add logic for building message body of unfurled commit links

The new file `slack_message.ml` should only contain generation
logic for messages that describe the state of some info on GH
(e.g., a description of a PR), and *not* an action taken on GH
(e.g, "XYZ opened a PR".

This decoupling is desired because we eventually want to reuse
code between link unfurl generation and event notification
generation, and link unfurl only concerns the former type of
info described above. Currently, it's difficult to do as
there is tight coupling in `slack.ml`.

Currently we support commit link unfurls. We can later easily
expand the selection of unfurlable links by adding onto
`slack_message.ml`.

Author: yasunariw

lib/colors.ml

@@ -0,0 +1,13 @@
+(* https://styleguide.github.com/primer/utilities/colors/#background-colors *)
+
+let gray = "#f6f8fa"
+
+let blue = "#0366d6"
+
+let yellow = "#ffd33d"
+
+let red = "#d73a49"
+
+let green = "#28a745"
+
+let purple = "#6f42c1"

lib/github.atd

@@ -24,6 +24,9 @@ type commit = {
 type github_user = {
   login: string;
   id: int;
+  url: string;
+  html_url: string;
+  avatar_url: string;
 }
 
 type repository = {
@@ -218,12 +221,19 @@ type file = {
   blob_url <ocaml name="url"> : string;
 }
 
+type api_commit_stats = {
+  total: int;
+  additions: int;
+  deletions: int;
+}
+
 type api_commit = {
   sha: commit_hash;
   commit: inner_commit;
   html_url <ocaml name="url"> : string;
   author: github_user;
   files: file list;
+  stats: api_commit_stats;
 }
 
 type commit_comment_notification = {

lib/slack.ml

@@ -1,6 +1,5 @@
 open Printf
 open Base
-open Devkit
 open Common
 open Github_j
 open Slack_j
@@ -33,6 +32,8 @@ let show_labels = function
   | (labels : label list) ->
     Some (sprintf "Labels: %s" @@ String.concat ~sep:", " (List.map ~f:(fun x -> x.name) labels))
 
+let pluralize name num suffix = if num = 1 then sprintf "%s" name else String.concat [ name; suffix ]
+
 let generate_pull_request_notification notification channel =
   let { action; number; sender; pull_request; repository } = notification in
   let ({ body; title; html_url; labels; _ } : pull_request) = pull_request in
@@ -302,8 +303,7 @@ let generate_status_notification (cfg : Config_t.config) (notification : status_
           [ main ]
         | _ -> notification_branches
       in
-      let pluralize s = if Int.equal (List.length branches) 1 then s else sprintf "%ses" s in
-      [ sprintf "*%s*: %s" (pluralize "Branch") (String.concat ~sep:", " branches) ]
+      [ sprintf "*%s*: %s" (pluralize "Branch" (List.length branches) "es") (String.concat ~sep:", " branches) ]
   in
   let summary =
     match target_url with

lib/slack_message.ml

@@ -0,0 +1,57 @@
+open Base
+open Printf
+open Github_t
+open Slack_t
+
+let git_short_sha_hash hash = String.sub ~pos:0 ~len:8 hash
+
+let empty_attachment =
+  {
+    mrkdwn_in = None;
+    fallback = None;
+    color = None;
+    pretext = None;
+    author_name = None;
+    author_link = None;
+    author_icon = None;
+    title = None;
+    title_link = None;
+    text = None;
+    fields = None;
+    image_url = None;
+    thumb_url = None;
+    ts = None;
+    footer = None;
+  }
+
+let base_attachment (repository : repository) =
+  { empty_attachment with footer = Some (sprintf "<%s|%s>" repository.url repository.full_name) }
+
+let pp_file (file : file) = sprintf "<%s|%s>" file.url (Mrkdwn.escape_mrkdwn file.filename)
+
+let populate_commit repository (commit : api_commit) =
+  let ({ sha; commit; url; author; files; stats } : api_commit) = commit in
+  let get_files () = List.map files ~f:pp_file in
+  let title = sprintf "`<%s|%s>` *%s - %s*" url (git_short_sha_hash sha) commit.message commit.author.name in
+  let num_change = List.length files in
+  let changes =
+    sprintf "%d %s with %d %s and %d %s:" num_change
+      (Slack.pluralize "changed file" num_change "s")
+      stats.additions
+      (Slack.pluralize "addition" stats.additions "s")
+      stats.deletions
+      (Slack.pluralize "deletion" stats.deletions "s")
+  in
+  let files = get_files () |> String.concat ~sep:"\n" in
+  let text = sprintf "%s\n%s\n%s" title changes files in
+  let fallback = sprintf "[%s] %s - %s" (git_short_sha_hash sha) commit.message commit.author.name in
+  {
+    (base_attachment repository) with
+    author_name = Some author.login;
+    author_link = Some author.html_url;
+    author_icon = Some author.avatar_url;
+    color = Some Colors.gray;
+    mrkdwn_in = Some [ "text" ];
+    text = Some text;
+    fallback = Some fallback;
+  }