utils.go

package xlsx

import "fmt"

func createIdentifierFromCoords(x, y int) string {
	letterPart := numericToLetters(x)
	numericPart := y + 1
	return fmt.Sprintf("%s%d", letterPart, numericPart)
}

func numericToLetters(colRef int) string {
	parts := intToBase26(colRef)
	return formatColumnName(smooshBase26Slice(parts))
}

func intToBase26(x int) (parts []int) {
	// Excel column codes are pure evil - in essence they're just
	// base26, but they don't represent the number 0.
	b26Denominator, _ := getLargestDenominator(x, 1, 26, 0)

	// This loop terminates because integer division of 1 / 26
	// returns 0.
	for d := b26Denominator; d > 0; d = d / 26 {
		value := x / d
		remainder := x % d
		parts = append(parts, value)
		x = remainder
	}
	return parts
}

func getLargestDenominator(numerator, multiple, baseDenominator, power int) (int, int) {
	if numerator/multiple == 0 {
		return 1, power
	}
	next, nextPower := getLargestDenominator(
		numerator, multiple*baseDenominator, baseDenominator, power+1)
	if next > multiple {
		return next, nextPower
	}
	return multiple, power
}

func smooshBase26Slice(b26 []int) []int {
	// Smoosh values together, eliminating 0s from all but the
	// least significant part.
	lastButOnePart := len(b26) - 2
	for i := lastButOnePart; i > 0; i-- {
		part := b26[i]
		if part == 0 {
			greaterPart := b26[i-1]
			if greaterPart > 0 {
				b26[i-1] = greaterPart - 1
				b26[i] = 26
			}
		}
	}
	return b26
}

func formatColumnName(colID []int) string {
	lastPart := len(colID) - 1

	result := ""
	for n, part := range colID {
		if n == lastPart {
			// The least significant number is in the
			// range 0-25, all other numbers are 1-26,
			// hence we use a different offset for the
			// last part.
			result += string(part + 65)
		} else {
			// Don't output leading 0s, as there is no
			// representation of 0 in this format.
			if part > 0 {
				result += string(part + 64)
			}
		}
	}
	return result
}