package gitutil

import (
	"fmt"
	"os/exec"
	"strings"
)

const zeroSHA = "0000000000000000000000000000000000000000"

type CommitMsg struct {
	SHA     string
	Message string
}

// RevListMessages returns sha and full message for commits reachable from
// new but not old, newest first, capped at max. An empty or zero old (new
// branch) lists from new alone, still capped.
func RevListMessages(dir, old, new string, max int) ([]CommitMsg, error) {
	args := []string{"-C", dir, "rev-list", fmt.Sprintf("-n%d", max), "--format=%B%x00", new}
	if old != "" && old != zeroSHA {
		args = append(args, "^"+old)
	}
	out, err := exec.Command("git", args...).Output()
	if err != nil {
		return nil, fmt.Errorf("rev-list messages: %w", err)
	}
	var msgs []CommitMsg
	for _, chunk := range strings.Split(string(out), "\x00") {
		chunk = strings.TrimLeft(chunk, "\n")
		if chunk == "" {
			continue
		}
		header, body, ok := strings.Cut(chunk, "\n")
		if !ok || !strings.HasPrefix(header, "commit ") {
			continue
		}
		msgs = append(msgs, CommitMsg{SHA: strings.TrimPrefix(header, "commit "), Message: strings.TrimSpace(body)})
	}
	return msgs, nil
}