move message construction and sending methods to the new NinePMessage class

1 files changed, 82 insertions, 0 deletions

diff --git a/src/main/kotlin/NinePMessage.kt b/src/main/kotlin/NinePMessage.kt
new file mode 100644
index 0000000..9edf361
--- /dev/null
+++ b/src/main/kotlin/NinePMessage.kt
@@ -0,0 +1,82 @@
+import kotlin.math.pow
+
+/**
+ * A 9P message with the given type, tag, and fields. The message size is calculated automatically.
+ *
+ * Important note: the field names in [fieldValuesInt] and [fieldValuesStr] (i.e. the keys of their maps) must be
+ * mutually exclusive and the union of these two maps' keys must result in a subset of (or a set equal to)
+ * [fieldNames]. Calling [write] when these conditions are not met throws an exception.
+ *
+ * @param type The 9P message type.
+ * @param tag The tag given to the message.
+ * @param fieldNames The names of the message fields, in the same order they are expected to be sent.
+ * @param fieldValuesInt A map of each field name into its value. This map only stores integer values.
+ * @param fieldValuesStr A map of each field name into its value. This map only stores string values.
+ */
+class NinePMessage(val type: NinePMessageType, val tag: UShort, val fieldNames: List<String>, val fieldValuesInt: Map<String, SizedMessageField>, val fieldValuesStr: Map<String, String>) {
+    /**
+     * Send the message using the given networking API.
+     *
+     * @param npt The networking API.
+     * @throws IllegalArgumentException if [fieldNames], [fieldValuesInt], and [fieldValuesStr] are incoherent.
+     */
+    fun write(npt: NetworkPacketTransporter) {
+        // shorthands
+        val insecInts = fieldNames.intersect(fieldValuesInt.keys)
+        val insecStrs = fieldNames.intersect(fieldValuesStr.keys)
+        // check that names in fieldNames exist as keys in either fieldValuesInt or fieldValuesStr but not both
+        require(insecInts.size == fieldNames.size - insecStrs.size)
+
+        val totalSize = 4 + 1 + 2 + insecInts.sumOf { fieldValuesInt[it]!!.size } + insecStrs.sumOf { 2 + fieldValuesStr[it]!!.length }
+
+        writeMessageSizeType(npt, totalSize, type)
+        writeInteger(npt, SizedMessageField(2, tag.toInt().toBigInteger()))
+        for (field in fieldNames) {
+            if (field in insecInts) {
+                writeInteger(npt, fieldValuesInt[field]!!)
+            } else {
+                writeString(npt, fieldValuesStr[field]!!)
+            }
+        }
+    }
+
+    /**
+     * Write an integer number to the connection.
+     *
+     * In 9P, binary numbers (non-textual) are specified in little-endian order (least significant byte first).
+     *
+     * @param npt The networking API.
+     * @param value The number's value. [SizedMessageField] defines both its actual value and its size.
+     */
+    private fun writeInteger(npt: NetworkPacketTransporter, value: SizedMessageField) {
+        npt.transmit(value.value.toByteArray().toList())
+    }
+
+    /**
+     * Write a string to the connection.
+     *
+     * In 9P, strings are represented as a 2-byte integer (the string's size) followed by the actual UTF-8 string. The
+     * null terminator is forbidden in 9P messages.
+     *
+     * @param npt The networking API.
+     * @param value The string.
+     * @throws IllegalArgumentException if the value of the string's size does not fit into 2 bytes.
+     */
+    private fun writeString(npt: NetworkPacketTransporter, value: String) {
+        require(value.length <= 2.0.pow(16.0) - 1)
+        writeInteger(npt, SizedMessageField(2, value.length.toBigInteger()))
+        npt.transmit(value.toByteArray().toList())
+    }
+
+    /**
+     * Write the message size and type.
+     *
+     * @param npt The networking API.
+     * @param size The total message size, including the 4 bytes of this parameter and the type's byte.
+     * @param type The 9P message type as a [NinePMessageType] constant.
+     */
+    private fun writeMessageSizeType(npt: NetworkPacketTransporter, size: Int, type: NinePMessageType) {
+        writeInteger(npt, SizedMessageField(4, size.toBigInteger()))
+        writeInteger(npt, SizedMessageField(1, type.value.toInt().toBigInteger()))
+    }
+}
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