go-qrcode/reedsolomon/gf_poly.go

// go-qrcode
// Copyright 2014 Tom Harwood

package reedsolomon

import (
	"fmt"
	"log"

	bitset "github.com/skip2/go-qrcode/bitset"
)

// gfPoly is a polynomial over GF(2^8).
type gfPoly struct {
	// The ith value is the coefficient of the ith degree of x.
	// term[0]*(x^0) + term[1]*(x^1) + term[2]*(x^2) ...
	term []gfElement
}

// newGFPolyFromData returns |data| as a polynomial over GF(2^8).
//
// Each data byte becomes the coefficient of an x term.
//
// For an n byte input the polynomial is:
// data[n-1]*(x^n-1) + data[n-2]*(x^n-2) ... + data[0]*(x^0).
func newGFPolyFromData(data *bitset.Bitset) gfPoly {
	numTotalBytes := data.Len() / 8
	if data.Len()%8 != 0 {
		numTotalBytes++
	}

	result := gfPoly{term: make([]gfElement, numTotalBytes)}

	i := numTotalBytes - 1
	for j := 0; j < data.Len(); j += 8 {
		result.term[i] = gfElement(data.ByteAt(j))
		i--
	}

	return result
}

// newGFPolyMonomial returns term*(x^degree).
func newGFPolyMonomial(term gfElement, degree int) gfPoly {
	if term == gfZero {
		return gfPoly{}
	}

	result := gfPoly{term: make([]gfElement, degree+1)}
	result.term[degree] = term

	return result
}

func (e gfPoly) data(numTerms int) []byte {
	result := make([]byte, numTerms)

	i := numTerms - len(e.term)
	for j := len(e.term) - 1; j >= 0; j-- {
		result[i] = byte(e.term[j])
		i++
	}

	return result
}

// numTerms returns the number of
func (e gfPoly) numTerms() int {
	return len(e.term)
}

// gfPolyMultiply returns a * b.
func gfPolyMultiply(a, b gfPoly) gfPoly {
	numATerms := a.numTerms()
	numBTerms := b.numTerms()

	result := gfPoly{term: make([]gfElement, numATerms+numBTerms)}

	for i := 0; i < numATerms; i++ {
		for j := 0; j < numBTerms; j++ {
			if a.term[i] != 0 && b.term[j] != 0 {
				monomial := gfPoly{term: make([]gfElement, i+j+1)}
				monomial.term[i+j] = gfMultiply(a.term[i], b.term[j])

				result = gfPolyAdd(result, monomial)
			}
		}
	}

	return result.normalised()
}

// gfPolyRemainder return the remainder of numerator / denominator.
func gfPolyRemainder(numerator, denominator gfPoly) gfPoly {
	if denominator.equals(gfPoly{}) {
		log.Panicln("Remainder by zero")
	}

	remainder := numerator

	for remainder.numTerms() >= denominator.numTerms() {
		degree := remainder.numTerms() - denominator.numTerms()
		coefficient := gfDivide(remainder.term[remainder.numTerms()-1],
			denominator.term[denominator.numTerms()-1])

		divisor := gfPolyMultiply(denominator,
			newGFPolyMonomial(coefficient, degree))

		remainder = gfPolyAdd(remainder, divisor)
	}

	return remainder.normalised()
}

// gfPolyAdd returns a + b.
func gfPolyAdd(a, b gfPoly) gfPoly {
	numATerms := a.numTerms()
	numBTerms := b.numTerms()

	numTerms := numATerms
	if numBTerms > numTerms {
		numTerms = numBTerms
	}

	result := gfPoly{term: make([]gfElement, numTerms)}

	for i := 0; i < numTerms; i++ {
		switch {
		case numATerms > i && numBTerms > i:
			result.term[i] = gfAdd(a.term[i], b.term[i])
		case numATerms > i:
			result.term[i] = a.term[i]
		default:
			result.term[i] = b.term[i]
		}
	}

	return result.normalised()
}

func (e gfPoly) normalised() gfPoly {
	numTerms := e.numTerms()
	maxNonzeroTerm := numTerms - 1

	for i := numTerms - 1; i >= 0; i-- {
		if e.term[i] != 0 {
			break
		}

		maxNonzeroTerm = i - 1
	}

	if maxNonzeroTerm < 0 {
		return gfPoly{}
	} else if maxNonzeroTerm < numTerms-1 {
		e.term = e.term[0 : maxNonzeroTerm+1]
	}

	return e
}

func (e gfPoly) string(useIndexForm bool) string {
	var str string
	numTerms := e.numTerms()

	for i := numTerms - 1; i >= 0; i-- {
		if e.term[i] > 0 {
			if len(str) > 0 {
				str += " + "
			}

			if !useIndexForm {
				str += fmt.Sprintf("%dx^%d", e.term[i], i)
			} else {
				str += fmt.Sprintf("a^%dx^%d", gfLogTable[e.term[i]], i)
			}
		}
	}

	if len(str) == 0 {
		str = "0"
	}

	return str
}

// equals returns true if e == other.
func (e gfPoly) equals(other gfPoly) bool {
	var minecPoly *gfPoly
	var maxecPoly *gfPoly

	if e.numTerms() > other.numTerms() {
		minecPoly = &other
		maxecPoly = &e
	} else {
		minecPoly = &e
		maxecPoly = &other
	}

	numMinTerms := minecPoly.numTerms()
	numMaxTerms := maxecPoly.numTerms()

	for i := 0; i < numMinTerms; i++ {
		if e.term[i] != other.term[i] {
			return false
		}
	}

	for i := numMinTerms; i < numMaxTerms; i++ {
		if maxecPoly.term[i] != 0 {
			return false
		}
	}

	return true
}