From f6746281fecd50ba9b093ab6ce385ff06fe3c6a2 Mon Sep 17 00:00:00 2001
From: Thomas Mesnard <thomas.mesnard@ens.fr>
Date: Mon, 21 Dec 2015 11:41:23 +0100
Subject: First complete CTC cost code

---
 ctc.py | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

(limited to 'ctc.py')

diff --git a/ctc.py b/ctc.py
index a0dab80..0ac620b 100644
--- a/ctc.py
+++ b/ctc.py
@@ -7,7 +7,7 @@ from blocks.bricks import Brick
 # L: OUTPUT_SEQUENCE_LENGTH
 # C: NUM_CLASSES
 class CTC(Brick):
-    def apply(l, probs, l_mask=None, probs_mask=None):
+    def apply(l, probs, l_len=None, probs_mask=None):
         """
         Numeration:
             Characters 0 to C-1 are true characters
@@ -15,7 +15,7 @@ class CTC(Brick):
         Inputs:
             l : L x B : the sequence labelling
             probs : T x B x C+1 : the probabilities output by the RNN
-            l_mask : L x B
+            l_len : B : the length of each labelling sequence
             probs_mask : T x B
         Output: the B probabilities of the labelling sequences
         Steps:
@@ -31,8 +31,9 @@ class CTC(Brick):
         B = l.shape[1]
         
         # l_blk = l with interleaved blanks
-        l_blk = tensor.zeros((S, B))
+        l_blk = C * tensor.ones((S, B))
         l_blk = tensor.set_subtensor(l_blk[1::2,:],l)
+        l_blk = l_blk.T     # now l_blk is B x S
 
         # dimension of alpha (corresponds to alpha hat in the paper) :
         #   T x B x S
@@ -48,17 +49,32 @@ class CTC(Brick):
         alpha0 = alpha0 / c0[:,None]
 
         # recursion
+        l_blk_2 = tensor.concatenate([-tensor.ones((B,2)), l_blk[:,:-2]], axis=1)
+        l_case2 = tensor.ne(l_blk, numpy.float32(C)) * tensor.ne(l_blk, l_blk_2)
+        # l_case2 is B x S
+
         def recursion(p, p_mask, prev_alpha, prev_c):
-            # TODO
-            return prev_alpha[-1], prev_c[-1]
+            prev_alpha = prev_alpha[-1]
+            # p is B x C+1
+            # prev_alpha is B x S 
+            prev_alpha_1 = tensor.concatenate([tensor.zeros((B,1)),prev_alpha[:,:-1]], axis=1)
+            prev_alpha_2 = tensor.concatenate([tensor.zeros((B,2)),prev_alpha[:,:-2]], axis=1)
+
+            alphabar = prev_alpha + prev_alpha1
+            alphabar = tensor.switch(l_case2, alphabar + prev_alpha2, alphabar)
+            next_alpha = alpha_bar * p[tensor.arange(B)[:,None].repeat(S,axis=1).flatten(), l_blk.flatten()].reshape((B,S))
+            next_alpha = tensor.switch(p_mask[:,None], next_alpha, prev_alpha]
+            next_c = next_alpha.sum(axis=1)
+            
+            return next_alpha / next_c[:, None], next_c
 
         # apply the recursion with scan
         alpha, c = tensor.scan(fn=recursion,
                                sequences=[probs, probs_mask],
                                outputs_info=[alpha0, c0])
 
-        # return the probability of the labellings
-
+        # return the log probability of the labellings
+        return tensor.log(c).sum(axis=0)
 
     
     def best_path_decoding(y_hat, y_hat_mask=None):
-- 
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