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TTSKit : guard sampleFromProbs against zero-sum top-k probabilities (#450) #459

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TTSKit : guard sampleFromProbs against zero-sum top-k probabilities (#450) #459

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12 changes: 12 additions & 0 deletions Sources/TTSKit/Utilities/Sampling.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -236,6 +236,14 @@ public class GreedyTokenSampler: TokenSampling, @unchecked Sendable {
let probsArray = await topKProbs.toFloatArray()
let idxArray = await topKIndices.toIntArray()
let probSum = probsArray.reduce(0, +)
// Numerical underflow at low temperature and small topK (e.g. 0.10
// + 15 over long-form generation) can round every top-k probability
// to zero. Float.random(in: 0..<0) crashes; fall back to greedy
// (the highest-probability token, which topK returns first).
// ref: https://github.com/argmaxinc/argmax-oss-swift/issues/450
guard probSum > 0 else {
return idxArray.first.map(Int32.init) ?? Int32(vocabSize - 1)
}
let randomValue = Float.random(in: 0..<probSum, using: &rng)
var cumulativeSum: Float = 0
for (i, probability) in probsArray.enumerated() {
Expand All @@ -245,6 +253,10 @@ public class GreedyTokenSampler: TokenSampling, @unchecked Sendable {
return idxArray.last.map(Int32.init) ?? Int32(vocabSize - 1)
} else {
let probsArray = await probs.toFloatArray()
let probSum = probsArray.reduce(0, +)
guard probSum > 0 else {
return Int32(vocabSize - 1)
}
let randomValue = Float.random(in: 0..<1, using: &rng)
var cumulativeSum: Float = 0
for (i, probability) in probsArray.enumerated() {
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