// Package git is THE boundary for every Git operation (AGENT.md §1.3). All Git
// access — read and, later, write — goes through here by shelling out to the
// system `git` binary via os/exec, so the user's credential helpers, SSH keys,
// hooks, and config apply exactly.
//
// The methods below are all READ-ONLY. Mutating operations (checkout, commit,
// push, …) will be added here as they are built; destructive ones must obey
// AGENT.md §1.4 (explicit, confirmed, never automatic, never a default).
package git

import (
	"bytes"
	"context"
	"os/exec"
	"strconv"
	"strings"
)

// CLI runs Git commands via the system binary.
type CLI struct {
	Bin string // path to git; "git" resolves from PATH
}

// New returns a CLI using the given binary (defaults to "git").
func New(bin string) *CLI {
	if bin == "" {
		bin = "git"
	}
	return &CLI{Bin: bin}
}

// run executes `git <args...>` in dir and returns trimmed stdout. On failure it
// returns an error whose message includes stderr.
func (c *CLI) run(ctx context.Context, dir string, args ...string) (string, error) {
	cmd := exec.CommandContext(ctx, c.Bin, args...)
	cmd.Dir = dir
	var stdout, stderr bytes.Buffer
	cmd.Stdout = &stdout
	cmd.Stderr = &stderr
	if err := cmd.Run(); err != nil {
		msg := strings.TrimSpace(stderr.String())
		if msg != "" {
			return "", &Error{Args: args, Stderr: msg, Err: err}
		}
		return "", &Error{Args: args, Err: err}
	}
	return strings.TrimSpace(stdout.String()), nil
}

// Error describes a failed git invocation.
type Error struct {
	Args   []string
	Stderr string
	Err    error
}

func (e *Error) Error() string {
	s := "git " + strings.Join(e.Args, " ") + ": " + e.Err.Error()
	if e.Stderr != "" {
		s += ": " + e.Stderr
	}
	return s
}

func (e *Error) Unwrap() error { return e.Err }

// Version returns the installed git version string.
func (c *CLI) Version(ctx context.Context) (string, error) {
	return c.run(ctx, "", "version")
}

// CurrentBranch returns the checked-out branch, or "HEAD" when detached.
func (c *CLI) CurrentBranch(ctx context.Context, dir string) (string, error) {
	return c.run(ctx, dir, "rev-parse", "--abbrev-ref", "HEAD")
}

// IsDirty reports whether the working tree has staged or unstaged changes.
func (c *CLI) IsDirty(ctx context.Context, dir string) (bool, error) {
	out, err := c.run(ctx, dir, "status", "--porcelain")
	if err != nil {
		return false, err
	}
	return out != "", nil
}

// AheadBehind returns how many commits HEAD is ahead of and behind its upstream.
// Both are 0 with no error when there is no configured upstream.
func (c *CLI) AheadBehind(ctx context.Context, dir string) (ahead, behind int, err error) {
	out, err := c.run(ctx, dir, "rev-list", "--left-right", "--count", "@{u}...HEAD")
	if err != nil {
		// No upstream is a normal state, not a failure to report.
		return 0, 0, nil
	}
	fields := strings.Fields(out)
	if len(fields) != 2 {
		return 0, 0, nil
	}
	behind, _ = strconv.Atoi(fields[0])
	ahead, _ = strconv.Atoi(fields[1])
	return ahead, behind, nil
}

// Remotes returns the configured remote names.
func (c *CLI) Remotes(ctx context.Context, dir string) ([]string, error) {
	out, err := c.run(ctx, dir, "remote")
	if err != nil {
		return nil, err
	}
	if out == "" {
		return nil, nil
	}
	return strings.Split(out, "\n"), nil
}

// Fetch updates remote-tracking refs. It does not modify the working tree, but
// it does touch the network, so the scanner only calls it when explicitly
// enabled (AGENT.md §5).
func (c *CLI) Fetch(ctx context.Context, dir string) error {
	_, err := c.run(ctx, dir, "fetch", "--quiet", "--all")
	return err
}

// Branch is a local branch and its upstream, if any.
type Branch struct {
	Name     string `json:"name"`
	Current  bool   `json:"current"`
	Upstream string `json:"upstream,omitempty"`
}

// Commit is a single log entry.
type Commit struct {
	Short   string `json:"short"`
	Author  string `json:"author"`
	Date    string `json:"date"`
	Subject string `json:"subject"`
}

// Remote is a named remote and its fetch URL.
type Remote struct {
	Name string `json:"name"`
	URL  string `json:"url"`
}

// LocalBranches lists local branches (refs/heads), flagging the current one and
// including each branch's upstream when set.
func (c *CLI) LocalBranches(ctx context.Context, dir string) ([]Branch, error) {
	const format = "%(refname:short)%09%(HEAD)%09%(upstream:short)"
	out, err := c.run(ctx, dir, "for-each-ref", "--format="+format, "refs/heads")
	if err != nil {
		return nil, err
	}
	var branches []Branch
	for _, line := range splitLines(out) {
		f := strings.Split(line, "\t")
		if len(f) < 1 || f[0] == "" {
			continue
		}
		b := Branch{Name: f[0]}
		if len(f) > 1 {
			b.Current = f[1] == "*"
		}
		if len(f) > 2 {
			b.Upstream = f[2]
		}
		branches = append(branches, b)
	}
	return branches, nil
}

// RecentCommits returns the newest n commits reachable from HEAD.
func (c *CLI) RecentCommits(ctx context.Context, dir string, n int) ([]Commit, error) {
	// Fields separated by TAB (%x09); records by newline.
	const format = "%h%x09%an%x09%ad%x09%s"
	out, err := c.run(ctx, dir, "log", "-n", strconv.Itoa(n), "--date=short", "--pretty=format:"+format)
	if err != nil {
		return nil, err
	}
	var commits []Commit
	for _, line := range splitLines(out) {
		f := strings.SplitN(line, "\t", 4)
		if len(f) < 4 {
			continue
		}
		commits = append(commits, Commit{Short: f[0], Author: f[1], Date: f[2], Subject: f[3]})
	}
	return commits, nil
}

// RemoteDetails returns each remote with its fetch URL.
func (c *CLI) RemoteDetails(ctx context.Context, dir string) ([]Remote, error) {
	out, err := c.run(ctx, dir, "remote", "-v")
	if err != nil {
		return nil, err
	}
	seen := make(map[string]struct{})
	var remotes []Remote
	for _, line := range splitLines(out) {
		// Format: "<name>\t<url> (fetch|push)"
		f := strings.Fields(line)
		if len(f) < 3 || f[2] != "(fetch)" {
			continue
		}
		if _, ok := seen[f[0]]; ok {
			continue
		}
		seen[f[0]] = struct{}{}
		remotes = append(remotes, Remote{Name: f[0], URL: f[1]})
	}
	return remotes, nil
}

// splitLines splits on newlines, dropping empty lines.
func splitLines(s string) []string {
	if s == "" {
		return nil
	}
	var out []string
	for _, line := range strings.Split(s, "\n") {
		if line != "" {
			out = append(out, line)
		}
	}
	return out
}