Document extractor service (#15665)

* Document extractor service

* Fixing vendor modules

* Addressing PR Review comments

* Some small simplifications

* Fixing a linter complain

* simplifying a bit the code using package variables

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

vendor/github.com/gigawattio/window/.gitignore

@@ -0,0 +1,67 @@
+##
+# Vim swap/working files.
+*.sw[opa]
+
+##
+# SublimeText files.
+*.sublime-project
+*.sublime-workspace
+
+##
+# IDEA IntelliJ files.
+*.idea
+*.iml
+
+##
+# Visual Studio Code files.
+.vscode
+
+##
+# Mac OS-X miscellany.
+.DS_Store
+.AppleDouble
+.LSOverride
+._*
+
+##
+# Windows image file caches
+Thumbs.db
+ehthumbs.db
+
+# Folder config file
+Desktop.ini
+
+##
+# Compiled Object files, Static and Dynamic libs (Shared Objects).
+*.o
+*.a
+*.so
+
+##
+# Folders.
+_obj
+_test
+
+##
+# Architecture specific extensions/prefixes.
+*.[568vq]
+[568vq].out
+
+*.cgo1.go
+*.cgo2.c
+_cgo_defun.c
+_cgo_gotypes.go
+_cgo_export.*
+
+_testmain.go
+
+*.exe
+*.test
+*.prof
+
+##
+# Certificate files.
+id_[rd]sa
+*.pem
+*.crt
+*.key

vendor/github.com/gigawattio/window/.travis.yml

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+language: go
+
+go:
+  - tip
+  - "1.10"
+  - 1.9
+  - 1.8
+  - 1.7
+
+script:
+  - go test ./...
+
+notifications:
+  email:
+    on_success: change
+    on_failure: always

vendor/github.com/gigawattio/window/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2018 GigawattIO
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

vendor/github.com/gigawattio/window/README.md

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+# window
+
+[![Documentation](https://godoc.org/github.com/gigawattio/window?status.svg)](https://godoc.org/github.com/gigawattio/window)
+[![Build Status](https://travis-ci.org/gigawattio/window.svg?branch=master)](https://travis-ci.org/gigawattio/window)
+[![Report Card](https://goreportcard.com/badge/github.com/gigawattio/window)](https://goreportcard.com/report/github.com/gigawattio/window)
+
+### About
+
+Window is a golang package for generating a rolling window of size N for a sequence of string tokens.
+
+Created by [Jay Taylor](https://jaytaylor.com/) and used by [Gigawatt](https://gigawatt.io/).
+
+### Requirements
+
+* Go version 1.7 or newer
+
+### Running the test suite
+
+    go test ./...
+
+#### License
+
+Permissive MIT license, see the [LICENSE](LICENSE) file for more information.

vendor/github.com/gigawattio/window/rolling.go

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+package window
+
+// Rolling generates a rolling window of size N for a sequence of string tokens.
+func Rolling(elements []string, n int) [][]string {
+	if len(elements) == 0 || len(elements) < n || n <= 0 {
+		return nil
+	}
+
+	var (
+		accum = make([][]string, len(elements)+1-n)
+		j     int
+	)
+
+	for i := 0; i < len(elements)+1-n; i++ {
+		win := make([]string, n)
+		win[0] = elements[i]
+		for j = 0; j+1 < n; j++ {
+			win[j+1] = elements[i+j+1]
+		}
+		accum[i] = win
+	}
+
+	return accum
+}