// Package continuity implements the validated, artifact-backed rotation contract.
package continuity

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"time"
)

type Dirty struct {
	Path   string `json:"path"`
	SHA256 string `json:"sha256"`
}
type Completed struct {
	What     string `json:"what"`
	Evidence string `json:"evidence"`
}
type DeadEnd struct {
	Tried     string `json:"tried"`
	WhyFailed string `json:"why_failed"`
}
type Anchor struct {
	GitSHA string  `json:"git_sha"`
	Branch string  `json:"branch"`
	Dirty  []Dirty `json:"dirty,omitempty"`
}
type Meta struct {
	ID            string `json:"id"`
	Parent        string `json:"parent,omitempty"`
	Reason        string `json:"reason"`
	RotationIndex int    `json:"rotation_index"`
}
type Handoff struct {
	Meta          Meta        `json:"meta"`
	Anchor        Anchor      `json:"anchor"`
	Goal          string      `json:"goal"`
	DoneWhen      []string    `json:"done_when"`
	Completed     []Completed `json:"completed"`
	Remaining     []string    `json:"remaining"`
	Action        string      `json:"action"`
	Command       string      `json:"command"`
	Files         []string    `json:"files"`
	Invariants    []string    `json:"invariants"`
	DeadEnds      []DeadEnd   `json:"dead_ends"`
	OpenQuestions []string    `json:"open_questions"`
	Build         string      `json:"build"`
	Test          string      `json:"test"`
	LastResult    Result      `json:"last_result"`
}
type Result struct {
	Command  string `json:"command"`
	ExitCode int    `json:"exit_code"`
	AtSHA    string `json:"at_sha"`
}

var reasons = map[string]bool{"threshold": true, "milestone": true, "thrash": true, "manual": true}

func (h Handoff) Validate() error {
	if strings.TrimSpace(h.Meta.ID) == "" || !reasons[h.Meta.Reason] || h.Meta.RotationIndex < 0 {
		return errors.New("invalid handoff meta")
	}
	if len(h.Anchor.GitSHA) != 40 || h.Anchor.Branch == "" || strings.TrimSpace(h.Goal) == "" || len(h.DoneWhen) == 0 || strings.TrimSpace(h.Action) == "" || strings.TrimSpace(h.Command) == "" {
		return errors.New("invalid handoff required fields")
	}
	for _, d := range h.Anchor.Dirty {
		if filepath.IsAbs(d.Path) || d.Path == "" || len(d.SHA256) != 64 {
			return errors.New("invalid dirty anchor")
		}
	}
	for _, d := range h.DeadEnds {
		if strings.TrimSpace(d.Tried) == "" || strings.TrimSpace(d.WhyFailed) == "" {
			return errors.New("invalid dead end")
		}
	}
	return nil
}
func Encode(h Handoff) ([]byte, error) {
	if err := h.Validate(); err != nil {
		return nil, err
	}
	return json.Marshal(h)
}
func Decode(b []byte) (Handoff, error) {
	var h Handoff
	d := json.NewDecoder(strings.NewReader(string(b)))
	d.DisallowUnknownFields()
	if err := d.Decode(&h); err != nil {
		return h, err
	}
	return h, h.Validate()
}

func ValidatePickup(root string, h Handoff, taskFileSHA string) error {
	if err := h.Validate(); err != nil {
		return err
	}
	out, err := exec.Command("git", "-C", root, "rev-parse", "HEAD").Output()
	if err != nil || strings.TrimSpace(string(out)) != h.Anchor.GitSHA {
		return errors.New("handoff anchor HEAD mismatch")
	}
	for _, d := range h.Anchor.Dirty {
		b, e := os.ReadFile(filepath.Join(root, d.Path))
		if e != nil {
			return e
		}
		sum := sha256.Sum256(b)
		if hex.EncodeToString(sum[:]) != d.SHA256 {
			return fmt.Errorf("handoff dirty file changed: %s", d.Path)
		}
	}
	if taskFileSHA != "" {
		b, e := os.ReadFile(filepath.Join(root, "TASK.md"))
		if e != nil {
			return e
		}
		sum := sha256.Sum256(b)
		if hex.EncodeToString(sum[:]) != taskFileSHA {
			return errors.New("TASK.md changed")
		}
	}
	return nil
}

// VerifyTaskFile ensures the worktree contains the original, immutable task.
func VerifyTaskFile(root, taskFileSHA string) error {
	if taskFileSHA == "" {
		return errors.New("TASK.md hash required")
	}
	b, err := os.ReadFile(filepath.Join(root, "TASK.md"))
	if err != nil {
		return err
	}
	sum := sha256.Sum256(b)
	if hex.EncodeToString(sum[:]) != taskFileSHA {
		return errors.New("TASK.md changed")
	}
	return nil
}

type CAS interface {
	PutArtifact([]byte) (string, error)
	Artifact(string) ([]byte, error)
}

func Save(h Handoff, cas CAS) (string, error) {
	b, e := Encode(h)
	if e != nil {
		return "", e
	}
	return cas.PutArtifact(b)
}
func Load(ref string, cas CAS) (Handoff, error) {
	b, e := cas.Artifact(ref)
	if e != nil {
		return Handoff{}, e
	}
	return Decode(b)
}

type Notice struct {
	Path   string
	SHA256 string
	At     time.Time
}

func MarkdownChanges(root string, paths []string) ([]Notice, error) {
	out := []Notice{}
	for _, p := range paths {
		if !strings.HasSuffix(strings.ToLower(p), ".md") {
			continue
		}
		b, e := os.ReadFile(filepath.Join(root, p))
		if e != nil {
			return nil, e
		}
		s := sha256.Sum256(b)
		out = append(out, Notice{p, hex.EncodeToString(s[:]), time.Now().UTC()})
	}
	return out, nil
}

// ScratchCommit records WIP atomically on a dedicated branch before rotation.
func ScratchCommit(root, branch, message string) error {
	if branch == "" || strings.ContainsAny(branch, " \t\n") {
		return errors.New("invalid scratch branch")
	}
	if strings.TrimSpace(message) == "" {
		return errors.New("scratch commit message required")
	}
	status, err := exec.Command("git", "-C", root, "status", "--porcelain", "--", "TASK.md").Output()
	if err != nil {
		return err
	}
	if len(status) != 0 {
		return errors.New("TASK.md is immutable")
	}
	if strings.TrimSpace(message) == "" {
		return errors.New("scratch commit message required")
	}
	for _, args := range [][]string{{"switch", "-c", branch}, {"add", "-A"}, {"commit", "-m", message}} {
		if err := exec.Command("git", append([]string{"-C", root}, args...)...).Run(); err != nil {
			return err
		}
	}
	return nil
}

func ScratchPush(root, branch, remote string) error {
	if branch == "" || remote == "" {
		return errors.New("scratch branch and remote required")
	}
	return exec.Command("git", "-C", root, "push", remote, branch).Run()
}

func ScratchPull(root, branch, remote string) error {
	if branch == "" || remote == "" {
		return errors.New("scratch branch and remote required")
	}
	if err := exec.Command("git", "-C", root, "fetch", remote, branch).Run(); err != nil {
		return err
	}
	return exec.Command("git", "-C", root, "merge", "--ff-only", "FETCH_HEAD").Run()
}

// ScratchSync pushes/pulls a scratch branch. Pull uses fast-forward-only to
// avoid silently merging independent WIP histories.
func ScratchSync(root, branch, remote string, push bool) error {
	if branch == "" || strings.ContainsAny(branch, " \t\n") || remote == "" {
		return errors.New("invalid scratch sync")
	}
	args := []string{"-C", root, "push", remote, branch}
	if !push {
		args = []string{"-C", root, "fetch", remote, branch}
	}
	if err := exec.Command("git", args...).Run(); err != nil {
		return err
	}
	if !push {
		return exec.Command("git", "-C", root, "merge", "--ff-only", "FETCH_HEAD").Run()
	}
	return nil
}