addition on 1-byte integers

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addition on 1-byte integers

Postby Roger|Dyalog on Thu Jun 03, 2021 5:26 pm

In previous posts, Membership, Index-Of, Interval Index, and Replicate, the game was to write functions g such that

      (x symbol y) ≡ x g y

for x and y of interest where g does not use the primitive symbol in question. Here, the game is to write a function h which does not use + or -, so that for 1-byte integer vectors x and y:

      all1 ← {¯128,(⍎∊(⊂' ¯'),¨⍕¨⌽1↓a),a←⍳128}  ⍝ all 1-byte ints
A ← all1 ⍬

x ← A[?5678 ⍴ ≢A]
y ← A[?5678 ⍴ ≢A]

(x+y) ≡ x h y

⎕io←0 is assumed. ⎕io delenda est!

Base Value

      p1b ← {1⊥⍺,[¯0.5]⍵}

(x+y) ≡ x p1b y

From the definition of ⍺⊥⍵ it can be shown that 1⊥⍺,⍵ ←→ ⍺+⍵ and more generally 1⊥⍺,[¯0.5]⍵ ←→ ⍺+⍵.

Table Look-Up

      plus1T ← {256 256 ↑ 256 514 ⍴ (⍎∊(⊂' ¯'),¨⍕¨⌽1↓v),v←⍳257}
⍝ addition table for 1-byte ints
A ← all1 ⍬
T ← plus1T ⍬

p1t ← {T[(A⍳⍺),¨(A⍳⍵)]}

(x+y) ≡ x p1t y

p1t is substantially slower than p1b.
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