Convert util/{HttpHeaderLink,PairedList,TimestampUtils,ThemeUtils} to Kotlin (#3046)

* Fix off-by-one error in HttpHeaderLink

Link headers with multiple URLs with multiple parameters were being parsed
incorrectly.

Detected by adding unit tests ahead of converting to Kotlin.

* Convert util/HttpHeaderLink from Java to Kotlin

* Convert util/ThemeUtils from Java to Kotlin

* Convert util/TimestampUtils from Java to Kotlin

* Add tests for PairedList

* Convert util/PairedList from Java to Kotlin

* Implement feedback from PR

* Relicense as GPL

app/src/main/java/com/keylesspalace/tusky/util/HttpHeaderLink.kt

@@ -0,0 +1,135 @@
+/* Copyright 2022 Tusky Contributors
+ *
+ * This file is a part of Tusky.
+ *
+ * This program is free software; you can redistribute it and/or modify it under the terms of the
+ * GNU General Public License as published by the Free Software Foundation; either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * Tusky is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
+ * the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with Tusky; if not,
+ * see <http://www.gnu.org/licenses>.
+ */
+package com.keylesspalace.tusky.util
+
+import android.net.Uri
+import androidx.annotation.VisibleForTesting
+import androidx.core.net.toUri
+
+/**
+ * Represents one link and its parameters from the link header of an HTTP message.
+ *
+ * @see [RFC5988](https://tools.ietf.org/html/rfc5988)
+ */
+class HttpHeaderLink @VisibleForTesting(otherwise = VisibleForTesting.PRIVATE) constructor(
+    uri: String
+) {
+    data class Parameter(val name: String, val value: String?)
+
+    private val parameters: MutableList<Parameter> = ArrayList()
+
+    val uri: Uri = uri.toUri()
+
+    private data class ValueResult(val value: String, val end: Int = -1)
+
+    companion object {
+        private fun findEndOfQuotedString(line: String, start: Int): Int {
+            var i = start
+            while (i < line.length) {
+                val c = line[i]
+                if (c == '\\') {
+                    i += 1
+                } else if (c == '"') {
+                    return i
+                }
+                i++
+            }
+            return -1
+        }
+
+        private fun parseValue(line: String, start: Int): ValueResult {
+            val foundIndex = line.indexOfAny(charArrayOf(';', ',', '"'), start, false)
+            if (foundIndex == -1) {
+                return ValueResult(line.substring(start).trim())
+            }
+            val c = line[foundIndex]
+            return if (c == ';' || c == ',') {
+                ValueResult(line.substring(start, foundIndex).trim(), foundIndex)
+            } else {
+                var quoteEnd = findEndOfQuotedString(line, foundIndex + 1)
+                if (quoteEnd == -1) {
+                    quoteEnd = line.length
+                }
+                ValueResult(line.substring(foundIndex + 1, quoteEnd).trim(), quoteEnd)
+            }
+        }
+
+        private fun parseParameters(line: String, start: Int, link: HttpHeaderLink): Int {
+            var i = start
+            while (i < line.length) {
+                val foundIndex = line.indexOfAny(charArrayOf('=', ','), i, false)
+                if (foundIndex == -1) {
+                    return -1
+                } else if (line[foundIndex] == ',') {
+                    return foundIndex
+                }
+                val name = line.substring(line.indexOf(';', i) + 1, foundIndex).trim()
+                val result = parseValue(line, foundIndex)
+                val value = result.value
+                val parameter = Parameter(name, value)
+                link.parameters.add(parameter)
+                i = if (result.end == -1) {
+                    return -1
+                } else {
+                    result.end
+                }
+            }
+            return -1
+        }
+
+        /**
+         * @param line the entire link header, not including the initial "Link:"
+         * @return all links found in the header
+         */
+        fun parse(line: String?): List<HttpHeaderLink> {
+            val links: MutableList<HttpHeaderLink> = mutableListOf()
+            line ?: return links
+
+            var i = 0
+            while (i < line.length) {
+                val uriEnd = line.indexOf('>', i)
+                val uri = line.substring(line.indexOf('<', i) + 1, uriEnd)
+                val link = HttpHeaderLink(uri)
+                links.add(link)
+                val parseEnd = parseParameters(line, uriEnd, link)
+                i = if (parseEnd == -1) {
+                    break
+                } else {
+                    parseEnd
+                }
+                i++
+            }
+
+            return links
+        }
+
+        /**
+         * @param links intended to be those returned by parse()
+         * @param relationType of the parameter "rel", commonly "next" or "prev"
+         * @return the link matching the given relation type
+         */
+        fun findByRelationType(
+            links: List<HttpHeaderLink>,
+            relationType: String
+        ): HttpHeaderLink? {
+            return links.find { link ->
+                link.parameters.any { parameter ->
+                    parameter.name == "rel" && parameter.value == relationType
+                }
+            }
+        }
+    }
+}