Encapsulated NDF logic

app/src/main/java/io/elixxir/dapp/session/data/NdfDataSource.kt

+package io.elixxir.dapp.session.data
+
+import android.app.Application
+import bindings.Bindings
+import io.elixxir.dapp.R
+import io.elixxir.dapp.session.models.Environment
+import kotlinx.coroutines.*
+typealias Ndf = String
+
+interface NdfDataSource {
+    suspend fun fetchNdf(): Ndf
+}
+
+class RemoteNdfDataSource internal constructor(
+    private val app: Application,
+    private val environment: Environment,
+    private val defaultDispatcher: CoroutineDispatcher = Dispatchers.IO
+) : NdfDataSource {
+
+    override suspend fun fetchNdf(): Ndf {
+        return retryFetchNdf()
+    }
+
+    private suspend fun retryFetchNdf(retries: Int = 0): Ndf = withContext(defaultDispatcher) {
+        val ndf = when (environment) {
+            Environment.MainNet -> {
+                downloadAndVerifySignedNdfWithUrl(
+                    NDF_URL_MAINNET,
+                    certificateFor(Environment.MainNet)
+                )
+            }
+            Environment.ReleaseNet -> {
+                downloadAndVerifySignedNdfWithUrl(
+                    NDF_URL_RELEASE,
+                    certificateFor(Environment.ReleaseNet)
+                )
+            }
+        }
+
+        if (ndf.isEmpty() && retries <= NDF_MAX_RETRIES) {
+            ensureActive()
+            delay(NDF_POLL_INTERVAL_MS)
+            retryFetchNdf(retries + 1)
+        } else {
+            ndf
+        }
+    }
+
+    private fun downloadAndVerifySignedNdfWithUrl(
+        url: String,
+        certificate: String
+    ): Ndf = String(Bindings.downloadAndVerifySignedNdfWithUrl(url, certificate))
+
+    private fun certificateFor(environment: Environment): String {
+        val certFile: Int = when (environment) {
+            Environment.MainNet -> R.raw.mainnet
+            Environment.ReleaseNet -> R.raw.release
+        }
+        val reader= app.resources.openRawResource(certFile)
+            .bufferedReader()
+        return reader.use { reader.readText() }
+    }
+
+    companion object {
+        private const val NDF_URL_MAINNET =
+            "https://elixxir-bins.s3.us-west-1.amazonaws.com/ndf/mainnet.json"
+        private const val NDF_URL_RELEASE =
+            "https://elixxir-bins.s3.us-west-1.amazonaws.com/ndf/release.json"
+        private const val NDF_MAX_RETRIES = 29
+        private const val NDF_POLL_INTERVAL_MS = 1000L
+    }
+}
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