# Universal Hirschberg for Width Bounded Dynamic Programs

arXiv: https://arxiv.org/abs/2512.10132 · HTML: https://mlsanity.com/paper/2512.10132/

## Key facts

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- First posted on arXiv 10 Dec 2025.

## Abstract (arXiv)

Hirschberg's algorithm (1975) reduces the space complexity for the longest common subsequence problem from O(N2) to O(N) via recursive midpoint bisection on a grid dynamic program (DP). We show that the underlying idea generalizes to a broad class of dynamic programs with local dependencies on directed acyclic graphs (DP DAGs). Modeling a DP as deterministic time evolution over a topologically ordered DAG with frontier width ω and bounded in-degree, and assuming a max-type semiring with deterministic tie breaking, we prove that in a standard offline random-access model any such DP admits deterministic traceback in space O(ω log T + (log T)O(1)) cells over a fixed finite alphabet, where T is the number of states. Our construction replaces backward dynamic programs by forward-only recomputation and organizes the time order into a height-compressed recursion tree whose nodes expose small "middle frontiers'' across which every optimal path must pass. The framework yields near-optimal traceback bounds for asymmetric and banded sequence alignment, one-dimensional recurrences, and dynamic-programming formulations on graphs of bounded pathwidth. We also show that an Omega(ω) space term (in bits) is unavoidable in forward single-pass models and discuss conjectured √T-type barriers in streaming settings, supporting the view that space-efficient traceback is a structural property of width-bounded DP DAGs rather than a peculiarity of grid-based algorithms.