package agent

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"strings"
)

// APIError is a non-2xx response from the Gitea API. It carries the status code
// so callers can react to specific failures (e.g. tolerate a 404 for a commit
// whose branch was deleted after a merge) instead of parsing error strings.
type APIError struct {
	Method     string
	Path       string
	StatusCode int
	Body       string
}

func (e *APIError) Error() string {
	return fmt.Sprintf("gitea %s %s: HTTP %d: %s", e.Method, e.Path, e.StatusCode, e.Body)
}

// IsNotFound reports whether err is a Gitea 404. Gitea hides repositories a
// caller may not see behind a 404 rather than a 403, so this also covers a repo
// that was renamed, deleted, or made private.
func IsNotFound(err error) bool {
	var apiErr *APIError
	return errors.As(err, &apiErr) && apiErr.StatusCode == http.StatusNotFound
}

// IsAuthError reports whether err is a Gitea 401/403: the token is expired or
// unauthorised, which retrying the same request cannot fix.
func IsAuthError(err error) bool {
	var apiErr *APIError
	return errors.As(err, &apiErr) &&
		(apiErr.StatusCode == http.StatusUnauthorized || apiErr.StatusCode == http.StatusForbidden)
}

// GiteaClient talks to the Gitea REST API as the agent user.
type GiteaClient struct {
	BaseURL string
	Token   string
	HTTP    *http.Client
	// Refresh mints a replacement token when the current one is rejected; Vault's
	// Gitea tokens expire in ~1h, far short of a watchpr run.
	Refresh func() (string, error)
}

// NewGiteaClient builds a client from the configured base URL and a Vault-minted
// token, re-minting from Vault when that token expires.
func NewGiteaClient(token string) *GiteaClient {
	return &GiteaClient{BaseURL: GiteaURL(), Token: token, HTTP: httpClient, Refresh: RefreshGiteaToken}
}

// do sends the request and, if the token was rejected, re-mints it once and
// replays the request with the fresh token.
func (c *GiteaClient) do(method, path string, body any, out any) error {
	var payload []byte
	if body != nil {
		b, err := json.Marshal(body)
		if err != nil {
			return err
		}
		payload = b
	}
	err := c.attempt(method, path, payload, out)
	if !IsAuthError(err) || c.Refresh == nil {
		return err
	}
	token, refreshErr := c.Refresh()
	if refreshErr != nil {
		return fmt.Errorf("%w; re-minting token: %v", err, refreshErr)
	}
	c.Token = token
	return c.attempt(method, path, payload, out)
}

func (c *GiteaClient) attempt(method, path string, body []byte, out any) error {
	var reader io.Reader
	if body != nil {
		reader = bytes.NewReader(body)
	}
	url := strings.TrimRight(c.BaseURL, "/") + path
	req, err := http.NewRequest(method, url, reader)
	if err != nil {
		return err
	}
	// An empty token means anonymous access, which public repos serve fine.
	if c.Token != "" {
		req.Header.Set("Authorization", "token "+c.Token)
	}
	req.Header.Set("Accept", "application/json")
	if body != nil {
		req.Header.Set("Content-Type", "application/json")
	}

	resp, err := c.HTTP.Do(req)
	if err != nil {
		return err
	}
	defer func() { _ = resp.Body.Close() }()
	data, _ := io.ReadAll(resp.Body)
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return &APIError{Method: method, Path: path, StatusCode: resp.StatusCode, Body: strings.TrimSpace(string(data))}
	}
	if out != nil && len(data) > 0 {
		if err := json.Unmarshal(data, out); err != nil {
			return fmt.Errorf("gitea %s %s: decoding response: %w", method, path, err)
		}
	}
	return nil
}

// User is the subset of the Gitea user object we care about.
type User struct {
	Login string `json:"login"`
	ID    int64  `json:"id"`
}

// Whoami returns the authenticated user (GET /api/v1/user).
func (c *GiteaClient) Whoami() (User, error) {
	var u User
	err := c.do(http.MethodGet, "/api/v1/user", nil, &u)
	return u, err
}

// PullRequest is the subset of Gitea's PR object we track.
type PullRequest struct {
	Number    int    `json:"number"`
	State     string `json:"state"`
	Title     string `json:"title"`
	Merged    bool   `json:"merged"`
	Mergeable bool   `json:"mergeable"`
	HTMLURL   string `json:"html_url"`
	Head      struct {
		Sha   string `json:"sha"`
		Ref   string `json:"ref"`
		Label string `json:"label"`
	} `json:"head"`
}

// prPageSize is the per-page limit for the pulls listing; maxPRPages caps how
// far back a listing walks.
const (
	prPageSize = 50
	maxPRPages = 20
)

// ErrPRListTruncated reports that a listing hit the page cap, so the returned
// pull requests are only the most recent ones and older PRs went unseen.
var ErrPRListTruncated = errors.New("pull request listing truncated at the page cap")

// ListPRs lists a repo's pull requests in the given state ("open", "closed" or
// "all"), following pagination. A repo with more PRs than the page cap returns
// the PRs it did read alongside ErrPRListTruncated.
func (c *GiteaClient) ListPRs(repoPath, state string) ([]PullRequest, error) {
	if state == "" {
		state = "all"
	}
	var all []PullRequest
	for page := 1; page <= maxPRPages; page++ {
		var batch []PullRequest
		path := fmt.Sprintf("/api/v1/repos/%s/pulls?state=%s&limit=%d&page=%d", repoPath, state, prPageSize, page)
		if err := c.do(http.MethodGet, path, nil, &batch); err != nil {
			return nil, err
		}
		all = append(all, batch...)
		if len(batch) < prPageSize {
			return all, nil
		}
	}
	return all, fmt.Errorf("%s: %w after %d pull requests", repoPath, ErrPRListTruncated, len(all))
}

// PRHeadBranch returns the branch a PR was opened from. Gitea rewrites head.ref
// to "refs/pull/<n>/head" once the branch is deleted (which merging does), so
// head.label — which keeps the original name — is authoritative.
func PRHeadBranch(pr PullRequest) string {
	if label := pr.Head.Label; label != "" && !strings.HasPrefix(label, "refs/pull/") {
		// Cross-repo PRs label as "<owner>:<branch>".
		if _, branch, ok := strings.Cut(label, ":"); ok {
			return branch
		}
		return label
	}
	ref := pr.Head.Ref
	if strings.HasPrefix(ref, "refs/pull/") {
		return ""
	}
	return strings.TrimPrefix(ref, "refs/heads/")
}

// IsOpen reports whether a PR is still open (not merged, not closed).
func (pr PullRequest) IsOpen() bool {
	return pr.State == "open" && !pr.Merged
}

// CreatePROptions are the fields for opening a PR.
type CreatePROptions struct {
	Base  string `json:"base"`
	Head  string `json:"head"`
	Title string `json:"title"`
	Body  string `json:"body"`
}

// CreatePR opens a pull request (POST /api/v1/repos/{owner}/{repo}/pulls).
func (c *GiteaClient) CreatePR(repoPath string, opts CreatePROptions) (PullRequest, error) {
	var pr PullRequest
	err := c.do(http.MethodPost, "/api/v1/repos/"+repoPath+"/pulls", opts, &pr)
	return pr, err
}

// EditPROptions are the fields an edit may change. Pointers so an unset field
// is omitted from the payload entirely, leaving that field as it is. The two
// fields are not symmetric: Gitea only applies a title when it is non-empty,
// so Title can be set but never cleared and a "" title is a silent no-op,
// while a pointer to "" Body really does blank the body.
type EditPROptions struct {
	Title *string `json:"title,omitempty"`
	Body  *string `json:"body,omitempty"`
}

// EditPR updates a pull request's title and/or body
// (PATCH /api/v1/repos/{owner}/{repo}/pulls/{index}).
func (c *GiteaClient) EditPR(repoPath string, number int, opts EditPROptions) (PullRequest, error) {
	var pr PullRequest
	err := c.do(http.MethodPatch, fmt.Sprintf("/api/v1/repos/%s/pulls/%d", repoPath, number), opts, &pr)
	return pr, err
}

// GetPR fetches a single pull request.
func (c *GiteaClient) GetPR(repoPath string, number int) (PullRequest, error) {
	var pr PullRequest
	err := c.do(http.MethodGet, fmt.Sprintf("/api/v1/repos/%s/pulls/%d", repoPath, number), nil, &pr)
	return pr, err
}

// Comment is the subset of an issue comment we track.
type Comment struct {
	ID   int64  `json:"id"`
	User User   `json:"user"`
	Body string `json:"body"`
}

// CreateComment posts a comment on the PR's issue thread
// (POST /api/v1/repos/{owner}/{repo}/issues/{n}/comments).
func (c *GiteaClient) CreateComment(repoPath string, number int, body string) (Comment, error) {
	var cm Comment
	payload := map[string]string{"body": body}
	err := c.do(http.MethodPost, fmt.Sprintf("/api/v1/repos/%s/issues/%d/comments", repoPath, number), payload, &cm)
	return cm, err
}

// ListComments lists the PR's issue comments.
func (c *GiteaClient) ListComments(repoPath string, number int) ([]Comment, error) {
	var out []Comment
	err := c.do(http.MethodGet, fmt.Sprintf("/api/v1/repos/%s/issues/%d/comments", repoPath, number), nil, &out)
	return out, err
}

// CombinedStatus is the combined commit status for a ref.
type CombinedStatus struct {
	State string `json:"state"`
}

// CommitStatus returns the combined CI status for a commit SHA
// (GET /api/v1/repos/{owner}/{repo}/commits/{sha}/status). An empty ref yields
// an empty state without an API call.
func (c *GiteaClient) CommitStatus(repoPath, sha string) (string, error) {
	if sha == "" {
		return "", nil
	}
	var cs CombinedStatus
	err := c.do(http.MethodGet, fmt.Sprintf("/api/v1/repos/%s/commits/%s/status", repoPath, sha), nil, &cs)
	return cs.State, err
}