A-M baby

Task/Long-multiplication/Go/long-multiplication.go

@@ -0,0 +1,79 @@
+// Long multiplication per WP article referenced by task description.
+// That is, multiplicand is multiplied by single digits of multiplier
+// to form intermediate results.  Intermediate results are accumulated
+// for the product.  Used here is the abacus method mentioned by the
+// article, of summing intermediate results as they are produced,
+// rather than all at once at the end.
+//
+// Limitations:  Negative numbers not supported, superfluous leading zeros
+// not generally removed.
+package main
+
+import "fmt"
+
+// argument validation
+func d(b byte) byte {
+    if b < '0' || b > '9' {
+        panic("digit 0-9 expected")
+    }
+    return b - '0'
+}
+
+// add two numbers as strings
+func add(x, y string) string {
+    if len(y) > len(x) {
+        x, y = y, x
+    }
+    b := make([]byte, len(x)+1)
+    var c byte
+    for i := 1; i <= len(x); i++ {
+        if i <= len(y) {
+            c += d(y[len(y)-i])
+        }
+        s := d(x[len(x)-i]) + c
+        c = s / 10
+        b[len(b)-i] = (s % 10) + '0'
+    }
+    if c == 0 {
+        return string(b[1:])
+    }
+    b[0] = c + '0'
+    return string(b)
+}
+
+// multipy a number by a single digit
+func mulDigit(x string, y byte) string {
+    if y == '0' {
+        return "0"
+    }
+    y = d(y)
+    b := make([]byte, len(x)+1)
+    var c byte
+    for i := 1; i <= len(x); i++ {
+        s := d(x[len(x)-i])*y + c
+        c = s / 10
+        b[len(b)-i] = (s % 10) + '0'
+    }
+    if c == 0 {
+        return string(b[1:])
+    }
+    b[0] = c + '0'
+    return string(b)
+}
+
+// multiply two numbers as strings
+func mul(x, y string) string {
+    result := mulDigit(x, y[len(y)-1])
+    for i, zeros := 2, ""; i <= len(y); i++ {
+        zeros += "0"
+        result = add(result, mulDigit(x, y[len(y)-i])+zeros)
+    }
+    return result
+}
+
+// requested output
+const n = "18446744073709551616"
+
+func main() {
+    fmt.Println(mul(n, n))
+}