collection: Support collecting posts

This adds support for moving posts to a collection. This change relies
on #13 so that we can create and delete collections during tests.

Couple notes:

- CollectPosts accepts a single struct rather than an alias and a list
  of structs. This makes it easy to add new optional parameters in the
  future.

- We're passing `[]*CollectPost` around rather than `*[]CollectPost`
  which is done in some of the existing APIs. The reasoning for this is:

  - Idiomatically, slices are passed by-value (`[]foo`) rather than
    by-pointer (`*[]foo`) because slices are already reference types.
    Plus they're really cheap to copy since they're just a triple:
    pointer, length, and capacity ([related blog post][1]). We need
    pointers to slices only when we're trying to modify the original
    slice reference and don't have the ability to return a slice, like
    with `json.Unmarshal` (see also [this post][2]).

  - If we're trying to reduce copying, `[]*foo` is better than `[]foo`
    because otherwise `foo` will be copied when manipulating or
    accessing entries in the slice (like ranging over it).

  Existing APIs were left as-is to avoid breaking them. I can switch to
  `*[]CollectPost` if you'd prefer that but `[]*CollectPost` is more
  idiomatic.

[1]: https://blog.golang.org/go-slices-usage-and-internals#TOC_4.
[2]: https://blog.golang.org/slices#TOC_5.

Resolves #4.

collection.go

@@ -35,6 +35,35 @@ type (
 	DeleteCollectionParams struct {
 		Alias string `json:"-"`
 	}
+
+	// CollectPostParams holds the parameters for moving posts to a collection.
+	CollectPostParams struct {
+		// Alias of the collection.
+		Alias string `json:"-"`
+
+		// Posts to move to this collection.
+		Posts []*CollectPost
+	}
+
+	// CollectPost is a post being moved to a collection.
+	CollectPost struct {
+		// ID of the post.
+		ID string `json:"id,omitempty"`
+
+		// The post's modify token.
+		//
+		// This is required if the post does not belong to the user making
+		// this request.
+		Token string `json:"token,omitempty"`
+	}
+
+	// CollectPostResult holds the result of moving a single post to a
+	// collection.
+	CollectPostResult struct {
+		Code         int    `json:"code,omitempty"`
+		ErrorMessage string `json:"error_msg,omitempty"`
+		Post         *Post  `json:"post,omitempty"`
+	}
 )
 
 // CreateCollection creates a new collection, returning a user-friendly error
@@ -165,3 +194,28 @@ func (c *Client) DeleteCollection(p *DeleteCollectionParams) error {
 		return fmt.Errorf("Problem deleting collection: %d. %s\n", status, env.ErrorMessage)
 	}
 }
+
+// CollectPosts adds a group of posts to a collection.
+//
+// See https://developers.write.as/docs/api/#move-a-post-to-a-collection.
+func (c *Client) CollectPosts(sp *CollectPostParams) ([]*CollectPostResult, error) {
+	endpoint := "/collections/" + sp.Alias + "/collect"
+
+	var p []*CollectPostResult
+	env, err := c.post(endpoint, sp.Posts, &p)
+	if err != nil {
+		return nil, err
+	}
+
+	status := env.Code
+	switch {
+	case status == http.StatusOK:
+		return p, nil
+	case c.isNotLoggedIn(status):
+		return nil, fmt.Errorf("Not authenticated.")
+	case status == http.StatusBadRequest:
+		return nil, fmt.Errorf("Bad request: %s", env.ErrorMessage)
+	default:
+		return nil, fmt.Errorf("Problem claiming post: %d. %s\n", status, env.ErrorMessage)
+	}
+}

collection_test.go

@@ -2,6 +2,7 @@ package writeas
 
 import (
 	"fmt"
+	"net/http"
 	"strings"
 	"testing"
 	"time"
@@ -103,3 +104,59 @@ func ExampleClient_GetCollection() {
 	fmt.Printf("%s", coll.Title)
 	// Output: write.as
 }
+
+func TestCollectPostsAnonymous(t *testing.T) {
+	// Create a post anonymously.
+	wac := NewDevClient()
+	p, err := wac.CreatePost(&PostParams{
+		Title:   "Title!",
+		Content: "This is a post.",
+		Font:    "sans",
+	})
+	if err != nil {
+		t.Errorf("Post create failed: %v", err)
+		return
+	}
+	t.Logf("Post created: %+v", p)
+
+	// Log in.
+	if _, err := wac.LogIn("demo", "demo"); err != nil {
+		t.Fatalf("Unable to log in: %v", err)
+	}
+	defer wac.LogOut()
+
+	now := time.Now().Unix()
+	alias := fmt.Sprintf("test-collection-%v", now)
+
+	// Create a collection.
+	_, err = wac.CreateCollection(&CollectionParams{
+		Alias: alias,
+		Title: fmt.Sprintf("Test Collection %v", now),
+	})
+	if err != nil {
+		t.Fatalf("Unable to create collection %q: %v", alias, err)
+	}
+	defer wac.DeleteCollection(&DeleteCollectionParams{Alias: alias})
+
+	// Move the anonymous post to this collection.
+	res, err := wac.CollectPosts(&CollectPostParams{
+		Alias: alias,
+		Posts: []*CollectPost{
+			{
+				ID:    p.ID,
+				Token: p.Token,
+			},
+		},
+	})
+	if err != nil {
+		t.Fatalf("Could not collect post %q: %v", p.ID, err)
+	}
+
+	for _, cr := range res {
+		if cr.Code != http.StatusOK {
+			t.Errorf("Failed to move post: %v", cr.ErrorMessage)
+		} else {
+			t.Logf("Moved post %q", cr.Post.ID)
+		}
+	}
+}