Source file `meta.ml`

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(*
 * Copyright (c) 2014, OCaml.org project
 * Copyright (c) 2015 KC Sivaramakrishnan <sk826@cl.cam.ac.uk>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *)

(** This module determines an image to be used as preview of a website.

    It does this by following the same logic Google+ and other websites use, and
    described in this article:
    https://www.raymondcamden.com/2011/07/26/How-are-Facebook-and-Google-creating-link-previews *)

let og_image html =
  let open Soup in
  let soup = parse html in
  try soup $ "meta[property=og:image]" |> R.attribute "content" |> Option.some
  with Failure _ -> None

let image_src html =
  let open Soup in
  let soup = parse html in
  try soup $ "link[rel=\"image_src\"]" |> R.attribute "href" |> Option.some
  with Failure _ -> None

let twitter_image html =
  let open Soup in
  let soup = parse html in
  try
    soup $ "meta[name=\"twitter:image\"]" |> R.attribute "content"
    |> Option.some
  with Failure _ -> None

let og_description html =
  let open Soup in
  let soup = parse html in
  try
    soup $ "meta[property=og:description]" |> R.attribute "content"
    |> Option.some
  with Failure _ -> None

let description html =
  let open Soup in
  let soup = parse html in
  try
    soup $ "meta[property=description]" |> R.attribute "content" |> Option.some
  with Failure _ -> None

let preview_image html =
  let preview_image =
    match og_image html with
    | None -> (
        match image_src html with
        | None -> twitter_image html
        | Some x -> Some x)
    | Some x -> Some x
  in
  match Option.map String.trim preview_image with
  | Some "" -> None
  | Some x -> Some x
  | None -> None

let description html =
  let preview_image =
    match og_description html with None -> description html | Some x -> Some x
  in
  match Option.map String.trim preview_image with
  | Some "" -> None
  | Some x -> Some x
  | None -> None