kaldi 源码阅读-transition-model

transition-model这个类定义在./src/hmm/transition-model.h中。先介绍一下相关的概念：

1. 每一个phone会有一个HMM topology，它会包含若干个hmmstate。
2. 每一个hmmstate上会有一个相关的pdf_class.这个pdf_class 在经过决策树聚类后，会被一个pdf_id代替。
3. 先定义了一个transition state=（phone，hmmstate，pdf_id），这是一个三元组，对应到某个phone的某个状态的pdf。
4. 上一篇关于hmm-topology.h的介绍中，定义了transition的概念，它对应一个transition-index。
5. 定义transition-id =（transition-state， transition-index）。

那么transition-model.h 实际上定义了上述的对应关系：

(phone, HMM-state, forward-pdf-id, self-loop-pdf-id) -> transition-state
(transition-state, transition-index) -> transition-id

涉及到的几个概念：

1. phone: a phone index (1, 2, 3 …)
2. hmmstate: a number (0, 1, 2…) that indexes TopologyEntry (see hmm-topology.h)
3. pdf-id: a number output by the Compute function of ContextDependency. Zero-based.
4. transition-state: the states for which we estimate transition probabilities for transitions out of them. In some topologies, will map one-to-one with pdf-ids. One-based, since it appears on FSTs.
5. transition-index: identifier of a transition (or final-prob) in the HMM. Zero-based.
6. transition-id: identifier of a unique parameter of the TransitionModel. =(transition-state, transition-index) pair. One-based, since it appears on FSTs.