add TagGenerator

1 files changed, 62 insertions, 0 deletions

diff --git a/src/main/kotlin/TagGenerator.kt b/src/main/kotlin/TagGenerator.kt
new file mode 100644
index 0000000..d7662e6
--- /dev/null
+++ b/src/main/kotlin/TagGenerator.kt
@@ -0,0 +1,62 @@
+import kotlin.random.Random
+
+/**
+ * Generate tags for 9P messages.
+ */
+class TagGenerator(val method: TagGenerationMethod, val initial: UShort) {
+
+    private var current: UShort = initial
+    private val rng: Random = Random(initial.toInt())
+    private val randomReturned: Set<UShort> = emptySet()
+
+    private val generationFunctions: Map<TagGenerationMethod, () -> UShort> = mapOf(
+        TagGenerationMethod.INCREMENTAL to this::generateIncremental,
+        TagGenerationMethod.RANDOM to this::generateRandom,
+        TagGenerationMethod.RANDOM_CHECKED to this::generateRandomChecked
+    )
+
+    /**
+     * How are tags generated?
+     */
+    enum class TagGenerationMethod {
+        /**
+         * Return the initial value on the first generation. Increment the value on each generation.
+         */
+        INCREMENTAL,
+
+        /**
+         * Use the initial value as a seed and generate random values from it.
+         */
+        RANDOM,
+
+        /**
+         * Same as [RANDOM], but checks are added to avoid generating the same value twice.
+         */
+        RANDOM_CHECKED,
+    }
+
+    /**
+     * Generate a new tag.
+     */
+    fun generate(): UShort {
+        return this.generationFunctions.getValue(method).invoke()
+    }
+
+    private fun generateIncremental(): UShort {
+        val tmp = this.current
+        this.current++
+        return tmp
+    }
+
+    private fun generateRandom(): UShort {
+        return this.rng.nextBits(16).toUShort()
+    }
+
+    private fun generateRandomChecked(): UShort {
+        var v: UShort
+        do {
+            v = generateRandom()
+        } while (v in randomReturned)
+        return v
+    }
+}
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