Merge Loop [BPE] #67
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Eamon2009
eamonsippy
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Merge Loop [BPE]
#67
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The Merge Loop: Explain the core logic of the BPE training loop: How does the priority queue (pq) and the merge_rank hash map work together to decide which two tokens should be merged next? |
Answered by
Eamon2009
Jul 28, 2026
Replies: 1 comment
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The code tracks all adjacent token pairs using a priority queue (pq). It pulls the pair with the lowest rank (the earliest BPE merge rule). If both tokens in the pair are still marked as active = 1 in the linked list, it replaces the left token's ID with the new merged ID, marks the right token as active = 0 (effectively deleting it), updates the prev and next pointers to bypass the deleted node, and pushes the newly formed adjacent pairs back into the queue. |
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The code tracks all adjacent token pairs using a priority queue (pq). It pulls the pair with the lowest rank (the earliest BPE merge rule). If both tokens in the pair are still marked as active = 1 in the linked list, it replaces the left token's ID with the new merged ID, marks the right token as active = 0 (effectively deleting it), updates the prev and next pointers to bypass the deleted node, and pushes the newly formed adjacent pairs back into the queue.