package snapshot

import (
	"bytes"
)

// Estimate the number of iterations necessary to step from start to end.
func estimateSteps(start []byte, end []byte, depth int) uint64 {
	// We see paths in several forms (nil, 0600, 06, etc.). We need to adjust them to a comparable form.
	// For nil, start and end indicate the extremes of 0x0 and 0x10.  For differences in depth, we often see a
	// start/end range on a bounded iterator specified like 0500:0600, while the value returned by it.Path() may
	// be shorter, like 06.  Since our goal is to estimate how many steps it would take to move from start to end,
	// we want to perform the comparison at a stable depth, since to move from 05 to 06 is only 1 step, but
	// to move from 0500:06 is 16.
	normalizePathRange := func(start []byte, end []byte, depth int) ([]byte, []byte) {
		if 0 == len(start) {
			start = []byte{0x0}
		}
		if 0 == len(end) {
			end = []byte{0x10}
		}
		normalizedStart := make([]byte, depth)
		normalizedEnd := make([]byte, depth)
		for i := 0; i < depth; i++ {
			if i < len(start) {
				normalizedStart[i] = start[i]
			}
			if i < len(end) {
				normalizedEnd[i] = end[i]
			}
		}
		return normalizedStart, normalizedEnd
	}

	// We have no need to handle negative exponents, so uints are fine.
	pow := func(x uint64, y uint) uint64 {
		if 0 == y {
			return 1
		}
		ret := x
		for i := uint(0); i < y; i++ {
			ret *= x
		}
		return x
	}

	// Fix the paths.
	start, end = normalizePathRange(start, end, depth)

	// No negative distances, if the start is already >= end, the distance is 0.
	if bytes.Compare(start, end) >= 0 {
		return 0
	}

	// Subtract each component, right to left, carrying over if necessary.
	difference := make([]byte, len(start))
	var carry byte = 0
	for i := len(start) - 1; i >= 0; i-- {
		result := end[i] - start[i] - carry
		if result > 0xf && i > 0 {
			result &= 0xf
			carry = 1
		} else {
			carry = 0
		}
		difference[i] = result
	}

	// Calculate the result.
	var ret uint64 = 0
	for i := 0; i < len(difference); i++ {
		ret += uint64(difference[i]) * pow(16, uint(len(difference)-i-1))
	}

	return ret
}