1 files changed, 89 insertions, 0 deletions
diff --git a/src/main/kotlin/moe/nea/firmament/util/TemplateUtil.kt b/src/main/kotlin/moe/nea/firmament/util/TemplateUtil.kt
new file mode 100644
index 0000000..f5e3466
--- /dev/null
+++ b/src/main/kotlin/moe/nea/firmament/util/TemplateUtil.kt
@@ -0,0 +1,89 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Linnea Gräf <nea@nea.moe>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+package moe.nea.firmament.util
+
+import java.util.*
+import kotlinx.serialization.DeserializationStrategy
+import kotlinx.serialization.SerializationStrategy
+import kotlinx.serialization.serializer
+import moe.nea.firmament.Firmament
+
+object TemplateUtil {
+
+    @JvmStatic
+    fun getTemplatePrefix(data: String): String? {
+        val decoded = maybeFromBase64Encoded(data) ?: return null
+        return decoded.replaceAfter("/", "", "").ifBlank { null }
+    }
+
+    @JvmStatic
+    fun intoBase64Encoded(raw: String): String {
+        return Base64.getEncoder().encodeToString(raw.encodeToByteArray())
+    }
+
+    private val base64Alphabet = charArrayOf(
+        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
+        'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
+        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
+        'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
+        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/', '='
+    )
+
+    @JvmStatic
+    fun maybeFromBase64Encoded(raw: String): String? {
+        val raw = raw.trim()
+        if (raw.any { it !in base64Alphabet }) {
+            return null
+        }
+        return try {
+            Base64.getDecoder().decode(raw).decodeToString()
+        } catch (ex: Exception) {
+            null
+        }
+    }
+
+
+    /**
+     * Returns a base64 encoded string, truncated such that for all `x`, `x.startsWith(prefix)` implies
+     * `base64Encoded(x).startsWith(getPrefixComparisonSafeBase64Encoding(prefix))`
+     * (however, the inverse may not always be true).
+     */
+    @JvmStatic
+    fun getPrefixComparisonSafeBase64Encoding(prefix: String): String {
+        val rawEncoded =
+            Base64.getEncoder().encodeToString(prefix.encodeToByteArray())
+                .replace("=", "")
+        return rawEncoded.substring(0, rawEncoded.length - rawEncoded.length % 4)
+    }
+
+    inline fun <reified T> encodeTemplate(sharePrefix: String, data: T): String =
+        encodeTemplate(sharePrefix, data, serializer())
+
+    fun <T> encodeTemplate(sharePrefix: String, data: T, serializer: SerializationStrategy<T>): String {
+        require(sharePrefix.endsWith("/"))
+        return intoBase64Encoded(sharePrefix + Firmament.json.encodeToString(serializer, data))
+    }
+
+    inline fun <reified T : Any> maybeDecodeTemplate(sharePrefix: String, data: String): T? =
+        maybeDecodeTemplate(sharePrefix, data, serializer())
+
+    fun <T : Any> maybeDecodeTemplate(sharePrefix: String, data: String, serializer: DeserializationStrategy<T>): T? {
+        require(sharePrefix.endsWith("/"))
+        val data = data.trim()
+        if (!data.startsWith(getPrefixComparisonSafeBase64Encoding(sharePrefix)))
+            return null
+        val decoded = maybeFromBase64Encoded(data) ?: return null
+        if (!decoded.startsWith(sharePrefix))
+            return null
+        return try {
+            Firmament.json.decodeFromString<T>(serializer, decoded.substring(sharePrefix.length))
+        } catch (e: Exception) {
+            null
+        }
+    }
+
+}