Look-ahead Tree Research Articles

Deterministic top-down tree transducers with iterated look-ahead

It is known that by iterating the look-ahead tree languages for deterministic top-down tree automata, more and more powerful recognizing devices are obtained. Let DR 0 = DR, where DR is the class of all tree languages recognizable by deterministic top-down tree automata, and let, for n ⩾ 1, DR n be the class of all tree languages recognizable by deterministic top-down tree automata with DR n−1 look-ahead. Then DR 0 ⊂ DR 1 ⊂ DR 2 ⊂…. Slutzki and Vágvölgyi (1993) showed that the composition powers of the class of all deterministic top-down tree transformations with deterministic top-down look-ahead ( DTT DR ) form a proper hierarchy, i.e. ( DTT DR ) n ⊂( DTT DR ) n + 1 for n ⩾ 0. Along the proof they studied the notion of the deterministic top-down tree transducer with DR n look-ahead (dtt DR n ) and showed that ( DTT DR ) n + 1 ⊆ DTT DR n ( n ⩾ 0), where DTT DR n stands for the class of all tree transformations induced by dtt DR n 'S. Our aim is to show the reversed inclusion, i.e. DTT DR n ⊆ ( DTT DR ) n + 1 ( n ⩾ 0). This implies a precise characterization DTT DR n = ( DTT DR ) n + 1 ( n ⩾ 0), and implies that the classes DTT DR n ( n⩾ 0) form a proper hierarchy.

A spanning tree carry lookahead adder

The design of the 56-b significant adder used in the Advanced Micro Devices Am29050 microprocessor is described. Originally implemented in a 1- mu m design role CMOS process, it evaluates 56-b sums in well under 4 ns. The adder employs a novel method for combining carries which does not require the back propagation associated with carry lookahead, and is not limited to radix-2 trees, as is the binary lookahead carry tree of R.P. Brent and H.T. Kung (1982). The adder also utilizes a hybrid carry lookahead-carry select structure which reduces the number of carriers that need to be derived in the carry lookahead tree. This approach produces a circuit well suited for CMOS implementation because of its balanced load distribution and regular layout.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Another optimization of alpha-beta search

An optimization of the alpha-beta search algorithm is proposed. When used in conjunction with a transposition table, the optimization can reduce the number of positions searched. Measured reductions in searching checkers lookahead trees range from 0% to 16%, with an average reduction of 7%.

Improved Speedup Bounds for Parallel Alpha-Beta Search

In this paper we investigate the ``mandatory-work-first'' approach to parallel alpha-beta search first proposed by Akl, Barnard, and Doran. This approach is based on a version of alpha-beta search without deep cutoffs and a two-stage evaluation process, the second stage of which is often pruned. Our analysis shows that for best-first ordering on the lookahead tree, this approach provides greater speedup than the Palphabeta tree-splitting technique, and that for worst-first ordering, mandatory work first provides only slightly worse speedup than Palphabeta.

Merged Arithmetic

The concept of merged arithmetic is introduced and demonstrated in the context of multiterm multiplication/addition. The merged approach involves synthesizing a composite arithmetic function (such as an inner product) directly instead of decomposing the function into discrete multiplication and addition operations. This approach provides equivalent arithmetic throughput with lower implementation complexity than conventional fast multipliers and carry look-ahead adder trees.

A program for timetable compilation by a look-ahead method

The problem of timetable compilation for a single-track railway is a job-shop scheduling problem but with differences that handicap the generation of feasible solutions. The paper states the problem and describes the algorithm and the experimental results. The idea of the algorithm is that a feasible solution is obtained by successive resolving of “conflicts” between trains, this process being interpreted as the generation of some tree T. The way to resolve a conflict is selected by a lookahead method which enables us to obtain good enough solutions by using a very rough estimate function. One specific feature of the algorithm is that the lookahead tree T′ is not a subtree of T; the other is the culs-de-sac on trees T and T′. When it reaches a cul-de-sac, the algorithm augments the tree with additional nodes.