package process

import (
	"context"
	"errors"
	"fmt"
	"os/exec"
	"sort"
	"strconv"
	"strings"

	"github.com/kandev/kandev/internal/common/unidiff"
)

// GitLogResult represents the result of a git log operation.
type GitLogResult struct {
	Success bool             `json:"success"`
	Commits []*GitCommitInfo `json:"commits"`
	Error   string           `json:"error,omitempty"`
	// ErrorCode is a bounded machine-readable failure reason. It is populated
	// for explicit comparison-target failures; raw git/provider errors stay in
	// server logs only.
	ErrorCode string `json:"error_code,omitempty"`
	// PerRepoErrors lists per-repo failures during a multi-repo log fan-out.
	// Empty/nil for single-repo responses or when every repo succeeded. The
	// merged response keeps Success=true as long as at least one repo
	// succeeded; callers inspect this slice to surface partial failures.
	PerRepoErrors []GitLogRepoError `json:"per_repo_errors,omitempty"`
}

// GitLogRepoError describes a single per-repo failure from a multi-repo log
// fan-out. It is omitted from the response entirely for single-repo responses.
type GitLogRepoError struct {
	RepositoryName string `json:"repository_name"`
	Error          string `json:"error"`
	ErrorCode      string `json:"error_code,omitempty"`
}

// GitCommitInfo represents a single commit in the log.
// Field names match GitCommitData and SessionCommit for frontend consistency.
type GitCommitInfo struct {
	CommitSHA     string `json:"commit_sha"`
	ParentSHA     string `json:"parent_sha"`
	CommitMessage string `json:"commit_message"`
	AuthorName    string `json:"author_name"`
	AuthorEmail   string `json:"author_email"`
	CommittedAt   string `json:"committed_at"`
	FilesChanged  int    `json:"files_changed"`
	Insertions    int    `json:"insertions"`
	Deletions     int    `json:"deletions"`
	// Pushed is true when the commit is reachable from the branch's upstream
	// tracking ref (i.e. present on the remote). Sourced from git itself —
	// not from any PR API — so it stays correct when no PR exists yet, after
	// rebases, and across multi-repo workspaces.
	Pushed bool `json:"pushed"`
	// RepositoryName tags commits returned from a multi-repo log fan-out.
	// Empty for single-repo workspaces. Set by the API layer after the call.
	RepositoryName string `json:"repository_name,omitempty"`
}

// CumulativeDiffResult represents the cumulative diff from base commit to HEAD.
type CumulativeDiffResult struct {
	Success      bool                   `json:"success"`
	BaseCommit   string                 `json:"base_commit"`
	HeadCommit   string                 `json:"head_commit"`
	TotalCommits int                    `json:"total_commits"`
	Files        map[string]interface{} `json:"files"`
	// TruncatedFilesCount is how many files were dropped from Files entirely
	// because the cumulative range exceeded maxCumulativeDiffFiles. Zero
	// (omitted) when the full file set fit. The frontend surfaces this as a
	// "N more files hidden" banner — a mid-rebase cumulative diff can otherwise
	// enumerate tens of thousands of files, one rendered row each.
	//
	// This counts only fully-dropped files. Files kept in the map but whose
	// diff was emptied by the byte budget (diff_skip_reason "budget_exceeded")
	// are NOT counted here — they remain listed and carry their own per-file
	// skip reason that the diff viewer renders, so they are visible-but-capped
	// rather than hidden.
	TruncatedFilesCount int    `json:"truncated_files_count,omitempty"`
	Error               string `json:"error,omitempty"`
	ErrorCode           string `json:"error_code,omitempty"`
}

// maxCumulativeDiffFiles bounds how many file entries GetCumulativeDiff returns.
// This is a safety valve for the rebase pathology (base→working-tree across a
// large range), not a UX limit — normal PRs stay well under it. Capping the
// count bounds the number of rendered file rows on the frontend regardless of
// the per-file/total byte budgets.
const maxCumulativeDiffFiles = 500

// Field and record separators for git log parsing.
// Using non-printable ASCII separators to avoid collision with commit message content.
const (
	fieldSep  = "\x1f" // Unit Separator (ASCII 31)
	recordSep = "\x1e" // Record Separator (ASCII 30)
)

// GetLog returns commits from baseCommit (exclusive) to HEAD (inclusive).
// If baseCommit is empty, returns recent commits (limited by limit parameter).
// Stats (files changed, insertions, deletions) are fetched in-band via --shortstat
// to avoid an N+1 git-show call per commit.
func (g *GitOperator) GetLog(ctx context.Context, baseCommit string, limit int) (*GitLogResult, error) {
	result := &GitLogResult{
		Commits: make([]*GitCommitInfo, 0),
	}

	// Build the git log command with non-printable separators and --shortstat.
	// The record separator (%x1e) is placed BEFORE the fields so that --shortstat
	// output (appended after the format) stays within the same record:
	//   \x1e<fields>\n <stat summary>\n\x1e<fields>\n <stat summary>\n...
	// Splitting on recordSep groups each commit's fields + stats together.
	args := []string{"log", "--format=%x1e%H%x1f%P%x1f%an%x1f%ae%x1f%s%x1f%aI", "--shortstat"}

	switch {
	case baseCommit != "":
		// --first-parent only on the divergence-range path: when the branch
		// has merged main back in, plain `git log <base>..HEAD` walks both
		// parents of the merge and surfaces commits brought in via main as
		// if they were the branch's own work. Following only the first parent
		// keeps the branch's line plus its merge commits and excludes the
		// merged-in side. We deliberately don't apply this to the open-ended
		// "recent N commits" path so future history-view callers (activity
		// widgets, etc.) keep getting the full graph.
		args = append(args, "--first-parent", baseCommit+"..HEAD")
	case limit > 0:
		args = append(args, fmt.Sprintf("-n%d", limit))
	default:
		// Default to last 50 commits if no base and no limit
		args = append(args, "-n50")
	}

	output, err := g.runGitCommand(ctx, args...)
	if err != nil {
		result.Error = fmt.Sprintf("failed to get git log: %s", err.Error())
		return result, nil
	}

	output = strings.TrimSpace(output)
	if output == "" {
		result.Success = true
		return result, nil
	}

	// Split by record separator. The first element is empty (separator is at the start).
	records := strings.Split(output, recordSep)
	for _, record := range records {
		record = strings.TrimSpace(record)
		if record == "" {
			continue
		}

		// The record may contain a trailing shortstat line after the commit fields.
		// Split on newline: first line has the fields, remaining lines may have the stat summary.
		lines := strings.SplitN(record, "\n", 2)
		fieldLine := strings.TrimSpace(lines[0])

		parts := strings.Split(fieldLine, fieldSep)
		if len(parts) < 6 {
			continue
		}

		sha := parts[0]
		parentSHA := parts[1]
		if idx := strings.Index(parentSHA, " "); idx > 0 {
			parentSHA = parentSHA[:idx]
		}

		// Parse inline shortstat if present
		var filesChanged, insertions, deletions int
		if len(lines) > 1 {
			statLine := strings.TrimSpace(lines[1])
			if statLine != "" {
				filesChanged, insertions, deletions = parseStatSummary(statLine)
			}
		}

		result.Commits = append(result.Commits, &GitCommitInfo{
			CommitSHA:     sha,
			ParentSHA:     parentSHA,
			AuthorName:    parts[2],
			AuthorEmail:   parts[3],
			CommitMessage: parts[4],
			CommittedAt:   parts[5],
			FilesChanged:  filesChanged,
			Insertions:    insertions,
			Deletions:     deletions,
		})
	}

	g.markPushedCommits(ctx, result.Commits)

	result.Success = true
	return result, nil
}

// markPushedCommits sets Pushed on each commit by looking up which commits in
// HEAD's history are NOT reachable from the upstream tracking ref. A commit is
// pushed iff it is not in that "ahead" set. When the branch has no upstream
// (never been pushed) or the lookup fails, all commits stay Pushed=false — the
// safer default than falsely claiming a commit is on the remote.
func (g *GitOperator) markPushedCommits(ctx context.Context, commits []*GitCommitInfo) {
	if len(commits) == 0 {
		return
	}
	upstream := g.getUpstreamRef(ctx)
	if upstream == "" {
		return
	}
	// Resolve to a SHA so we can pass it to rev-list as `^<sha>`. The caret
	// prefix means the arg doesn't match LooksLikeCommitSHA; it falls through
	// to the non-branch catch-all in runGitCommand, which passes it unchanged.
	// Safe: upstreamSHA is local rev-parse output, not user input.
	upstreamSHA, err := g.runGitCommand(ctx, "rev-parse", upstream)
	if err != nil {
		return
	}
	upstreamSHA = strings.TrimSpace(upstreamSHA)
	if upstreamSHA == "" {
		return
	}
	// Cap the walk to the number of commits we're marking. Without this, a
	// branch with many local-only commits would walk unbounded history per
	// GetLog call. rev-list walks newest-first the same way GetLog does, so
	// the N most recent unpushed SHAs cover the N commits in our result.
	output, err := g.runGitCommand(ctx, "rev-list",
		fmt.Sprintf("-n%d", len(commits)), "HEAD", "^"+upstreamSHA)
	if err != nil {
		return
	}
	unpushed := make(map[string]struct{})
	for _, line := range strings.Split(strings.TrimSpace(output), "\n") {
		sha := strings.TrimSpace(line)
		if sha != "" {
			unpushed[sha] = struct{}{}
		}
	}
	for _, c := range commits {
		if _, isUnpushed := unpushed[c.CommitSHA]; !isUnpushed {
			c.Pushed = true
		}
	}
}

// GetCumulativeDiff returns the cumulative diff from baseCommit to the working tree
// (including uncommitted/unstaged changes).
func (g *GitOperator) GetCumulativeDiff(ctx context.Context, baseCommit string) (*CumulativeDiffResult, error) {
	result := &CumulativeDiffResult{
		Files: make(map[string]interface{}),
	}

	if baseCommit == "" {
		result.Error = "base_commit is required"
		return result, nil
	}

	result.BaseCommit = baseCommit

	// Get current HEAD
	headOutput, err := g.runGitCommand(ctx, "rev-parse", "HEAD")
	if err != nil {
		result.Error = fmt.Sprintf("failed to get HEAD: %s", err.Error())
		return result, nil
	}
	result.HeadCommit = strings.TrimSpace(headOutput)

	// Count commits between base and HEAD
	countOutput, err := g.runGitCommand(ctx, "rev-list", "--count", baseCommit+"..HEAD")
	if err == nil {
		_, _ = fmt.Sscanf(strings.TrimSpace(countOutput), "%d", &result.TotalCommits)
	}

	// Get the cumulative diff (base → working tree, includes uncommitted changes).
	// Deliberately no --numstat here: parseCommitDiffWithOptions falls back to
	// unidiff.CountLines, which was checked against git's own numstat over 600
	// commits of this repo's history with zero disagreements on line counts.
	// (Binary files were excluded from that comparison — numstat reports them as
	// `-` with nothing to compare — but both routes report 0/0 for them anyway,
	// since a binary block carries no hunk.) Adding the flag would change a
	// production git invocation for a difference no test can observe.
	diffOutput, err := g.runGitCommand(
		ctx,
		"diff",
		"--src-prefix=a/",
		"--dst-prefix=b/",
		baseCommit,
	)
	if err != nil {
		result.Error = fmt.Sprintf("failed to get diff: %s", err.Error())
		return result, nil
	}

	// Parse the diff output into files, capping per-file (256 KB) and total
	// (2 MB) diff bytes. Unlike ShowCommit (single, bounded commit) a
	// multi-commit cumulative range can produce tens of MB of patch text.
	result.Files = g.parseCommitDiffWithOptions(diffOutput, parseCommitDiffOptions{
		perFileMaxBytes: maxDiffOutputSize,
		totalMaxBytes:   maxTotalDiffBytes,
	})

	// Cap the number of file entries too — the byte budget bounds payload size
	// but not the row count, and the frontend renders one component per file.
	result.Files, result.TruncatedFilesCount = capCumulativeDiffFiles(result.Files, maxCumulativeDiffFiles)

	result.Success = true
	return result, nil
}

// capCumulativeDiffFiles keeps at most maxFiles entries (deterministically, by
// sorted path) and returns the kept map plus the number dropped.
func capCumulativeDiffFiles(files map[string]interface{}, maxFiles int) (map[string]interface{}, int) {
	if maxFiles <= 0 || len(files) <= maxFiles {
		return files, 0
	}

	paths := make([]string, 0, len(files))
	for path := range files {
		paths = append(paths, path)
	}
	sort.Strings(paths)

	capped := make(map[string]interface{}, maxFiles)
	for _, path := range paths[:maxFiles] {
		capped[path] = files[path]
	}
	return capped, len(files) - maxFiles
}

// CommitDiffResult represents the result of getting a commit's diff.
type CommitDiffResult struct {
	Success      bool                   `json:"success"`
	CommitSHA    string                 `json:"commit_sha"`
	Message      string                 `json:"message"`
	Author       string                 `json:"author"`
	Date         string                 `json:"date"`
	Files        map[string]interface{} `json:"files"` // FileInfo objects with diff content
	FilesChanged int                    `json:"files_changed"`
	Insertions   int                    `json:"insertions"`
	Deletions    int                    `json:"deletions"`
	Error        string                 `json:"error,omitempty"`
}

// isHexChar reports whether r is a valid hexadecimal digit (0-9, a-f, A-F).
func isHexChar(r rune) bool {
	return (r >= '0' && r <= '9') || (r >= 'a' && r <= 'f') || (r >= 'A' && r <= 'F')
}

// validateCommitSHA validates a commit SHA.
// Returns an error message if invalid, empty string if valid.
func validateCommitSHA(sha string) string {
	if sha == "" {
		return "commit SHA is required"
	}

	// Must be between 4 and 40 hex characters
	if len(sha) < 4 || len(sha) > 40 {
		return "commit SHA must be between 4 and 40 characters"
	}

	for _, r := range sha {
		if !isHexChar(r) {
			return "commit SHA contains invalid characters"
		}
	}
	return ""
}

// sumFileDiffStats sums additions and deletions from a file diff map.
func sumFileDiffStats(files map[string]interface{}) (insertions, deletions int) {
	for _, f := range files {
		fi, ok := f.(map[string]interface{})
		if !ok {
			continue
		}
		if additions, ok := fi["additions"].(int); ok {
			insertions += additions
		}
		if dels, ok := fi["deletions"].(int); ok {
			deletions += dels
		}
	}
	return insertions, deletions
}

// ShowCommit gets the diff for a specific commit using git show.
func (g *GitOperator) ShowCommit(ctx context.Context, commitSHA string) (*CommitDiffResult, error) {
	result := &CommitDiffResult{
		CommitSHA: commitSHA,
	}

	// Validate commit SHA (basic validation - alphanumeric only)
	if errMsg := validateCommitSHA(commitSHA); errMsg != "" {
		result.Error = errMsg
		return result, nil
	}

	// Get commit metadata
	formatOutput, err := g.runGitCommand(ctx, "show", "--no-patch", "--format=%H%n%s%n%an <%ae>%n%aI", commitSHA)
	if err != nil {
		result.Error = fmt.Sprintf("failed to get commit info: %s", err.Error())
		return result, nil
	}

	lines := strings.Split(strings.TrimSpace(formatOutput), "\n")
	if len(lines) >= 4 {
		result.CommitSHA = lines[0]
		result.Message = lines[1]
		result.Author = lines[2]
		result.Date = lines[3]
	}

	// Get the diff with stats
	// Merge commits must be compared with their first parent so the patch
	// matches the branch history shown by the commit list and GitHub.
	diffOutput, err := g.runGitCommand(
		ctx,
		"show",
		"--first-parent",
		"--format=",
		"--stat",
		"--numstat",
		"-p",
		"--src-prefix=a/",
		"--dst-prefix=b/",
		commitSHA,
	)
	if err != nil {
		result.Error = fmt.Sprintf("failed to get commit diff: %s", err.Error())
		return result, nil
	}

	// Parse the diff output into files
	result.Files = g.parseCommitDiff(diffOutput)
	result.FilesChanged = len(result.Files)
	result.Insertions, result.Deletions = sumFileDiffStats(result.Files)

	result.Success = true
	return result, nil
}

// parseCommitDiffOptions controls per-file and cumulative budget enforcement
// while parsing a `git diff`/`git show` output into the file info map. The zero
// value applies no caps, which preserves the ShowCommit contract (single-commit
// detail views expect full diffs). The cumulative-diff path passes non-zero
// budgets because a multi-commit range can produce tens of MB of patch text
// that, JSON-encoded and shipped over the WS, grinds the browser.
type parseCommitDiffOptions struct {
	perFileMaxBytes int
	totalMaxBytes   int
}

// parseCommitDiff parses git show output into file info map. No budget caps —
// callers that need them use parseCommitDiffWithOptions.
func (g *GitOperator) parseCommitDiff(output string) map[string]interface{} {
	return g.parseCommitDiffWithOptions(output, parseCommitDiffOptions{})
}

// parseCommitDiffWithOptions parses git diff/show output into a file info map,
// applying the budgets in opts. Diffs are truncated (per-file) or dropped
// (once the cumulative budget is exceeded); additions/deletions counts are
// always computed from the full per-file content because they're cheap and
// the frontend relies on them.
func (g *GitOperator) parseCommitDiffWithOptions(output string, opts parseCommitDiffOptions) map[string]interface{} {
	files := make(map[string]interface{})

	// Split into per-file sections. The split is line-aware: only a line that
	// STARTS with `diff --git ` begins a new section, because git only emits
	// that token at the start of a section header. Splitting the raw byte
	// stream would truncate a real section when a patch body or filename
	// contains the literal sequence (e.g. an added line
	// `+diff --git a/example b/example`) and fabricate a bogus section.
	prefix, sections := splitDiffSections(output)
	if len(sections) == 0 {
		return files
	}

	// When the caller's git command included --numstat, git printed authoritative
	// per-file counts ahead of the first "diff --git" — exactly the text the split
	// discards. Recover them; files without a row fall back to counting
	// their own patch body.
	numstat := numstatByPath(prefix)

	totalDiffBytes := 0
	for _, diffContent := range sections {
		// Extract the file path. git's per-file `+++ b/<path>` line is the
		// authoritative source: it carries exactly one path, so spaces and
		// literal ` b/` sequences inside a filename can't be confused with
		// the old/new separator, and git C-quotes paths containing tabs,
		// non-ASCII bytes, quotes, or backslashes (core.quotePath, default
		// on) — a header-only split would drop those entirely.
		filePath, ok := diffSectionPath(diffContent)
		if !ok {
			continue
		}

		// Determine file status from diff content
		status := fileStatusModified
		switch {
		case strings.Contains(diffContent, "new file mode"):
			status = "added"
		case strings.Contains(diffContent, "deleted file mode"):
			status = fileStatusDeleted
		case strings.Contains(diffContent, "rename from"):
			status = "renamed"
		}

		// Count additions and deletions (always from the full content).
		additions, deletions := fileLineCounts(numstat, filePath, diffContent)

		diffOut, skipReason := applyDiffBudget(diffContent, totalDiffBytes, opts)
		totalDiffBytes += len(diffOut)

		fileEntry := map[string]interface{}{
			"path":      filePath,
			"status":    status,
			"staged":    false,
			"additions": additions,
			"deletions": deletions,
			"diff":      diffOut,
		}
		if skipReason != "" {
			fileEntry["diff_skip_reason"] = skipReason
		}
		files[filePath] = fileEntry
	}

	return files
}

// diffSectionPath extracts the new-side path of one `diff --git` section.
// The `+++ b/<path>` file line is the authoritative source: git writes it
// once per section, exactly one path per line, so spaces and literal ` b/`
// sequences inside filenames can't confuse the old/new split, and git
// C-quotes paths containing special bytes (tabs, non-ASCII, quotes,
// backslashes) the same way it quotes the header. Sections that carry no
// file lines fall back to other unambiguous per-section lines before the
// `diff --git` header:
//   - deleted files: `--- a/<path>` (the `+++` side is `/dev/null`);
//   - pure renames without hunks: `rename to <path>`;
//   - binary sections: `Binary files a/<old> and b/<new> differ`;
//   - mode-only changes: the quoted/unquoted header itself.
//
// A single trailing tab on a file line is git's marker for paths that end in
// whitespace; it is stripped but any real trailing whitespace is kept.
func diffSectionPath(diffContent string) (string, bool) {
	lines := strings.Split(diffContent, "\n")

	// First pass prefers the `+++ b/<path>` line: in a rename section the
	// `--- a/<old>` line precedes it, and the new-side path is what the
	// changes panel shows. `+++ /dev/null` (deleted files) yields nothing,
	// so the second pass falls back to the `--- a/<path>` old-side line.
	for _, line := range lines {
		if path, ok := diffFileLinePath("+++", line); ok {
			return path, true
		}
	}
	for _, line := range lines {
		if path, ok := diffFileLinePath("---", line); ok {
			return path, true
		}
	}
	// Pure renames (similarity 100%, no hunks) emit rename from/to lines with
	// no ---/+++ file lines; `rename to <path>` is one path per line.
	for _, line := range lines {
		if path, ok := diffWholeLinePath("rename to ", line); ok {
			return path, true
		}
	}
	// Binary sections emit `Binary files a/<old> and b/<new> differ` with no
	// ---/+++ lines; parse the new-side path off that line.
	for _, line := range lines {
		if path, ok := diffBinaryFilesNewPath(line); ok {
			return path, true
		}
	}
	// Mode-only changes (old mode/new mode) have no path-bearing line at all;
	// parse the header, handling git's C-quoted form.
	return diffHeaderNewPath(lines[0])
}

// splitDiffSections splits git diff/show output into the pre-header prefix
// (numstat rows when --numstat was used) and one section per file, each
// section starting with its `diff --git ` header line. The split only breaks
// at lines that begin with the token — git's actual section-boundary
// grammar — so the token appearing inside a patch body or filename cannot
// truncate a real section or create a bogus one.
func splitDiffSections(output string) (prefix string, sections []string) {
	first := firstDiffHeaderIndex(output)
	prefix = output[:first]
	rest := output[first:]
	for rest != "" {
		// rest starts at a `diff --git ` header line; the next section
		// begins at the next line that starts with the token.
		next := strings.Index(rest, "\ndiff --git ")
		if next == -1 {
			sections = append(sections, rest)
			break
		}
		sections = append(sections, rest[:next+1])
		rest = rest[next+1:]
	}
	return prefix, sections
}

// firstDiffHeaderIndex returns the byte offset of the first line that starts
// with `diff --git `, or len(s) when there is none.
func firstDiffHeaderIndex(s string) int {
	for i := 0; i < len(s); {
		lineEnd := i
		for lineEnd < len(s) && s[lineEnd] != '\n' {
			lineEnd++
		}
		if strings.HasPrefix(s[i:lineEnd], "diff --git ") {
			return i
		}
		if lineEnd >= len(s) {
			return len(s)
		}
		i = lineEnd + 1
	}
	return len(s)
}

// diffHeaderNewPath parses the new-side path out of a `diff --git` header,
// supporting git's C-quoted form. The unquoted form (`a/<old> b/<new>`) can
// only be split at the first ` b/` — a path that itself contains ` b/`
// renders unquoted and is inherently ambiguous in git's text output; the
// per-line sources above disambiguate every section that carries them.
func diffHeaderNewPath(header string) (string, bool) {
	rest, ok := strings.CutPrefix(header, "diff --git ")
	if !ok {
		return "", false
	}
	if strings.HasPrefix(rest, `"`) {
		// Quoted: `"a/<old>" "b/<new>"`. Both tokens are C-quoted, so scan
		// the first token's closing quote escape-aware: a `\"` inside the
		// path must not be read as the end of the token. The second token
		// starts right after the closing quote and the separating space.
		oldEnd := quotedTokenEnd(rest)
		if oldEnd < 0 || oldEnd >= len(rest) || rest[oldEnd] != ' ' {
			return "", false
		}
		after := rest[oldEnd+1:]
		if !strings.HasPrefix(after, `"`) {
			return "", false
		}
		newEnd := quotedTokenEnd(after)
		if newEnd < 0 {
			return "", false
		}
		newPath := unquoteGitPath(after[:newEnd])
		return strings.TrimPrefix(newPath, "b/"), true
	}
	// Unquoted: `a/<old> b/<new>`. A path that itself contains ` b/` renders
	// unquoted and the first occurrence splits inside the old path. Mode-only
	// changes (the main consumer of this fallback) always carry identical
	// old/new paths, so scan candidates and prefer the separator whose two
	// sides unquote to equal paths — the same rule the binary parser uses.
	// Fall back to the first candidate when none match (a hypothetical
	// rename-without-rename-lines would land here).
	firstSep := -1
	search := rest
	base := 0
	for {
		sep := strings.Index(search, " b/")
		if sep < 0 {
			break
		}
		// `abs` points at the space; the `b/` prefix of the new side starts
		// one byte later, so the right side (prefix included) is rest[abs+1:].
		abs := base + sep
		if firstSep == -1 {
			firstSep = abs
		}
		left, leftOK := headerSidePath(rest[:abs], "a/")
		right, rightOK := headerSidePath(rest[abs+1:], "b/")
		if leftOK && rightOK && left == right {
			return right, true
		}
		base = abs + 3
		search = rest[base:]
	}
	if firstSep == -1 {
		return "", false
	}
	return unquoteGitPath(rest[firstSep+3:]), true
}

// headerSidePath strips a fixed a/ or b/ prefix (optionally inside git's
// C-quotes) off one side of a diff header or Binary files line, returning the
// unquoted path.
func headerSidePath(side, prefix string) (string, bool) {
	if strings.HasPrefix(side, `"`) {
		unquoted, err := strconv.Unquote(side)
		if err != nil {
			return "", false
		}
		side = unquoted
	}
	if !strings.HasPrefix(side, prefix) {
		return "", false
	}
	return strings.TrimPrefix(side, prefix), true
}

// diffWholeLinePath reads a path off a line whose entire remainder is the
// path (`rename to <path>`, `rename from <path>`), C-unquoting when git
// quoted it. One path per line, so spaces inside the path are unambiguous.
func diffWholeLinePath(prefix, line string) (string, bool) {
	rest, ok := strings.CutPrefix(line, prefix)
	if !ok {
		return "", false
	}
	if rest == "" {
		return "", false
	}
	return unquoteGitPath(rest), true
}

// quotedTokenEnd returns the index just past the closing quote of the first
// C-quoted token in s (which must start with `"`), or -1 when unterminated.
// Escapes are honored: a `\"` inside the path is content, not a terminator.
func quotedTokenEnd(s string) int {
	if !strings.HasPrefix(s, `"`) {
		return -1
	}
	for i := 1; i < len(s); i++ {
		switch s[i] {
		case '\\':
			i++ // skip the escaped byte
		case '"':
			return i + 1
		}
	}
	return -1
}

// diffBinaryFilesNewPath parses the new-side path off a
// `Binary files a/<old> and b/<new> differ` line (git's shape for binary
// sections, which carry no ---/+++ file lines). Both sides may be C-quoted:
// `Binary files "a/<old>" and "b/<new>" differ`.
//
// For a binary content change git emits the SAME path on both sides, so the
// separator whose two sides unquote to equal paths is authoritative — this
// resolves paths that themselves contain ` and b/` (a valid filename; the
// first-candidate scan would split inside the old path). When no candidate
// matches (binary rename with differing paths), the first ` and ` whose
// right side starts the `b/` token wins.
func diffBinaryFilesNewPath(line string) (string, bool) {
	rest, ok := strings.CutPrefix(line, "Binary files ")
	if !ok {
		return "", false
	}
	rest = strings.TrimSuffix(rest, " differ")
	if rest == "" {
		return "", false
	}

	firstSep := -1
	search := rest
	base := 0
	for {
		sep := strings.Index(search, " and ")
		if sep < 0 {
			break
		}
		abs := base + sep
		right := search[sep+5:]
		rightBare := strings.TrimPrefix(right, `"`)
		if strings.HasPrefix(rightBare, "b/") {
			if firstSep == -1 {
				firstSep = abs
			}
			// Equal-paths check: the left side is everything before this
			// candidate separator; both sides may be quoted.
			leftPath, leftOK := binarySidePath(rest[:abs])
			rightPath, rightOK := binarySidePath(right)
			if leftOK && rightOK && leftPath == rightPath {
				return rightPath, true
			}
		}
		base = abs + 5
		search = rest[base:]
	}
	if firstSep == -1 {
		return "", false
	}
	newSide, ok := binarySidePath(rest[firstSep+5:])
	if !ok {
		return "", false
	}
	return newSide, true
}

// binarySidePath strips the `a/` or `b/` prefix (optionally inside git's
// C-quotes) off one side of a `Binary files` line, returning the unquoted
// path.
func binarySidePath(side string) (string, bool) {
	if strings.HasPrefix(side, `"`) {
		unquoted, err := strconv.Unquote(side)
		if err != nil {
			return "", false
		}
		side = unquoted
	}
	if !strings.HasPrefix(side, "a/") && !strings.HasPrefix(side, "b/") {
		return "", false
	}
	return side[2:], true
}

// diffFileLinePath reads the path off one `+++ <path>` or `--- <path>` file
// line (per git's per-section file headers), returning ok=false for
// `/dev/null` sides and non-file lines. Quoted paths (git C-quoting) carry
// the `a/`/`b/` prefix inside the quotes; the trailing-tab whitespace marker
// sits outside them, so it is stripped before unquoting.
func diffFileLinePath(prefix, line string) (string, bool) {
	rest, ok := strings.CutPrefix(line, prefix+" ")
	if !ok {
		return "", false
	}
	if strings.HasPrefix(rest, "/dev/null") {
		return "", false
	}
	path := unquoteGitPath(strings.TrimSuffix(rest, "\t"))
	if prefix == "+++" {
		return strings.TrimPrefix(path, "b/"), true
	}
	return strings.TrimPrefix(path, "a/"), true
}

// numstatByPath indexes the `<adds>\t<dels>\t<path>` rows git writes for
// --numstat by their new-side path (parseNumstatEntry already expands git's
// `dir/{old => new}` rename notation, so the key matches the path
// parseCommitDiffWithOptions reads out of the `diff --git` header). Rows that
// are not numstat — the `--stat` table, which is tab-free — are skipped, and a
// binary file's `-\t-\t<path>` row yields a zero pair, which is the correct
// answer since binary changes have no lines.
func numstatByPath(block string) map[string]numstatEntry {
	rows := make(map[string]numstatEntry)
	for _, line := range strings.Split(block, "\n") {
		entry, ok := parseNumstatEntry(line)
		if !ok {
			continue
		}
		rows[entry.path] = entry
	}
	return rows
}

// fileLineCounts returns the add/delete counts for one file section: git's own
// --numstat row when the caller's command supplied one, otherwise a count of the
// patch body. Counting is positional (in-hunk), never textual — a removed
// `-- seed data` line arrives as `--- seed data` and a prefix test drops it.
func fileLineCounts(numstat map[string]numstatEntry, filePath, diffContent string) (additions, deletions int) {
	if entry, ok := numstat[filePath]; ok {
		return entry.additions, entry.deletions
	}
	return unidiff.CountLines(diffContent)
}

// applyDiffBudget enforces the per-file and cumulative byte budgets in opts and
// returns the (possibly truncated/empty) diff content plus a skip reason. A zero
// budget means "no cap" for that dimension. The cumulative budget is strict: the
// running total never exceeds totalMaxBytes — a file that would cross the
// boundary is clamped to the remaining budget rather than emitted in full.
func applyDiffBudget(diffContent string, totalSoFar int, opts parseCommitDiffOptions) (diff, skipReason string) {
	limit := len(diffContent)
	truncated := false

	if opts.totalMaxBytes > 0 {
		remaining := opts.totalMaxBytes - totalSoFar
		if remaining <= 0 {
			return "", diffSkipReasonBudgetExceeded
		}
		if remaining < limit {
			limit = remaining
			truncated = true
		}
	}

	if opts.perFileMaxBytes > 0 && opts.perFileMaxBytes < limit {
		limit = opts.perFileMaxBytes
		truncated = true
	}

	if truncated {
		return diffContent[:limit], diffSkipReasonTruncated
	}
	return diffContent, ""
}

// GetMergeBase returns the merge-base commit SHA between two refs (e.g., HEAD and origin/main).
// This is used to determine the common ancestor for filtering commits.
func (g *GitOperator) GetMergeBase(ctx context.Context, ref1, ref2 string) (string, error) {
	output, err := g.runGitCommand(ctx, "merge-base", ref1, ref2)
	if err != nil {
		return "", fmt.Errorf("failed to compute merge-base: %w", err)
	}
	return strings.TrimSpace(output), nil
}

// IsAncestor reports whether `ancestor` is an ancestor of `descendant`
// (git merge-base --is-ancestor). Exit 0 => true, exit 1 => false, any other
// exit status => error. git treats a commit as an ancestor of itself, so an
// equal SHA returns true; callers wanting a STRICT ancestor check compare the
// SHAs themselves first. Empty ancestor or descendant returns (false, nil)
// without invoking git.
func (g *GitOperator) IsAncestor(ctx context.Context, ancestor, descendant string) (bool, error) {
	if ancestor == "" || descendant == "" {
		return false, nil
	}
	_, err := g.runGitCommand(ctx, "merge-base", "--is-ancestor", ancestor, descendant)
	if err == nil {
		return true, nil
	}
	// runGitCommand wraps the underlying *exec.ExitError with fmt.Errorf("%w: ..."),
	// so errors.As recovers the exit code. Exit 1 is git's "not an ancestor"
	// signal; anything else is a genuine failure (bad repo, unknown SHA).
	var exitErr *exec.ExitError
	if errors.As(err, &exitErr) && exitErr.ExitCode() == 1 {
		return false, nil
	}
	return false, err
}

// GetRevParse resolves a ref name to its commit SHA. Returns the empty
// string when the ref doesn't exist; used as a no-common-ancestor fallback
// for the commits panel so a base branch with unrelated history still
// produces a stable anchor for `git log <tip>..HEAD`.
func (g *GitOperator) GetRevParse(ctx context.Context, ref string) (string, error) {
	output, err := g.runGitCommand(ctx, "rev-parse", ref)
	if err != nil {
		return "", fmt.Errorf("failed to rev-parse %q: %w", ref, err)
	}
	return strings.TrimSpace(output), nil
}