feat: progress

pkg/questions/multiply_strings/main.go

@@ -0,0 +1,115 @@
+package multiplystrings
+
+func addDigits(a, b byte) (byte, byte) {
+	abInt := int(a-'0') + int(b-'0')
+	return byte(abInt/10) + '0', byte(abInt%10) + '0'
+}
+
+func addByDigit(dst *[]byte, src byte) {
+	for i, b := range *dst {
+		if src == '0' {
+			return
+		}
+
+		src, (*dst)[i] = addDigits(b, src)
+	}
+
+	if src != '0' {
+		*dst = append(*dst, src)
+	}
+}
+
+func add(dst *[]byte, src []byte) {
+	carry := byte('0')
+
+	for i := range max(len(*dst), len(src)) {
+		d, s := byte('0'), byte('0')
+		if len(*dst) > i {
+			d = (*dst)[i]
+		}
+		if len(src) > i {
+			s = src[i]
+		}
+
+		hi, lo := addDigits(d, s)
+		total := []byte{lo, hi}
+		addByDigit(&total, carry)
+		(*dst)[i], carry = total[0], total[1]
+	}
+
+	if carry != '0' {
+		*dst = append(*dst, carry)
+	}
+}
+
+func multiplyDigits(a, b byte) (byte, byte) {
+	abInt := int(a-'0') * int(b-'0')
+	return byte(abInt/10) + '0', byte(abInt%10) + '0'
+}
+
+func multiplyByDigit(and []byte, ier byte) []byte {
+	result, carry := []byte{}, byte('0')
+	for _, b := range and {
+		hi, lo := multiplyDigits(b, ier)
+
+		// Fold the incoming carry into the low digit; any overflow rolls
+		// into hi, which is small enough that it never overflows again.
+		c, lo := addDigits(lo, carry)
+		_, carry = addDigits(hi, c)
+
+		result = append(result, lo)
+	}
+
+	if carry != '0' {
+		result = append(result, carry)
+	}
+
+	return result
+}
+
+func reverse(s string) []byte {
+	digits := make([]byte, len(s))
+	for i := range s {
+		digits[len(s)-1-i] = s[i]
+	}
+
+	return digits
+}
+
+func multiply(a, b string) string {
+	if a == "0" || b == "0" {
+		return "0"
+	}
+
+	and := reverse(a)
+	result := make([]byte, len(a)+len(b))
+	for i := range result {
+		result[i] = '0'
+	}
+
+	for i := range len(b) {
+		ier := b[len(b)-1-i]
+		partial := multiplyByDigit(and, ier)
+
+		// Shift left by i places (little-endian: i leading zeros).
+		shifted := make([]byte, i+len(partial))
+		for j := range i {
+			shifted[j] = '0'
+		}
+		copy(shifted[i:], partial)
+
+		add(&result, shifted)
+	}
+
+	end := len(result)
+	for end > 1 && result[end-1] == '0' {
+		end--
+	}
+
+	out := make([]byte, end)
+	for i := range end {
+		out[i] = result[end-1-i]
+	}
+
+	return string(out)
+}