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Task/Long-multiplication/EchoLisp/long-multiplication.echolisp

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+(lib 'math) ;; for poly multiplication
+
+;; convert string of decimal digits to polynomial
+;; "1234" → x^3 +2x^2 +3x +4
+;; least-significant digit first
+(define (string->long N)
+	(reverse (map string->number (string->list N))))
+	
+;; convert polynomial to string
+(define (long->string N)
+(if (pair? N)
+   (string-append (number->string (first N)) (long->string (rest N)))  ""))
+
+;; convert poly coefficients to base 10
+(define (poly->10 P (carry 0))
+(append
+	(for/list ((coeff P))
+		(set! coeff (+ carry coeff ))
+		(set! carry (quotient coeff 10)) ;; new carry
+		(modulo coeff 10))
+	(if(zero? carry) null (list carry)))) ;; remove leading 0 if any
+
+;; long multiplication
+;; convert input - strings of decimal digits - to polynomials
+;; perform poly multiplication in base 10
+;; convert result to string of decimal digits
+
+(define (long-mul A B )
+ (long->string (reverse  (poly->10 (poly-mul (string->long A) (string->long B))))))
+
+(define two-64 "18446744073709551616")
+(long-mul two-64 two-64)
+    → "340282366920938463463374607431768211456"
+
+;; check it
+(lib 'bigint)
+Lib: bigint.lib loaded.
+(expt 2 128)
+   → 340282366920938463463374607431768211456