @@ type KeybindingFilesConfig struct {
 	CommitChangesWithoutHook string `yaml:"commitChangesWithoutHook"`
 	AmendLastCommit          string `yaml:"amendLastCommit"`
 	CommitChangesWithEditor  string `yaml:"commitChangesWithEditor"`
+	FindBaseCommitForFixup   string `yaml:"findBaseCommitForFixup"`
 	ConfirmDiscard           string `yaml:"confirmDiscard"`
 	IgnoreFile               string `yaml:"ignoreFile"`
 	RefreshFiles             string `yaml:"refreshFiles"`
@@ -762,6 +763,7 @@ func GetDefaultConfig() *UserConfig {
 				CommitChangesWithoutHook: "w",
 				AmendLastCommit:          "A",
 				CommitChangesWithEditor:  "C",
+				FindBaseCommitForFixup:   "<c-f>",
 				IgnoreFile:               "i",
 				RefreshFiles:             "r",
 				StashAllChanges:          "s",
diff --git a/pkg/gui/controllers.go b/pkg/gui/controllers.go
index 37c54a655..b4cbec79e 100644
--- a/pkg/gui/controllers.go
+++ b/pkg/gui/controllers.go
@@ -103,6 +103,7 @@ func (gui *Gui) resetHelpersAndControllers() {
 		CherryPick:      cherryPickHelper,
 		Upstream:        helpers.NewUpstreamHelper(helperCommon, suggestionsHelper.GetRemoteBranchesSuggestionsFunc),
 		AmendHelper:     helpers.NewAmendHelper(helperCommon, gpgHelper),
+		FixupHelper:     helpers.NewFixupHelper(helperCommon),
 		Commits:         commitsHelper,
 		Snake:           helpers.NewSnakeHelper(helperCommon),
 		Diff:            diffHelper,
diff --git a/pkg/gui/controllers/files_controller.go b/pkg/gui/controllers/files_controller.go
index 0db509ca3..d60773ec1 100644
--- a/pkg/gui/controllers/files_controller.go
+++ b/pkg/gui/controllers/files_controller.go
@@ -65,6 +65,12 @@ func (self *FilesController) GetKeybindings(opts types.KeybindingsOpts) []*types
 			Description: self.c.Tr.CommitChangesWithEditor,
 		},
 		{
+			Key:         opts.GetKey(opts.Config.Files.FindBaseCommitForFixup),
+			Handler:     self.c.Helpers().FixupHelper.HandleFindBaseCommitForFixupPress,
+			Description: self.c.Tr.FindBaseCommitForFixup,
+			Tooltip:     self.c.Tr.FindBaseCommitForFixupTooltip,
+		},
+		{
 			Key:         opts.GetKey(opts.Config.Universal.Edit),
 			Handler:     self.checkSelectedFileNode(self.edit),
 			Description: self.c.Tr.EditFile,
diff --git a/pkg/gui/controllers/helpers/fixup_helper.go b/pkg/gui/controllers/helpers/fixup_helper.go
new file mode 100644
index 000000000..2198d11cb
--- /dev/null
+++ b/pkg/gui/controllers/helpers/fixup_helper.go
@@ -0,0 +1,178 @@
+package helpers
+
+import (
+	"regexp"
+	"strings"
+	"sync"
+
+	"github.com/jesseduffield/generics/set"
+	"github.com/jesseduffield/lazygit/pkg/commands/models"
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/utils"
+	"github.com/samber/lo"
+)
+
+type FixupHelper struct {
+	c *HelperCommon
+}
+
+func NewFixupHelper(
+	c *HelperCommon,
+) *FixupHelper {
+	return &FixupHelper{
+		c: c,
+	}
+}
+
+type deletedLineInfo struct {
+	filename     string
+	startLineIdx int
+	numLines     int
+}
+
+func (self *FixupHelper) HandleFindBaseCommitForFixupPress() error {
+	diff, hasStagedChanges, err := self.getDiff()
+	if err != nil {
+		return err
+	}
+	if diff == "" {
+		return self.c.ErrorMsg(self.c.Tr.NoChangedFiles)
+	}
+
+	deletedLineInfos := self.parseDiff(diff)
+	if len(deletedLineInfos) == 0 {
+		return self.c.ErrorMsg(self.c.Tr.NoDeletedLinesInDiff)
+	}
+
+	shas := self.blameDeletedLines(deletedLineInfos)
+
+	if len(shas) == 0 {
+		// This should never happen
+		return self.c.ErrorMsg(self.c.Tr.NoBaseCommitsFound)
+	}
+	if len(shas) > 1 {
+		subjects, err := self.c.Git().Commit.GetShasAndCommitMessagesFirstLine(shas)
+		if err != nil {
+			return err
+		}
+		message := lo.Ternary(hasStagedChanges,
+			self.c.Tr.MultipleBaseCommitsFoundStaged,
+			self.c.Tr.MultipleBaseCommitsFoundUnstaged)
+		return self.c.ErrorMsg(message + "\n\n" + subjects)
+	}
+
+	commit, index, ok := lo.FindIndexOf(self.c.Model().Commits, func(commit *models.Commit) bool {
+		return commit.Sha == shas[0]
+	})
+	if !ok {
+		commits := self.c.Model().Commits
+		if commits[len(commits)-1].Status == models.StatusMerged {
+			// If the commit is not found, it's most likely because it's already
+			// merged, and more than 300 commits away. Check if the last known
+			// commit is already merged; if so, show the "already merged" error.
+			return self.c.ErrorMsg(self.c.Tr.BaseCommitIsAlreadyOnMainBranch)
+		}
+		// If we get here, the current branch must have more then 300 commits. Unlikely...
+		return self.c.ErrorMsg(self.c.Tr.BaseCommitIsNotInCurrentView)
+	}
+	if commit.Status == models.StatusMerged {
+		return self.c.ErrorMsg(self.c.Tr.BaseCommitIsAlreadyOnMainBranch)
+	}
+
+	if !useIndex {
+		if err := self.c.Git().WorkingTree.StageAll(); err != nil {
+			return err
+		}
+		_ = self.c.Refresh(types.RefreshOptions{Mode: types.SYNC, Scope: []types.RefreshableView{types.FILES}})
+	}
+
+	self.c.Contexts().LocalCommits.SetSelectedLineIdx(index)
+	return self.c.PushContext(self.c.Contexts().LocalCommits)
+}
+
+func (self *FixupHelper) getDiff() (string, bool, error) {
+	args := []string{"-U0", "--ignore-submodules=all", "HEAD", "--"}
+
+	// Try staged changes first
+	hasStagedChanges := true
+	diff, err := self.c.Git().Diff.DiffIndexCmdObj(append([]string{"--cached"}, args...)...).RunWithOutput()
+
+	if err == nil && diff == "" {
+		hasStagedChanges = false
+		// If there are no staged changes, try unstaged changes
+		diff, err = self.c.Git().Diff.DiffIndexCmdObj(args...).RunWithOutput()
+	}
+
+	return diff, hasStagedChanges, err
+}
+
+func (self *FixupHelper) parseDiff(diff string) []*deletedLineInfo {
+	lines := strings.Split(strings.TrimSuffix(diff, "\n"), "\n")
+
+	deletedLineInfos := []*deletedLineInfo{}
+
+	hunkHeaderRegexp := regexp.MustCompile(`@@ -(\d+)(?:,\d+)? \+\d+(?:,\d+)? @@`)
+
+	var filename string
+	var currentLineInfo *deletedLineInfo
+	finishHunk := func() {
+		if currentLineInfo != nil && currentLineInfo.numLines > 0 {
+			deletedLineInfos = append(deletedLineInfos, currentLineInfo)
+		}
+	}
+	for _, line := range lines {
+		if strings.HasPrefix(line, "diff --git") {
+			finishHunk()
+			currentLineInfo = nil
+		} else if strings.HasPrefix(line, "--- ") {
+			// For some reason, the line ends with a tab character if the file
+			// name contains spaces
+			filename = strings.TrimRight(line[6:], "\t")
+		} else if strings.HasPrefix(line, "@@ ") {
+			finishHunk()
+			match := hunkHeaderRegexp.FindStringSubmatch(line)
+			startIdx := utils.MustConvertToInt(match[1])
+			currentLineInfo = &deletedLineInfo{filename, startIdx, 0}
+		} else if currentLineInfo != nil && line[0] == '-' {
+			currentLineInfo.numLines++
+		}
+	}
+	finishHunk()
+
+	return deletedLineInfos
+}
+
+// returns the list of commit hashes that introduced the lines which have now been deleted
+func (self *FixupHelper) blameDeletedLines(deletedLineInfos []*deletedLineInfo) []string {
+	var wg sync.WaitGroup
+	shaChan := make(chan string)
+
+	for _, info := range deletedLineInfos {
+		wg.Add(1)
+		go func(info *deletedLineInfo) {
+			defer wg.Done()
+
+			blameOutput, err := self.c.Git().Blame.BlameLineRange(info.filename, "HEAD", info.startLineIdx, info.numLines)
+			if err != nil {
+				self.c.Log.Errorf("Error blaming file '%s': %v", info.filename, err)
+				return
+			}
+			blameLines := strings.Split(strings.TrimSuffix(blameOutput, "\n"), "\n")
+			for _, line := range blameLines {
+				shaChan <- strings.Split(line, " ")[0]
+			}
+		}(info)
+	}
+
+	go func() {
+		wg.Wait()
+		close(shaChan)
+	}()
+
+	result := set.New[string]()
+	for sha := range shaChan {
+		result.Add(sha)
+	}
+
+	return result.ToSlice()
+}
diff --git a/pkg/gui/controllers/helpers/helpers.go b/pkg/gui/controllers/helpers/helpers.go
index 9e05ba163..1f1050dc9 100644
--- a/pkg/gui/controllers/helpers/helpers.go
+++ b/pkg/gui/controllers/helpers/helpers.go
@@ -33,6 +33,7 @@ type Helpers struct {
 	GPG            *GpgHelper
 	Upstream       *UpstreamHelper
 	AmendHelper    *AmendHelper
+	FixupHelper    *FixupHelper
 	Commits        *CommitsHelper
 	Snake          *SnakeHelper
 	// lives in context package because our contexts need it to render to main
@@ -70,6 +71,7 @@ func NewStubHelpers() *Helpers {
 		GPG:               &GpgHelper{},
 		Upstream:          &UpstreamHelper{},
 		AmendHelper:       &AmendHelper{},
+		FixupHelper:       &FixupHelper{},
 		Commits:           &CommitsHelper{},
 		Snake:             &SnakeHelper{},
 		Diff:              &DiffHelper{},
diff --git a/pkg/i18n/english.go b/pkg/i18n/english.go
index 34df4aba1..e2559f3d2 100644
--- a/pkg/i18n/english.go
+++ b/pkg/i18n/english.go
@@ -39,6 +39,14 @@ type TranslationSet struct {
 	SureToAmend                         string
 	NoCommitToAmend                     string
 	CommitChangesWithEditor             string
+	FindBaseCommitForFixup              string
+	FindBaseCommitForFixupTooltip       string
+	NoDeletedLinesInDiff                string
+	NoBaseCommitsFound                  string
+	MultipleBaseCommitsFoundStaged      string
+	MultipleBaseCommitsFoundUnstaged    string
+	BaseCommitIsAlreadyOnMainBranch     string
+	BaseCommitIsNotInCurrentView        string
 	StatusTitle                         string
 	GlobalTitle                         string
 	Menu                                string
@@ -858,6 +866,14 @@ func EnglishTranslationSet() TranslationSet {
 		SureToAmend:                         "Are you sure you want to amend last commit? Afterwards, you can change the commit message from the commits panel.",
 		NoCommitToAmend:                     "There's no commit to amend.",
 		CommitChangesWithEditor:             "Commit changes using git editor",
+		FindBaseCommitForFixup:              "Find base commit for fixup",
+		FindBaseCommitForFixupTooltip:       "Find the commit that your current changes are building upon, for the sake of amending/fixing up the commit. This spares you from having to look through your branch's commits one-by-one to see which commit should be amended/fixed up. See docs: <https://github.com/jesseduffield/lazygit/tree/master/docs/Fixup_Commits.md>",
+		NoDeletedLinesInDiff:                "No deleted lines in diff",
+		NoBaseCommitsFound:                  "No base commits found",
+		MultipleBaseCommitsFoundStaged:      "Multiple base commits found. (Try staging fewer changes at once)",
+		MultipleBaseCommitsFoundUnstaged:    "Multiple base commits found. (Try staging some of the changes)",
+		BaseCommitIsAlreadyOnMainBranch:     "The base commit for this change is already on the main branch",
+		BaseCommitIsNotInCurrentView:        "Base commit is not in current view",
 		StatusTitle:                         "Status",
 		Menu:                                "Menu",
 		Execute:                             "Execute",
diff --git a/pkg/integration/tests/commit/find_base_commit_for_fixup.go b/pkg/integration/tests/commit/find_base_commit_for_fixup.go
new file mode 100644
index 000000000..4440932e9
--- /dev/null
+++ b/pkg/integration/tests/commit/find_base_commit_for_fixup.go
@@ -0,0 +1,79 @@
+package commit
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var FindBaseCommitForFixup = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Finds the base commit to create a fixup for",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.NewBranch("mybranch").
+			EmptyCommit("1st commit").
+			CreateFileAndAdd("file1", "file1 content\n").
+			Commit("2nd commit").
+			CreateFileAndAdd("file2", "file2 content\n").
+			Commit("3rd commit").
+			UpdateFile("file1", "file1 changed content").
+			UpdateFile("file2", "file2 changed content")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Lines(
+				Contains("3rd commit"),
+				Contains("2nd commit"),
+				Contains("1st commit"),
+			)
+
+		// Two changes from different commits: this fails
+		t.Views().Files().
+			Focus().
+			Press(keys.Files.FindBaseCommitForFixup)
+
+		t.ExpectPopup().Alert().
+			Title(Equals("Error")).
+			Content(
+				Contains("Multiple base commits found").
+					Contains("2nd commit").
+					Contains("3rd commit"),
+			).
+			Confirm()
+
+		// Stage only one of the files: this succeeds
+		t.Views().Files().
+			IsFocused().
+			NavigateToLine(Contains("file1")).
+			PressPrimaryAction().
+			Press(keys.Files.FindBaseCommitForFixup)
+
+		t.Views().Commits().
+			IsFocused().
+			Lines(
+				Contains("3rd commit"),
+				Contains("2nd commit").IsSelected(),
+				Contains("1st commit"),
+			).
+			Press(keys.Commits.AmendToCommit)
+
+		t.ExpectPopup().Confirmation().
+			Title(Equals("Amend commit")).
+			Content(Contains("Are you sure you want to amend this commit with your staged files?")).
+			Confirm()
+
+		// Now only the other file is modified (and unstaged); this works now
+		t.Views().Files().
+			Focus().
+			Press(keys.Files.FindBaseCommitForFixup)
+
+		t.Views().Commits().
+			IsFocused().
+			Lines(
+				Contains("3rd commit").IsSelected(),
+				Contains("2nd commit"),
+				Contains("1st commit"),
+			)
+	},
+})
diff --git a/pkg/integration/tests/test_list.go b/pkg/integration/tests/test_list.go
index ccefd1dd2..c018d84e7 100644
--- a/pkg/integration/tests/test_list.go
+++ b/pkg/integration/tests/test_list.go
@@ -70,6 +70,7 @@ var tests = []*components.IntegrationTest{
 	commit.CommitWithPrefix,
 	commit.CreateTag,
 	commit.DiscardOldFileChange,
+	commit.FindBaseCommitForFixup,
 	commit.Highlight,
 	commit.History,
 	commit.HistoryComplex,
diff --git a/schema/config.json b/schema/config.json
index a5cbc0b67..1251240a1 100644
--- a/sch