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The masked mean of one sequence's token embeddings.

public static float[] MeanPool(ReadOnlySpan<float> tokenEmbeddings, int seqLen, int dim, ReadOnlySpan<long> attentionMask)

Parameters — tokenEmbeddings is the encoder's output, row-major. seqLen is the number of token positions, dim the embedding dimension, and attentionMask marks a real token with a non-zero value and padding with zero. attentionMask has length seqLen.

Returns — float[] of length dim, the mean of the masked token vectors. Not normalized.

Exceptions — ArgumentException when the spans do not match the shape the other arguments declare. ArgumentOutOfRangeException when seqLen or dim is negative.

Example — three positions, one of them padding.

using Lodestar.Embeddings.Pooling;

float[] tokens = { 2f, 0f, 4f, 0f, 99f, 99f };   // three positions, dim 2
long[] mask = { 1L, 1L, 0L };

float[] pooled = Pooler.MeanPool(tokens, seqLen: 3, dim: 2, mask);
float first = pooled[0];  // => 3
float second = pooled[1];  // => 0

Remarks — The divisor is the number of real tokens, not seqLen: two here, not three, which is why the mean of 2 and 4 is 3 and the padding row never enters it. Dividing by seqLen instead — the mistake this method exists to prevent — would have given 2.

The formula matches sentence-transformers' mean_pooling exactly, clamp included: sum(embeddings × mask) / max(sum(mask), 1e-9). That clamp is what an all-padding sequence meets; it yields a zero vector rather than a division by zero, and L2Normalize leaves a zero vector alone.

Use MeanPoolAndNormalize unless you specifically want the unnormalized mean.

Applies to — net10.0, netstandard2.0.

See also — Pooler.MeanPoolAndNormalize, Pooler.MeanPoolBatch, Pooler.