Factored out a safer llama_sample_apply_guidance method based on spans

Author: martindevans

LLama/Native/LLamaTokenDataArray.cs

@@ -217,14 +217,7 @@ public void Guidance(SafeLLamaContextHandle context, ReadOnlySpan<float> guidanc
                 }
 
                 // Apply guidance
-                unsafe
-                {
-                    fixed (float* logitsPtr = logits)
-                    fixed (float* guidanceLogitsPtr = guidanceLogits)
-                    {
-                        NativeApi.llama_sample_apply_guidance(context, logitsPtr, guidanceLogitsPtr, guidance);
-                    }
-                }
+                NativeApi.llama_sample_apply_guidance(context, logits, guidanceLogits, guidance);
 
                 // Copy logits back into data array
                 for (var i = 0; i < data.Length; i++)

LLama/Native/NativeApi.Sampling.cs

@@ -1,4 +1,5 @@
-using System.Runtime.InteropServices;
+using System;
+using System.Runtime.InteropServices;
 
 namespace LLama.Native
 {
@@ -23,6 +24,33 @@ public static extern unsafe void llama_sample_repetition_penalties(SafeLLamaCont
                                                                     float penalty_freq,
                                                                     float penalty_present);
 
+        /// <summary>
+        /// Apply classifier-free guidance to the logits as described in academic paper "Stay on topic with Classifier-Free Guidance" https://arxiv.org/abs/2306.17806
+        /// </summary>
+        /// <param name="ctx"></param>
+        /// <param name="logits">Logits extracted from the original generation context.</param>
+        /// <param name="logits_guidance">Logits extracted from a separate context from the same model.
+        /// Other than a negative prompt at the beginning, it should have all generated and user input tokens copied from the main context.</param>
+        /// <param name="scale">Guidance strength. 1.0f means no guidance. Higher values mean stronger guidance.</param>
+        public static void llama_sample_apply_guidance(SafeLLamaContextHandle ctx, Span<float> logits, ReadOnlySpan<float> logits_guidance, float scale)
+        {
+            if (logits == null)
+                throw new ArgumentNullException(nameof(logits));
+            if (logits_guidance == null)
+                throw new ArgumentNullException(nameof(logits_guidance));
+            if (logits.Length != ctx.VocabCount)
+                throw new ArgumentException("Logits count must have equal context vocab size", nameof(logits));
+            if (logits_guidance.Length != ctx.VocabCount)
+                throw new ArgumentException("Guidance logits count must have equal context vocab size", nameof(logits_guidance));
+
+            unsafe
+            {
+                fixed (float* logitsPtr = logits)
+                fixed (float* logitsGuidancePtr = logits_guidance)
+                    llama_sample_apply_guidance(ctx, logitsPtr, logitsGuidancePtr, scale);
+            }
+        }
+
         /// <summary>
         /// Apply classifier-free guidance to the logits as described in academic paper "Stay on topic with Classifier-Free Guidance" https://arxiv.org/abs/2306.17806
         /// </summary>