// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package proxy

import (
	"archive/zip"
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
	"net/url"
	"strings"
	"time"

	"go.opencensus.io/plugin/ochttp"
	"golang.org/x/discovery/internal"
	"golang.org/x/discovery/internal/derrors"
	"golang.org/x/mod/module"
	"golang.org/x/net/context/ctxhttp"
	"golang.org/x/xerrors"
)

// A Client is used by the fetch service to communicate with a module
// proxy. It handles all methods defined by go help goproxy.
type Client struct {
	// URL of the module proxy web server
	url string

	// client used for HTTP requests. It is mutable for testing purposes.
	httpClient *http.Client
}

// A VersionInfo contains metadata about a given version of a module.
type VersionInfo struct {
	Version string
	Time    time.Time
}

// New constructs a *Client using the provided rawurl, which is expected to
// be an absolute URI that can be directly passed to http.Get.
func New(rawurl string) (_ *Client, err error) {
	derrors.Wrap(&err, "proxy.New(%q)", rawurl)
	url, err := url.Parse(rawurl)
	if err != nil {
		return nil, fmt.Errorf("url.Parse: %v", err)
	}
	if url.Scheme != "https" {
		return nil, fmt.Errorf("scheme must be https (got %s)", url.Scheme)
	}
	cleanURL := strings.TrimRight(rawurl, "/")
	return &Client{url: cleanURL, httpClient: &http.Client{Transport: &ochttp.Transport{}}}, nil
}

// GetInfo makes a request to $GOPROXY/<module>/@v/<version>.info and
// transforms that data into a *VersionInfo.
func (c *Client) GetInfo(ctx context.Context, modulePath, version string) (_ *VersionInfo, err error) {
	defer derrors.Wrap(&err, "proxy.Client.GetInfo(%q, %q)", modulePath, version)
	u, err := c.escapedURL(modulePath, version, "info")
	if err != nil {
		return nil, err
	}
	var v VersionInfo
	err = c.executeRequest(ctx, u, func(body io.Reader) error {
		if err := json.NewDecoder(body).Decode(&v); err != nil {
			return err
		}
		return nil
	})
	if err != nil {
		return nil, err
	}
	return &v, nil
}

// GetZip makes a request to $GOPROXY/<path>/@v/<version>.zip and transforms
// that data into a *zip.Reader. <version> is obtained by first making a
// request to $GOPROXY/<path>/@v/<version>.info to obtained the valid
// semantic version.
func (c *Client) GetZip(ctx context.Context, requestedPath, requestedVersion string) (_ *zip.Reader, err error) {
	defer derrors.Wrap(&err, "proxy.Client.GetZip(ctx, %q, %q)", requestedPath, requestedVersion)

	info, err := c.GetInfo(ctx, requestedPath, requestedVersion)
	if err != nil {
		return nil, err
	}
	u, err := c.escapedURL(requestedPath, info.Version, "zip")
	if err != nil {
		return nil, err
	}
	var bodyBytes []byte
	err = c.executeRequest(ctx, u, func(body io.Reader) error {
		var err error
		bodyBytes, err = ioutil.ReadAll(body)
		if err != nil {
			return fmt.Errorf("ioutil.ReadAll: %v", err)
		}
		return nil
	})
	if err != nil {
		return nil, err
	}
	zipReader, err := zip.NewReader(bytes.NewReader(bodyBytes), int64(len(bodyBytes)))
	if err != nil {
		return nil, fmt.Errorf("zip.NewReader: %v", err)
	}
	return zipReader, nil
}

func (c *Client) escapedURL(modulePath, version, suffix string) (_ string, err error) {
	defer func() {
		derrors.Wrap(&err, "Client.escapedURL(%q, %q, %q)", modulePath, version, suffix)
	}()

	if suffix != "info" && suffix != "zip" {
		return "", errors.New(`suffix must be "info" or "zip"`)
	}
	escapedPath, err := module.EscapePath(modulePath)
	if err != nil {
		return "", xerrors.Errorf("path: %v: %w", err, derrors.InvalidArgument)
	}
	if version == internal.LatestVersion {
		if suffix != "info" {
			return "", fmt.Errorf("cannot ask for latest with suffix %q", suffix)
		}
		return fmt.Sprintf("%s/%s/@latest", c.url, escapedPath), nil
	}
	escapedVersion, err := module.EscapeVersion(version)
	if err != nil {
		return "", xerrors.Errorf("version: %v: %w", err, derrors.InvalidArgument)
	}
	return fmt.Sprintf("%s/%s/@v/%s.%s", c.url, escapedPath, escapedVersion, suffix), nil
}

// ListVersions makes a request to $GOPROXY/<path>/@v/list and returns the
// resulting version strings.
func (c *Client) ListVersions(ctx context.Context, modulePath string) ([]string, error) {
	escapedPath, err := module.EscapePath(modulePath)
	if err != nil {
		return nil, xerrors.Errorf("module.EscapePath(%q): %w", modulePath, derrors.InvalidArgument)
	}
	u := fmt.Sprintf("%s/%s/@v/list", c.url, escapedPath)
	var versions []string
	collect := func(body io.Reader) error {
		scanner := bufio.NewScanner(body)
		for scanner.Scan() {
			versions = append(versions, scanner.Text())
		}
		return scanner.Err()
	}
	if err := c.executeRequest(ctx, u, collect); err != nil {
		return nil, err
	}
	return versions, nil
}

// executeRequest executes an HTTP GET request for u, then calls the bodyFunc
// on the response body, if no error occurred.
func (c *Client) executeRequest(ctx context.Context, u string, bodyFunc func(body io.Reader) error) error {
	r, err := ctxhttp.Get(ctx, c.httpClient, u)
	if err != nil {
		return fmt.Errorf("ctxhttp.Get(ctx, client, %q): %v", u, err)
	}
	defer r.Body.Close()
	switch {
	case 200 <= r.StatusCode && r.StatusCode < 300:
		// OK.
	case r.StatusCode == http.StatusNotFound,
		r.StatusCode == http.StatusGone:
		// Treat both 404 Not Found and 410 Gone responses
		// from the proxy as a "not found" error category.
		return xerrors.Errorf("ctxhttp.Get(ctx, client, %q): %w", u, derrors.NotFound)
	default:
		return fmt.Errorf("ctxhttp.Get(ctx, client, %q): unexpected status %d %s", u, r.StatusCode, r.Status)
	}
	return bodyFunc(r.Body)
}