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Task/Long-multiplication/Nim/long-multiplication.nim

@@ -0,0 +1,41 @@
+import strutils
+
+proc ti(a): int = ord(a) - ord('0')
+
+proc longmulti(a, b: string): string =
+  var
+    i, j, n, carry, la, lb = 0
+    k = false
+
+  # either is zero, return "0"
+  if a == "0" or b == "0":
+    return "0"
+
+  # see if either a or b is negative
+  if a[0] == '-':
+    i = 1; k = not k
+  if b[0] == '-':
+    j = 1; k = not k
+
+  # if yes, prepend minus sign if needed and skip the sign
+  if i > 0 or j > 0:
+    result = if k: "-" else: ""
+    result.add longmulti(a[i..a.high], b[j..b.high])
+    return
+
+  result = repeatChar(a.len + b.len, '0')
+
+  for i in countdown(a.high, 0):
+    var carry = 0
+    var k = i + b.len
+    for j in countdown(b.high, 0):
+      let n = ti(a[i]) * ti(b[j]) + ti(result[k]) + carry
+      carry = n div 10
+      result[k] = chr(n mod 10 + ord('0'))
+      dec k
+    result[k] = chr(ord(result[k]) + carry)
+
+  if result[0] == '0':
+    result[0..result.high-1] = result[1..result.high]
+
+echo longmulti("-18446744073709551616", "-18446744073709551616")