package registries

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"time"

	"github.com/modelcontextprotocol/registry/pkg/model"
)

var (
	ErrMissingIdentifierForCargo = errors.New("package identifier is required for Cargo packages")
	ErrMissingVersionForCargo    = errors.New("package version is required for Cargo packages")
)

// cargoUserAgent identifies the validator to crates.io. crates.io's crawler
// policy expects a non-generic User-Agent with a contact URL; a bare UA may be
// rate-limited or blocked. (Distinct from the package-level userAgent constant
// used by the NuGet validator, which has no contact URL.)
const cargoUserAgent = "MCP-Registry-Validator/1.0 (https://registry.modelcontextprotocol.io)"

// cargoStaticHost is the CDN host crates.io serves rendered READMEs from.
const cargoStaticHost = "static.crates.io"

// maxCargoReadmeBytes caps how much of a rendered README we buffer, so a hostile
// or oversized response cannot exhaust validator memory.
const maxCargoReadmeBytes = 5 << 20 // 5 MiB

// CargoReadmeMetaResponse is the structure returned by the crates.io readme metadata endpoint.
//
// With `Accept: application/json`, crates.io's /api/v1/crates/{name}/{version}/readme
// endpoint returns 200 OK with a JSON body containing a `url` field that points to the
// rendered README on the static CDN. (Without the Accept header, or via HEAD, the same
// endpoint emits a 302 redirect to the CDN URL — the validator uses the JSON path so
// that crates.io controls where the README lives.) Validators must follow the pointer
// to retrieve the actual README content.
type CargoReadmeMetaResponse struct {
	URL string `json:"url"`
}

// ValidateCargo validates that a Cargo (crates.io) package contains the correct MCP server name.
//
// Verification mechanism: the `mcp-name: <server-name>` token is searched for in the package's
// rendered README. This mirrors the PyPI validator's README-token approach (see ValidatePyPI),
// requiring no Cargo.toml parsing on the registry side. Crate authors add a single line
// `mcp-name: io.github.OWNER/REPO` to their README before publishing.
//
// Two-call retrieval pattern:
//  1. GET https://crates.io/api/v1/crates/{name}/{version}/readme
//     → 200 OK with JSON: {"url": "https://static.crates.io/readmes/.../...html"}
//  2. GET <url from step 1>
//     → 200 OK with rendered README HTML, or 403 if the crate/version is missing
//
// The two-call pattern stays on the documented crates.io API surface rather than relying
// on the CDN URL layout being stable.
func ValidateCargo(ctx context.Context, pkg model.Package, serverName string) error {
	// Set default registry base URL if empty
	if pkg.RegistryBaseURL == "" {
		pkg.RegistryBaseURL = model.RegistryURLCrates
	}

	if pkg.Identifier == "" {
		return ErrMissingIdentifierForCargo
	}

	if pkg.Version == "" {
		return ErrMissingVersionForCargo
	}

	// Validate that MCPB-specific fields are not present
	if pkg.FileSHA256 != "" {
		return fmt.Errorf("cargo packages must not have 'fileSha256' field - this is only for MCPB packages")
	}

	// Validate that the registry base URL matches crates.io exactly
	if pkg.RegistryBaseURL != model.RegistryURLCrates {
		return fmt.Errorf("registry type and base URL do not match: '%s' is not valid for registry type '%s'. Expected: %s",
			pkg.RegistryBaseURL, model.RegistryTypeCargo, model.RegistryURLCrates)
	}

	return validateCargoREADME(ctx, pkg, serverName)
}

// cargoAllowedHosts returns the set of hosts the validator is permitted to talk
// to for a given base URL. For the real crates.io base this is crates.io (the API)
// plus static.crates.io (the rendered-README CDN). For any other base — only the
// httptest-driven tests, since the public ValidateCargo pins the base to
// crates.io — it is the base host itself, so mock servers keep working.
//
// This is the allowlist enforced both on the README pointer (step 2 URL) and on
// every redirect hop, so a metadata response or redirect cannot steer the
// validator at an internal or attacker-chosen host (SSRF).
func cargoAllowedHosts(baseURL string) map[string]struct{} {
	hosts := map[string]struct{}{}
	if u, err := url.Parse(baseURL); err == nil && u.Hostname() != "" {
		hosts[u.Hostname()] = struct{}{}
	}
	if baseURL == model.RegistryURLCrates {
		hosts[cargoStaticHost] = struct{}{}
	}
	return hosts
}

// cargoURLAllowed reports whether u is a URL the validator may fetch for the
// given base. The host must be in allowedHosts; additionally, for the real
// crates.io base, the scheme must be https and the port must be the default —
// so a metadata response or redirect cannot downgrade to http or steer the
// validator at a non-standard port on an otherwise-trusted host. For test bases
// (httptest servers) only the host is checked, so mocks keep working.
func cargoURLAllowed(u *url.URL, baseURL string, allowedHosts map[string]struct{}) bool {
	if _, ok := allowedHosts[u.Hostname()]; !ok {
		return false
	}
	if baseURL == model.RegistryURLCrates {
		if u.Scheme != "https" {
			return false
		}
		if p := u.Port(); p != "" && p != "443" {
			return false
		}
	}
	return true
}

// newCargoHTTPClient builds the client used for all crates.io calls. The
// CheckRedirect policy pins every redirect hop via cargoURLAllowed, so even
// though the initial URL is checked, an upstream 3xx cannot redirect the
// validator to an unexpected host, scheme, or port.
func newCargoHTTPClient(baseURL string, allowedHosts map[string]struct{}) *http.Client {
	return &http.Client{
		Timeout: 10 * time.Second,
		CheckRedirect: func(req *http.Request, via []*http.Request) error {
			if !cargoURLAllowed(req.URL, baseURL, allowedHosts) {
				return fmt.Errorf("refusing redirect to unexpected URL %q", req.URL.Redacted())
			}
			if len(via) >= 10 {
				return errors.New("stopped after 10 redirects")
			}
			return nil
		},
	}
}

// cargoVersionState is the outcome of probing the crate-version metadata
// endpoint, used to disambiguate a 403 from the README CDN.
type cargoVersionState int

const (
	// cargoVersionUnknown: the probe returned an unexpected status we can't classify.
	cargoVersionUnknown cargoVersionState = iota
	// cargoVersionExists: the crate version exists (200).
	cargoVersionExists
	// cargoVersionMissing: the crate version does not exist (404).
	cargoVersionMissing
	// cargoVersionTransient: the probe failed for a retryable reason (network
	// error, 429, or 5xx) — existence is undetermined and the caller should not
	// report "not found".
	cargoVersionTransient
)

// probeCargoVersion checks whether a specific crate version exists on crates.io.
// static.crates.io (S3) returns 403 both for a genuinely-missing crate/version
// AND for a crate that exists but has no rendered README, so a 403 from the CDN
// alone cannot tell a publisher which it is; this probe disambiguates, while
// distinguishing a transient failure from a definitive missing/exists answer.
func probeCargoVersion(ctx context.Context, client *http.Client, baseURL, identifier, version string) cargoVersionState {
	versionURL := fmt.Sprintf("%s/api/v1/crates/%s/%s",
		baseURL, url.PathEscape(identifier), url.PathEscape(version))
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, versionURL, nil)
	if err != nil {
		return cargoVersionUnknown
	}
	req.Header.Set("User-Agent", cargoUserAgent)
	req.Header.Set("Accept", "application/json")

	resp, err := client.Do(req)
	if err != nil {
		return cargoVersionTransient
	}
	defer resp.Body.Close()

	switch {
	case resp.StatusCode == http.StatusOK:
		return cargoVersionExists
	case resp.StatusCode == http.StatusNotFound:
		return cargoVersionMissing
	case resp.StatusCode == http.StatusTooManyRequests, resp.StatusCode >= 500 && resp.StatusCode < 600:
		return cargoVersionTransient
	default:
		return cargoVersionUnknown
	}
}

// cargoMetadataStatusError maps a non-200 status from the metadata endpoint to a
// caller-actionable error, distinguishing transient upstream conditions (429,
// 5xx) from a genuine fetch failure.
func cargoMetadataStatusError(identifier string, status int) error {
	switch {
	case status == http.StatusTooManyRequests:
		return fmt.Errorf("crates.io rate-limited the metadata request for cargo package '%s' (status: 429) — likely transient, retry later", identifier)
	case status >= 500 && status < 600:
		return fmt.Errorf("crates.io upstream error fetching metadata for cargo package '%s' (status: %d) — likely transient, retry later", identifier, status)
	default:
		return fmt.Errorf("cargo package '%s' metadata fetch failed (status: %d)", identifier, status)
	}
}

// cargoReadmeStatusError maps a non-200 status from the README CDN to a
// caller-actionable error. 429/5xx are transient; 403 is disambiguated (see
// cargoReadme403Error) because static.crates.io returns it both for a missing
// crate/version and for a crate that has no rendered README.
func cargoReadmeStatusError(ctx context.Context, client *http.Client, pkg model.Package, serverName string, status int) error {
	switch {
	case status == http.StatusTooManyRequests:
		return fmt.Errorf("crates.io rate-limited the README fetch for cargo package '%s' version '%s' (status: 429) — likely transient, retry later", pkg.Identifier, pkg.Version)
	case status >= 500 && status < 600:
		return fmt.Errorf("crates.io upstream error fetching README for cargo package '%s' version '%s' (status: %d) — likely transient, retry later", pkg.Identifier, pkg.Version, status)
	case status == http.StatusForbidden:
		return cargoReadme403Error(ctx, client, pkg, serverName)
	default:
		return fmt.Errorf("cargo package '%s' version '%s' README fetch failed (status: %d)", pkg.Identifier, pkg.Version, status)
	}
}

// cargoReadme403Error disambiguates a 403 from static.crates.io (S3's default
// for a missing key): a genuinely-missing crate/version versus a crate that
// exists but has no rendered README. It probes the crate-version metadata
// endpoint so the publisher gets an actionable message rather than a blanket
// "not found".
func cargoReadme403Error(ctx context.Context, client *http.Client, pkg model.Package, serverName string) error {
	switch probeCargoVersion(ctx, client, pkg.RegistryBaseURL, pkg.Identifier, pkg.Version) {
	case cargoVersionExists:
		// The crate/version exists but the README CDN returned 403. The likely
		// cause is a missing README, but a 403 is not definitive proof (e.g. a
		// transient CDN/WAF block), so don't flatly assert "no README".
		return fmt.Errorf("cargo package '%s' version '%s' exists on crates.io, but its rendered README could not be retrieved (status: 403). If it has no README, add one containing 'mcp-name: %s' and publish a new version", pkg.Identifier, pkg.Version, serverName)
	case cargoVersionMissing:
		return fmt.Errorf("cargo package '%s' version '%s' not found on crates.io", pkg.Identifier, pkg.Version)
	case cargoVersionTransient:
		return fmt.Errorf("crates.io could not confirm cargo package '%s' version '%s' (README status: 403, version check inconclusive) — likely transient, retry later", pkg.Identifier, pkg.Version)
	case cargoVersionUnknown:
		// Probe returned an unclassifiable status — fall through to the
		// best-effort message below.
	}
	return fmt.Errorf("cargo package '%s' version '%s' not found on crates.io (status: 403)", pkg.Identifier, pkg.Version)
}

// validateCargoREADME performs the two-call README fetch and the mcp-name token
// check. It is split out from ValidateCargo so that httptest-based tests can
// drive the HTTP pipeline against a mock server (exposed via export_test.go),
// bypassing the exact-baseURL guard that ValidateCargo enforces for callers.
func validateCargoREADME(ctx context.Context, pkg model.Package, serverName string) error {
	allowedHosts := cargoAllowedHosts(pkg.RegistryBaseURL)
	client := newCargoHTTPClient(pkg.RegistryBaseURL, allowedHosts)

	// Step 1: fetch the README pointer from the documented API endpoint.
	metaURL := fmt.Sprintf("%s/api/v1/crates/%s/%s/readme",
		pkg.RegistryBaseURL,
		url.PathEscape(pkg.Identifier),
		url.PathEscape(pkg.Version))

	metaReq, err := http.NewRequestWithContext(ctx, http.MethodGet, metaURL, nil)
	if err != nil {
		return fmt.Errorf("failed to create crates.io metadata request: %w", err)
	}
	metaReq.Header.Set("User-Agent", cargoUserAgent)
	metaReq.Header.Set("Accept", "application/json")

	metaResp, err := client.Do(metaReq)
	if err != nil {
		return fmt.Errorf("failed to fetch package metadata from crates.io: %w", err)
	}
	defer metaResp.Body.Close()

	if metaResp.StatusCode != http.StatusOK {
		return cargoMetadataStatusError(pkg.Identifier, metaResp.StatusCode)
	}

	var meta CargoReadmeMetaResponse
	if err := json.NewDecoder(metaResp.Body).Decode(&meta); err != nil {
		return fmt.Errorf("failed to parse crates.io readme metadata: %w", err)
	}
	if meta.URL == "" {
		return fmt.Errorf("cargo package '%s' metadata response missing 'url' field", pkg.Identifier)
	}

	// Pin the README pointer to an allowed host/scheme/port before fetching it, so
	// a metadata response cannot steer the validator at an internal or
	// attacker-chosen URL.
	readmeParsed, err := url.Parse(meta.URL)
	if err != nil || readmeParsed.Hostname() == "" {
		return fmt.Errorf("cargo package '%s': crates.io returned an unparseable README URL", pkg.Identifier)
	}
	if !cargoURLAllowed(readmeParsed, pkg.RegistryBaseURL, allowedHosts) {
		return fmt.Errorf("cargo package '%s': crates.io returned a README URL on an unexpected host/scheme %q — refusing to fetch", pkg.Identifier, readmeParsed.Redacted())
	}

	// Step 2: fetch the rendered README from the (now host-validated) URL.
	readmeReq, err := http.NewRequestWithContext(ctx, http.MethodGet, meta.URL, nil)
	if err != nil {
		return fmt.Errorf("failed to create crates.io readme request: %w", err)
	}
	readmeReq.Header.Set("User-Agent", cargoUserAgent)
	readmeReq.Header.Set("Accept", "text/html")

	readmeResp, err := client.Do(readmeReq)
	if err != nil {
		return fmt.Errorf("failed to fetch rendered README from crates.io: %w", err)
	}
	defer readmeResp.Body.Close()

	if readmeResp.StatusCode != http.StatusOK {
		return cargoReadmeStatusError(ctx, client, pkg, serverName, readmeResp.StatusCode)
	}

	body, err := io.ReadAll(io.LimitReader(readmeResp.Body, maxCargoReadmeBytes))
	if err != nil {
		return fmt.Errorf("failed to read rendered README: %w", err)
	}

	// Search for the mcp-name: <server-name> ownership token. The token contains no
	// characters that get HTML-escaped during README rendering (no <, >, &, ", '),
	// so matching against the rendered HTML is reliable; containsMCPNameToken
	// additionally requires a trailing boundary to avoid prefix confusion.
	if containsMCPNameToken(string(body), serverName) {
		return nil
	}

	// If the token IS present but glued to a trailing character, explain that
	// rather than telling the publisher to add a token they can already see.
	if trailing, glued := mcpNameTokenGluedTrailing(string(body), serverName); glued {
		return fmt.Errorf("cargo package '%s' ownership validation failed: found 'mcp-name: %s' in the README, but it is immediately followed by %q rather than a boundary. The token must be followed by a space, newline, or an HTML tag — put it on its own line and publish a new version", pkg.Identifier, serverName, trailing)
	}

	return fmt.Errorf("cargo package '%s' ownership validation failed. The server name '%s' must appear as 'mcp-name: %s' in the package README", pkg.Identifier, serverName, serverName)
}